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HOLOGRAPHY
A BEGINNER'S GUIDE
Douglas E. Tyler
© 1999 Douglas E. Tyler
No part of this publication may be reproduced in any manner or by any means without the
prior written permission of the author.
CAUTION:
The use of lasers, photochemistry and other materials involved in
the creation of holograms may be hazardous. Participants in this workshop
should exercise caution in dealing with all materials as instructed by staff of the
workshop. In particular, participants should be aware of the following cautions:
GENERAL SAFETY PRECAUTIONS FOR WORKSHOP PARTICIPANTS
A. Never stare directly into a beam of laser light or at it's reflection off of shiny
surfaces. Laser light is a concentrated form of energy which can cause damage
to the human eye.
B. Avoid direct contact with those photochemicals used in processing
holograms. Use protective gloves or photo instruments while handling these
materials. Use photochemicals only in well ventilated areas and avoid prolonged
breathing of vapors.
C. In instances where elecrical instruments, including timers, dryers and so forth
are used near wet processing areas participants must be certain that their
hands are dry before handling these instruments.
HOLOGRAPHY
A BEGINNER'S GUIDE
HOLO / GRAPHY
WHOLE MESSAGE
The essential theoretical principles of holography were proposed by Dr. Dennis Gabor in the late
1940's. Gabor did not originate the term holography, however, instead, in his early papers, he
referred to it by the rather complex, descriptive name "wave-front reconstruction." Although it
is not absolutely clear who first introduced the term holography to refer to Gabor's discovery
there is reason to believe that Gordon Rogers, an early pioneer in the field, deserves this credit.
Rogers, of Dundee, Scotland, apparently used this term because the theory of "wave-front
reconstruction" provided a technique for a more complete form of image recording. Over time,
a simple vocabulary has developed within the medium wherein the general process of wavefront
reconstruction is referred to as holography.
HOLOGRAMS (not holographs):
In contrast to the process of holography, a single image created through holographic means is
referred to as a hologram ! This is obviously simple terminology, but its proper use helps to
clarify meaning while discussing the medium. The term "holograph" has been used to describe a
singular image as well and in fact is considered proper usage according to the Oxford English
Dictionary. However, a "holograph" is also defined as a written document such as a will,
contract, etc. and because this presents some confusion it seems more practical to refer the
singular image as a hologram.
A DEFINITION OF HOLOGRAPHY:
Although it is difficult, if not presumptuous, to try to define sophisticated scientific principles
and natural phenomena of the holographic process in a few short sentences, the following gives
an adequate overview of the primary components in this technology.
"Holography is the recording (construction) of wavefront interference patterns created by coherent (laser) light sources and
the replay (reconstruction) of these patterns as visual images or
other forms of wave media."
In some instances of definition, it is as important to clarify what you are not describing as what
you are. This is certainly the case in holography. The popular awareness of three-dimensionality
in the holographic image has led to many confusing associations of the medium with other, less
sophisticated, technologies.
Among these are the following:
A.
Lenticular Devices - There are a great variety of these and most of us have
encountered them in post card form at some time. These are the popular cards which either
have a three-dimensional appearance or that jump rapidly from one flat image to another as the
card is moved in the hand. Often "lenticulars" can by distinguished by their highly textured
surface (these are the lenses which form the image you see). In recent times the "NIMSLO"
camera has offered a related process which allows you to create these sorts of images by using
their special
camera and processing procedures.
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B.
Film Animation - Fictional, future-oriented, projections of holography have often
gone far beyond the real capabilities of the medium. Wanting to satisfy the dreams of movie
audiences however, filmmakers have resorted to special animation techniques and trick
photography to simulate holography as it might be at some point in the future. The most
popular examples of this are the scenes from such films as "Star Wars." Many of us wish that
holography was at such an advanced stage in its development but, sadly, it isn't. Real
holograms have been used in films such as "Logan's Run" and "The Man Who Fell Earth," but
because film is a two-dimensional medium it is incapable of carrying the same level of
information as a hologram (i.e. a 2-D image of a 3-D image is still 2-D).
C.
The Floating Penny and Other Optical Illusions - Long before the development
of holography, physicists and artists used special mirror and optical configurations to project
images into and through space. Various novelties sold today still rely upon this, as do some of
the popular diversions in the haunted house at Disneyworld and other theme parks. These are
not holograms, simply sophisticated visual tricks for your viewing pleasure !
D.
Stereo Photography (anaglyphs, stereo pairs, et al) – Arguably the earliest
form of 3-D imaging. A pair of images created from slightly separated vantage points are
reconstructed with the aid of a viewing device.
E.
Random Dot Stereograms – a popular visual diversion in which a seemingly endless
series of small images (or dots) when viewed properly reconstructs for the eye/brain as a 3-D
form.
F.
Virtual reality – A sophisticated form of digital stereo imaging in which with the aid
of a stereo viewing device one may navigate seemingly real 3-D space. The more sophisticated
forms incorporate body sensing elements that permit the participant to interact with this
“virtual space.”
FEATURES OF A HOLOGRAM:
Because of the many varieties of holograms which are possible with the present technology, it is
difficulty to describe what a hologram is. The following features may indicate that what you are
looking at is a hologram. Keep in mind that these are just a few – there are many others.
A.
There is some surface into which you are looking. The physical portion of a hologram
is a photographic emulsion and it needs some support surface. Generally when looking at
holograms the image you see is related to a tangible material (glass plate or piece of film).
B.
The image is "vaporous" and does not appear to have a physical presence. You may
be able to touch the image itself, but you feel nothing when you do.
C.
Some special lighting is involved. A hologram is an image comprised of light, created
by the holograms ability to modulate (bend, that is) light in a highly sophisticated and controlled
manner. In order for the proper replay of the holographic image a special form and arrangement
of illumination may be required. In certain instances this may involve cumbersome and
noticeable paraphernalia.
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OPTICS: SOME BASIC BACKGROUND:
The dictionary defines Optics as "the science of light . . . that deals with its genesis and
propagation and the effects that it undergoes and produces." Essentially, the study of optics
has to do with understanding the properties and actions of light as it moves through or comes
in contact with various materials. Within the field of optics there are essentially three primary
divisions which focus upon these specific properties of light. In ascending order of complexity
they are:
Geometrical optics - this area of optics operates on the primary assumption that light travels
in a straight line. Although this is an abstraction of reality, it nevertheless permits rather direct
and accurate explanation and prediction of certain optical phenomena such as the focusing of
light by lenses. In this text, the use of geometrical optics relies heavily upon the principle of
refraction to explain the interaction of light with various substances.
Wave optics - is also sometimes referred to as physical optics because it seeks to explain the
action and effects of light through the mechanism of waves, a more physical phenomenon.
Although less abstracted than geometrical optics, it nevertheless relies more upon mathematical
models, and thus is still distanced from the physical realities involved in the actions of light. For
our purposes the primary value of wave optics will be in explaining the concept of diffraction.
Quantum optics - this field of optics concerns itself with the interaction of light and matter
on the atomic level. Although such information may be useful to scientists in explaining certain
events these effects are not easy to demonstrate in a visible form and therefore don't recieve
significant attention in these notes.
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GOMETRICAL OPTICS AND REFRACTION:
As stated previously, geometrical optics assumes that light travels in a straight line and in this
form is referred to as a ray. There is no concern here with the substance of the light itself;
however, there is considerable interest in this field of optics with the effects which differing
substances will have upon a ray of light as it interacts with them. In this regard there are
essentially three common and well-known properties of physical materials that describe their
effect upon light. They are:
Opaque - light striking these materials may be reflected in greater or lesser degree but usually
the material absorbs a certain portion of it. Critical to concern in optics is the fact that such
materials do not permit light to pass through them.
Translucent - the physical structure of these materials does permit rays of light to pass
through them, but often distorts the direction of their movement so drastically that images may
not be recognizable through it. Examples of such a material are ground glass, milk glass and so
forth.
Transparent - such materials as these permit the passage of rays of light in a relatively
unhindered fashion. In most instances the movement of light rays through transparent
materials is also accompanied by distortions and changes in the directions of the rays
movement. Of great importance here, however, is the fact that such movement and distortion
can often be controlled in order to produce very specific optical effects.
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REFRACTION:
The effect of transparent materials upon rays of light moving in a given direction is usually
described in geometrical optics by the principle of refraction. This phenomena is widely known
and often demonstrated and observed by the effect which water has upon the appearance of
objects, a pencil is the popular example, placed in a glass of water. The dynamic effects of
refraction are more dramatically demonstrated in the "floating penny" demonstration. In this
demonstration a coin is placed in the bottom of a dish and the observer asked to sight along the
top of the dish. As water gradually fills the bowl, light reflected off the coin on the bottom is
bent toward the direction of the observer and the coin thus appears to them, as if floating on
the surface of the water.
EMPTY GLASS
NO REFRACTION
WATER REFRACTS
LIGHT OF PENCIL
The significance of this refraction demonstration lies not only in its evidencing the ability of
certain materials to bend light, but also the spatial positioning of images in optical systems. If
the observer in this situation attempts to touch or locate the coin in the image they are
viewing, they will discover that it appears to come from the surface of the water. Although this
location
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may be correct to the eye, it obviously does not correspond to the physical position of the coin.
Thus, it must be understood from the outset that the image of an object does not always
correspond to its physical location in space. This simple coin experiment may help us to
understand what eventually occurs in a holographic image. Light, modulated by the recording
emulsion of the hologram reconstructs waves of light which appear to come from a specific object
located at a specific point in space. Thus, the images which we see are mere patterns of light and
if through some technique we are able to recreate those patterns we are able to make images
appear without their commonly associated object.
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Understanding this, we must develop some understanding of the mechanism of refraction. In
the simplest terms it may be stated that when a ray of light moving through space passes into a
medium of different density the direction of its movement will be altered so long as it does not
enter the medium at a perpendicular (90) angle. A physical explanation for this may be that
light seeks the shortest time/distance between two points. Thus, it may be that the path of a
ray of light may be more efficient if it assumes the "bent" path than if it continues in a straight
line.
The degree to which a ray of light is bent while passing through a transparent material is
dependent upon the density of the material which it enters. The denser the material the more
drastically the ray will be bent or diverted from its original course. This fact is often expressed
in the refractive index of the material which is a ratio of the speed of light through a vacuum
compared to the speed of light through any more dense material. The refractive index is usually
symbolized by the letter "n" and the refractive index of a vacuum as base is referred to as 1.0
or n = 1.0. In comparison, water has a refractive index of about n = 1.33 and glass of about n =
1.50.
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The direction in which light bends when passing between materials of differing densities is easily
predicted by its relationship to the surface which it enters. If a ray of light enters at 90˚ or
perpendicular to the surface (Fig. 1) there is no change brought about in the direction of its
movement. This perpendicular line is often referred to as the NORMAL and is used as a basis
for expressing other directional relationships. If a ray passes from a medium of lesser density to
a medium of greater density its path or direction is bent toward the normal (Fig. 2).
Obviously, if a beam passes from a more dense to a less dense material the opposite is true and
the path of the ray is diverted away from the normal (Fig. 3). Generally speaking, the sharper the
angle of entry (the greater the angle in relationship to the normal) the more sharply a ray will be
bent when entering the new material.
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The relationship here can be described in a simple mathematical model or equation:
n x sine θ = n' x sine θ'
Thus if we know the refractive indices of two materials, say air and water, we can predict the
bending which will occur when a ray of light moves between these two materials. Following are
some sample equations to demonstrate how simply this formula may be worked.
1.
a ray of light traveling through air (n = 1.0) is incident at an angle of 45˚ on a smooth
water surface (n = 1.3). Given this information determine the angle of refraction of this
light ray.
n X sine θ = n' X sine θ'
so
1 X sine 45˚ = 1.3 X sine θ'
Then sine 45˚ = 1.3 X sine θ'
and sine 45 = sine θ'
1.3
(note: the sine of a 45˚ angle equals .7071)
Then
.5439
sine θ' =
the corresponding angle for this sine is 32.9514.
Then the angle O' = 32.9514 or 33˚ .
The diagram below illustrates how these various angle measurement relate.
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This simple equation is known as Snell's Law and may be worked on an infinite variety of
examples. Try working this example. A ray of light traveling through air is incident on a smooth
glass surface at an angle of 13˚. If the refractive index of this glass is 1.5358 what will the
angle be at which the ray is refracted within the glass?
n x sine θ = n' x sine θ'
so
1 x sine 13˚
then
sine 13˚
and
.2246 = sine θ'
1.5258
then
sine θ' =
=
=
1.5258 x sine θ'
1.5258 x sine θ'
(note: the sin of a 13˚ angle = .2246)
.1472
the corresponding angle for this sine is 8.46
then the angle θ' = 8.46 or
9˚
The diagram below illustrates how these various angle measurements relate.
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REFRACTION AND COLOR DISPERSION:
It might be appropriate at this stage to point out that not all wavelengths of light are refracted
in the same manner as each other. In examining the refractive response of different colors of
light it becomes clear that light sources with shorter wavelengths, and thus higher energy
levels, are refracted more radically than are those with longer wavelengths. Therefore, if white
light passes through various materials at the appropriate angles, the various wavelengths of
light which comprise the white-light source will be diverted to specific points in space, a
phenomena which we experience when viewing a prism spectrum or rainbow.
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Having observed the effect of transparent materials upon the movement of a ray of light it
might be of interest to inquire about the opposing action i.e. what happens to a ray of light as
it passes from a material of greater density to one of lesser density. By backtracking through
our example we discover that the opposite effect is observed. When a ray of light passes from
glass into air it bends away from the normal. In examing this phenomena we soon realize that
this must be the case, what occurs with light moving in one direction must be repeated when
the direction of the light is reversed. In optics this is referred to as the "principle of
reversibility" and we may observe it by reworking the formula previously given for Snell's law.
A ray of light is passing through a glass of water and is incident upon an air surface at an angle
of 32.9154. At what angle or direction (relative to the normal) will the beam travel in air?
Knowing the refractive index figures for air and glass, we may plug them into the formula for the
answer.
n
x
so:
then:
then
sine θ
=
n' x
1 x sine θ =
sine θ
sine θ =
=
1.3
1.3
x
sin θ'
x sine 32.9514
.5439
(if the sine of 32.9514˚ is .5439)
.7071057
and 45˚ is the corresponding angle for this sine.
then the angle θ = 45˚
The diagram below illustrates how these various angle measurements relate.
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The principle of reversibility is important to understand because it accounts for some optical
phenomena that may plague the holographer. The phenomena in question is referred to as
"TOTAL INTERNAL REFLECTION" and occurs when a beam of light moving from a dense material
into a less dense material is incident on this interface at too steep an angle. The ray of light in
this case cannot exit the denser material and is 100 % totally reflected internally. For this
simple reason this is referred to as total internal refection and the angle at which rays of light
begin to be totally internally reflected is referred to as the "CRITICAL ANGLE". In glass the
critical angle is approximately 41˚. Thus any ray of light incident on a glass air interface at an
angle greater than 41˚ (relative to the normal) will be reflected back into the glass. Again the
formula expressing Snell's law reflects this phenomena, however, for brevity a diagram is
included to demonstrate the concept.
The phenomena of Total Internal Reflection has practical implications when we you create
holograms. The support materials for holographic emulsions are usually glass or some form of
plastic. Frequently the laser light being used to construct a hologram will enter the supporting
glass or plastic from the side of the material. Those rays that enter your cover glass from the
side are often subject to total internal reflection. The result is, in the case of single beam
reflection holograms, a series of progressively stepped bands of color in the image. This
undesirable effect can be prevented by blocking the edges of the cover glass during the
exposure of the actual hologram.
13
The principle of fiber optics is reliant on the concept of total internal reflection. As may be
observed in the diagram below, in constructing fiber optic materials, glass or plastic materials
are heated and drawn into very fine fibers. In the process of drawing out these fibers a glass of
a lower index of refraction is layered on the outside of a glass fiber of a higher index of
refraction. So long as the rays of light which pass through these fibers do not fall below the
critical angle, they will always be totally internally reflected and thus light rays entering one end
of the fiber stream will continue along it without leaving the fiber
It should be noted here that fiber optics represent a powerful tool to aid the holographer in
creating the special interference patterns that establish her images. It is possible that in the
near future, holographers will employ fiber optics to direct laser beams in their recording
apparatus. This may ultimately be more effecient and less costly than the present system of
mirrors and lenses commonly in use today.
14
REFRACTION
OPTICS &
LENSES
15
REFRACTION, OPTICS, AND LENSES:
Perhaps the most common and practical application of the priniples and laws of refraction is
with respect to lenses used in various optical systems (eyeglasses, camera lenses and so forth).
Relying on simply the basic laws of refraction we may predict the response of light rays as they
pass through various types of lenses. In order to do this we might look at the two most
common lenses (Fig. 1 & Fig. 2):
The positive lens (Fig.1) is typified by an expansive curvature and is therefore thicker in the mid
region. This type of lens is also referred to as convex and is the type commonly found in
magnifying glasses. The effect which such a lens shape has upon parallel rays of light which
enter it is easily understood if we refer back to the model of a normal. Those rays which enter
close to the center of the lens are nearly perpendicular to the normal and thus pass through
almost directly. Those on the periphery, however, strike the surface at increasingly large angles
to the normal and thus are increasingly bent toward it. The result of this is that the various
rays which enter parallel in direction to one another are forced to converge at some point on
the opposite side of the lens. The distance from the center of the lens to the point at which
these rays are focused is referred to as the FOCAL LENGTH (F.L.) of the lens. Because the rays
of light are focused a measurable distance from the center of the lens, this focal length is
expressed in positive terms, and thus it is referred to as a positive (or convex) lens.
16
A simple means for relating the focusing action of the positive lens is to relate it to the
refracting action of a prism. The positive lens is similar in effect to two prisms stacked together
(see Fig. 2) base to base. Parallel rays of light directed at such a construct would converge.
17
The second major type of lens is a negative (or concave) lens whose profile reveals an inward
curvature in the center. This type of lens is most frequently used in viewing devices and is also
employed in various compound lenses. The effects which such a lens has upon parallel rays of
light is understandably the reverse of the effect of a positive lens. Parallel rays which strike the
center of the lens at a perpendicular to the surface pass through almost directly. Those rays
which strike the periphery however are now forced to diverge rather than converge as in the
case of the positive lens. The interesting result of this is that such a lens does not have a
focusing capability. Interestingly though, it does have a focal length which is expressed in
negative terms. This focal length is derived by reversing the direction of the diverged rays to
determine an imaginary point of convergence. The focal length in this instance being expressed
as F.L. -13 mm.
NEGATIVE LENS
PARALELL LIGHT RAYS
LIGHT PATH
F. L. = FOCAL LENGTH
18
REAL AND VIRTUAL - IMAGES IN SPACE:
Perhaps the most fascinating possibility afforded by the focusing capability of lenses is the
ability to project images to various points in space so that they appear either in front of or
behind the imaging lens. Most instructive in this regard is the positive lens because of its
versatility. If we examine the focusing pattern of a positive lens (Fig. 1) we notice that the
image of the object is focused in space at a location on the opposite side of the lens as the
object which is being imaged. This will be the case if the object's distance from the lens is
greater than the focal length of the lens only. It is also important to notice that the image will
appear to be both upside down and laterally reversed (flipped from side to side.). An image
such as this is referred to as a REAL image since it exists in the "real" space of the viewer and
actually is focused in space (it may be observed easily with a sheet of ground glass or
waxed/greased paper).
In addition to real, this type of image is also referred to a
"pseudoscopic".
If, unlike the previous example, the object is located closer to the lens than the focal length of
the lens, then the image of the object will appear in virtual format. In this form, the image of
the object will be upright rather than inverted as with the real image.
19
In the case of a negative lens, only one type of image may be created and this is the type
commonly referred to as a virtual image. Because the negative lens is incapable of converging
light rays to focus them in space the images which it creates appear to be focused behind the
lens surface. These images are never inverted or laterally reversed, but instead appear upright
and in their proper lateral orientation. In addition, these images are characteristically reduced in
size, appearing to present a large field of view from a tiny point in space. Because the image
which we see is not focused in space, but originates from an imaginary focal point, this type of
image is called virtual or "orthoscopic".
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An awareness of real and virtual images often produces questions about where the various
images formed by these lenses exist in space. With real images it is possible to demonstrate
their presence by viewing the projected image on ground glass (as was done with some earlier
cameras), thin tissue papers or onto a white card. The images created by virtual formation may
not be viewed in thi way and therefore are more difficult to visualize through physical
demonstrations.
Despite the nature of a lens and its image forming properties, predicting where images focused
by a lens will exist in space is made relatively easy by the application of a simple mathematical
equation known as the "LENS FORMULA". In its simplest form the equation states
1
________
d1
+
1
_______
d2
=
1
_________
F. L.
21
that the sum of inverses of the focal points for a lens will be equal to the inverse of the focal
length for that lens. Armed with this equation we can determine a great deal about a lens,
among which, the following information may prove useful.
1. If supplied with a source of collimated light in which the rays are parallel, we may
the focal length of a simple lens by applying the formula given above. To do
must note that since the rays comprising the distance d in our diagram are all
they are originating at INFINITY (represented as a lazy or sideways "8") which
equivalent to zero in our equation. As a result the equation may be simplified to the
1
d2
=
determine
this
we
parallel,
is
form:
1
F.L.
Given this simplified formula it is obvious that the F.L. will equal the distance required
for parallel rays entering the lens to converge to a point, thus d and F.L. are equivalent!
22
2. Using the same "LENS FORMULA", we may predict the position of an objects image in
space given only the distance of the object (d) from the lens and the focal length of the
lens. To make the calculation easier ,however,
equation given earlier to the following form:
we may rearrange or transpose the
d 2 = d 1 X F.L.
d 1 - F.L.
Therefore, if we place an object 15cm from a positive lens with a focal length of 10 cm,
the location of the real image created by this lens may be easily calculated.
d2=
15 X 10
=
+30cm
15 - 10
Because this image exists on the opposite side of the lens as the object, it is known as a
real image. Note here that d is a positive number. If d had been a negative number this
would indicate that the image was virtual or orthoscopic and that its true focal point was
an imaginary one located on the same side of the lens as the object.
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3. Here is a sample problem for you to solve. Its easy since you only need to plug the
appropriate figures in the equation you've been given and calculate the answer.
An object (in this case a small model of a tree) is located 15 cm from a lens with a focal
length of 20 cm. Using the lens formula below, place the appropriate figures in the
spaces and complete the calculation of the equation.
d2 =
d1 X F.L.
d1 - F.L.
Notice that the answer you have calculated has a negative sign before it. This indicates
that the image will not be a real one but instead will be a virtual orthoscopic image. Thus,
the image of the object will appear to be on the same side fo the lens as the object itself,
located at a distance 60 cm behind the lens in the diagram. It should be noted here that
when an object is placed between a lens and the focal point, a virtual image is created
rather than a real image.
Try completing the diagram below illustrating the direction of rays as they pass through
the lens.
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MAGNIFICATION:
When either a positive or negative lens refracts the light of an object, it does so in a manner
that affects the scale of the image in relation to the object itself. These changes in size from
that of the original object size are known as lateral magnifications and may be either
enhancements or reductions in size of the original objects, like the lens formula presented
earlier, the formula for lateral magnification is quite simple:
m = - d2
d1
If we relate this back to our first example, we could easily compute that the magnification for
this lens would be - 2.0. The minus sign in this calculation indicates that this is a real image
that would be both inverted and laterally reversed. The value two indicates that the image
would be approximately twice the size of the object.
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WAVE OPTICS AND DIFFRACTION
26
WAVE OPTICS AND DIFFRACTION:
As was pointed out earlier, many of the basic laws of lenses and their effect upon light are
based upon the concept of refraction and of light as a "ray" phenomena. In wave optics, light is
conceptualized as a wave of electromagnetic energy propogating through space at tremendous
speed. According to this model, light is a disturbance of the electromagnetic fields which exist
in our environment. These disturbances are much like the effect seen when a pebble is dropped
in a pond. As the pebble penetrates the surface of still water, it sends out spherical waves in all
directions. When static fields of electromagnetic energy are disturbed, they likewise send out
waves that continue through the electromagnetic field.
What is important about this different model is that it permits us to explain and predict certain
phenomena that are incongruous with the geometrical model of light. In particular wave optics
more
27
more accurately predicts the effects of light when it passes by the edges of objects. If we were
to use the "ray" concept it is easy to believe that an object (Fig. 1), placed between a point
light source and a fixed wall surface, would cast a sharp and distinctive shadow on the wall
behind it. In reality however, we seldom find this to be the case. Although some shadows may
appear sharp, on closer inspection we recognize that light is blurred on the edges of the
shadow, indicating that in passing the object's edge, the rays of light are somehow bent. The
geometrical model of light cannot predict or explain this behavior.
GEOMETRICAL (RAY) THEORY
HARD EDGED SHADOW
POINT SOURCE
(LASER)
OPAQUE OBJECT
REALITY
RAYS BENDING AT EDGE
OF OBJECT
WALL SURFACE
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LIGHT - THE WAVE MODEL:
Although light is obviously essential for our survival, many individuals know little about it. In
past centuries, light was important commercially for its role in illuminating certain activities,
whereas in our own times it is becoming important as a tool in its own right, performing some of
the very activities it previously made visible to us. In performing these functions light often
does so at vastly increased speeds and with far greater economy. Holography is an excellent
example of this gradual evolution toward the use of light as a tool in modern technology.
Therefore, in studying holography it is important to develop a basic understanding of light, the
mechanisms and theories which explain its behavior, and its significance in relation to the theory
and applied principles of holography.
To discuss light we might begin by recognizing that in the sciences there are essentially two
models used to explain the actions of light. One of these is the QUANTUM model which
essentially describes the actions of light as resulting from the effect of discrete packets of
energy with clearly defined properties. These packets of energy are referred to as photons. In
the second conceptualization of light, the form is defined as a wave of electro-magnetic energy
which is propagated through space in a cyclical fashion, its energy level rising and falling in a
fashion most often visualized as a sine wave (fig.1). Wave theory is of more practical use in
explaining some basic phenomena in holography and therefore we focus upon it here.
In describing light as a wave, several terms are commonly used to denote parts of the wave:
CREST - the highest point of each wave cycle.
TROUGH - the lowest point of each wave cycle
AMPLITUDE -the intensity of the wave. It is usually measured from the center line to the top of
a crest.
29
Generally the sine wave has two important features which are used in defining its character and
its effects:
WAVELENGTH -The distance between two consecutive and similar points on the wave. In
holography and optics, the common measure for wavelengths is the nanometer. A nanomter is
-9
equivalent to one billionth of a meter, - 10 meters or .000000001 meters. In red light for
instance, the length of waves would be around 600 nanometers. The greek symbol lambda “λ”
is often used to designate wavelength.
FREQUENCY -Is the number of cycles (ups and downs) a wave goes through during a fixed
period of time. This period of time is commonly set as one second, and the number of cycles
-8
per second are referred to as Hertz (abbreviated Hz). Light waves oscillate at about 10
-4
10
Hz –
Hz.
c = Hz X λ
This small formula expresses the relationship between frequency and wavelength and equates
them to the constant c, a universal symbol for the speed of light (3 X 10 meters/second,
300,000,000 meters/second or 186,000 miles per second).
30
The sine wave can be somewhat deceiving. Not all waves of electromagnetic radiation occur in
such a precise form in nature. Rather, various forces, collisions with objects, the affects of
other forms of energy and interference with other waves may modify the original wave. When
waves are altered by such forces we refer to them as modulated.
It is important to note before proceeding further that many phenomena, besides visible light,
are the result of waves of electromagnetic radiation. Varying wavelengths of such energy
account for such phenomena as radio waves, ultraviolet light, x-rays and so forth. The various
types, their wavelengths and their common applications. Although holograms can be recorded
using many types of waves, including sound waves, our interest will be limited to waves in the
visible spectrum, or from about 400-700 nanometers (violet to red that is!). The chart below
shows the wavelengths for visible colors.
AMPLITUDE MODULATED
FREQUENCY MODULATED
AMPLITUDE AND FREQUENCY MODULATED
31
WAVELENGTH AND COLOR :
Early on, in Junior High School, we learn that the colors we perceive are the result of various
frequencies (or wavelengths) of electromagnetic energy (waves) stimulating certain nerves in the
eye. These various colors and their relationship to certain wavelength or frequencies can be
useful to remember in holography. Essentially, red wavelengths are longer and therefore have a
correspondingly lower frequency and energy level. Wavelengths in the violet and ultraviolet range
are much shorter with a much higher frequency and energy level.
3 seconds (one cycle)
------------------------------------------------------------------
3 seconds (three cycles)
------------------------------------------------------------------
3 seconds (ten cycles)
------------------------------------------------------------------
3 seconds (30 cycles)
------------------------------------------------------------------
3 seconds (100 cycles)
------------------------------------------------------------------
32
LIGHT AND INTERFERENCE
The coherent light of lasers is quite useful in itself for industrial research, communications and
various other applications. For holography, however, the coherent character of a light source is
essential. This is so because the technology of holography is made possible by the nature of
light waves and the manner in which they interact with one another. The effect of light waves
upon one another is generally described in a relationship known as interference.
If we think of light waves as water waves (there are some great similarities between them) we
can demonstrate some of their interactive effects. If two wave sources collide with one another
(Fig.1) the phase conditions of these waves (the relation of their crests and the phase troughs)
mutually interact with one another. The rising action of the crests may be reduced if they
coincide with the trough segment of another wave and vice versa, or two crests coinciding may
create an amplified crest and trough in the wave. The results constitute a combined effect. In
the illustration, wave A and B combine in such a manner that both waves reach a cresting stage
at the same time. The result of this is that the two waves magnify each others intensity
(amplitude) as a result of their additive relationship. The wave which results from this
combination is represented in C. Although the wavelength of the original waves has not
changed, the height of its crest and depth of its trough has, as if the energy in the separate
waves had been combined.
This additive effect is referred to as CONSTRUCTIVE
INTERFERENCE.
WAVE A
WAVE A & B COMBINED
WAVE B
33
In contrast to this situation, it is possible for another event to occur in which two waves of
opposite phase collide with one another. In this case, something very curious occurs. Because
the energy states of the two waves oppose each other (one is cresting while the other is
troughing) the two waves process to cancel each other out. Because of destruction or
nullification of the wave's energy at this point, this phenomena is referred to as DESTRUCTIVE
INTERFERENCE.
In reality, it is somewhat unlikely that two waves would meet so neatly to form a new wave of
such a consistent configuration. Instead, it is more likely for them to be slightly out of phase
with one another and therefore forming somewhat irregular patterns of constructive and
destructive interference.
In describing the interference mechanism of light it is important for us to realize that this effect
is essentially characteristic of light, whether the source is coherent or incoherent. Thus
interference effects can be demonstrated using ordinary incandescent sources of light (this is
what Thomas Young did in his famous double slit experiment). However, as we now know,
incoherent light is not so clearly organized and structured as coherent light and this makes it
difficult to create highly ordered patterns of interference from incoherent light. Since the
mechanism of holography relies upon the creation and recording of the interference
relationships a coherent light source provides a more orderly pattern and thus a more highly
resolved image.
34
DIFFRACTION AND INTERFERENCE:
It is this phenomena of light bending at the edge of objects which is rather clearly explained in
the wave theory of light. According to this theory, light is transmitted as a wave and as
explained in the description of constructive and destructive interference, these waves have the
ability to turn each other on and off depending upon how they interact with one another. This
interaction has been the object of scrutiny by many scientists over the past several centuries.
Huygens claimed that light was a pulse phenomenon, a concept which laid the foundation for a
wave theory of light. However, it was the work of the English scientist Thomas Young and his
now famous "double slit" experiment near the beginning of the 19th Century that lent great
credibility to the theory.
In Young's ingenious experiment, a light source was first passed through a narrow slit to
generate a single source effect. Close behind this slit was located a barrier containing an
additional two slits. Given the design of this experiment, it appeared to those supporting the
ray optics model of light that , since light traveled in a straight line, no light would be observed
beyond the barrier with the double slit. In reality however, light was visible, but with a very
peculiar pattern present, later known as "Young's fringes".
SPHERICAL WAVES
FORMED BY SLIT
SPHERICAL WAVES
INTERFERING
PLANE WAVES
APPROACHING SLIT
35
The pattern created was a very distinctive series of light and dark bands whose position and
intensity could not be accounted for by the "rays" of light employed in geometrical optics. The
pattern could, however, be accounted for by a new priciple termed "diffraction". This theory
postulated that light moves as waves and that a wave leaving slit A will travel the same
distance as a wave leaving slit B when they meet at the centerpoint C (between the two slit
position). Because they travel the same distance these waves woud meet "in phase" with one
another. The constructive interference which would result would cause this location to be well
illuminated. Further, if we select yet another point, say C', we can see that the distance from
slit A to this point is different than the distance of slit B to this point. However, if the distance
(d) is exactly one wavelength longer, then these two waves will also interfere constructively and
form a bright spot. In this case, because the difference in distance is only one wavelength
greater it is known as the firest order of diffraction.
36
Likewise, we can appreciate that any waves meeting in such fashion that the differences in their
distances (path lengths) were more or less than one wavelength would be out of phase, thus
canceling one another out and leaving a dark spot of limited or no illumination. We can
understand that any waves meeting here would be out of phase and their destructive
interference would leave a dark area at this location. These various bright and dark areas
created in Young's double slit experiment are known as diffraction pattern and the areas of dark
and light represent the various orders of diffraction which are numbered from the center area
out, beginning with the central zone being referred to as the zeroeth order, followed by the first
order, second and so forth, symmetrically about the axis. The illustration below describes this
pattern.
d
d'
A
C
C = ZERO ORDER OF
DIFFRACTION
B
C'
C' = FIRST ORDER OF
DIFFRACTION
WAVE "A" IN FIRST ORDER TRAVELS
ONE WAVELENGTH FURTHER THAN
WAVE "B" (LENGTH NOTED AS "D") S0
WAVES INTERFERE CONSTRUCTIVELY
FORMING A BRIGHT AREA AT C'.
D
A
d'
d
B
C'
37
From this it could be observed that light possessed a wave-like property which was useful in
explaining its behavior. Whatsmore, it would be seen that light, even if thought to be
propagating in all directions, could have a straight-line nature. Light emitted from a source at
each point in its propagation acts as a source emitting still other waves. However, if it is
considered that light interferes with light it may be understood that these various expanding
wave forms do interfere with each other as well and in fact their interference is so complete as
to eradicate, by destructive interference, any opposing waves coming from other sources.
Therefore, although light is being propagated in all directions, only those portions of the waves
advancing directly forward remain since all others are cancelled out by destructive interference
effects.
WAVEFRONTS CANCEL
ONE ANOTHER
WAVEFRONT MOVING STRAIGHT
FORWARD IS NOT CANCELLED
38
The complementary wave cancellation described above does not occur in situations where a
portion of the light source has been eliminated. This is true since any disruption of the
accompanying waves eliminates the cancellation source for many other waves. Without their
necessary cancellation counterparts, waves begin to propagate at various angles from their
straing path.
Because these waves continue to interfere with other waves which were not blocked by the
original object, a certain amount of cancellation continues to occur but not the total
cancellation experienced prior to the objects blocking the light path. The cancellation which
does occur has a very special pattern which can be referred to as a diffraction pattern in which
light is brightest in the proximity of the edge of the objects shadow and dimmer at further
distances from it. The effect of this is a finely graded shadow with edges which we experience
when light strikes and object and casts its shadow onto a surface.
39
The diffraction pattern which Thomas Young first observed can be explained using the wave
model of light, and it is these same diffraction graduations which we observe join the edges of
shadows, being very minute, however, and disguised by many wavelengths mixing together the
effect is less distict.
The diffraction pattern created by Young's double slit device is dependent upon the spacing
between the two slits. The greater this spacing, the more narrow or compressed the pattern
will appear to be and thus light will appear to have been bent less (Fig. 4). A simple formula
expresses this relationship and may be used to predict the physical separation between light and
dark fringes and angle at which the various orders are bent out from the central axis:
Fringe spacing =
λ x
D
d
Therefore, if we have light from a helium Neon laser at .6328 mm incident on a slit with a slit
separation of 1 mm and a distance to a reading surface of 1 decimeter (or 100 mm) then the
equation is easy to solve:
Fringe spacing = .6328 x 100
Then Fringe spacing = 63.28 mm
1
40
Another formula which predicts the angle at which the various orders of diffraction will occur is
just as useful and equally as simple to employ. It requires the following elements for calculation.
m = the order of diffraction
λ = the wavelength of light employed (helium-Neon laser 632.8 nM)
d = the fringe separation distance
θ = the angle at which light will be diffracted from the normal represented in the zeroeth order.
In its primary form the equation is usually written as:
M x λ =
d
x
sine θ
Thus if light from a helium helium-neon laser at 632.8 nM is incident on a slit with d = 1_m (or
1000 nM) then the degree of diffraction of the first order (M = 1) would be:
1 x 6328 = 1000 x Sin θ
Then
Then
Sin θ = 632.8
1000
and Sin θ = .6328
θ = _____
The slits used in this example may be replaced by line elements whose edges act as the
diffracting elements. The more lines which occur (the higher the frequency) the smaller the
spacing between lines and the greater the bending effect of the various orders. In the sciences
special glass plates are prepared in which these lines are carefully etched. These devices are
referred to as Diffraction gratings. Many materials have this fine grating property as an
inherent characteristic of their structure. The fine fabrics used in serigraphic printing (also
known as silkscreen printing) create startling diffraction patterns, as do the surfaces of the
popular new compact discs! Today, these diffraction gratings can be generated optically by a
holographic process.
41
It should be noted here that the degree of shift in a diffracted wavefront is related to it's
wavelength. Thus, different colors of light will be diffracted to greater or lesser degrees
depending upon their wavelength. Blue light had a shorter wavelength than red. As a result of
this, two blue sources of light travel a shorter distance than two sources of red light in order to
create an area of constructive interference. The result of this is that the 1st order diffraction
for a blue wave of light may be located closer to the zeroeth order of diffraction. What this
means, is that blue light is bent less radically than red light. This is the opposite of the effect
observed in refraction and can help in distinguishing the means being employed to manipulate
light.
So it is apparent from this wave model, that light may be manipulated in a variety of ways
depending upon its wave-like properties. By the use of diffraction gratings light may be focused
to points in space as with a common glass lens, or broken into its spectral hues as with a prism.
For the application of the concept of diffraction to the optical manipulation of light perhaps no
other technology is more promising than that of holography.
42
CREATING LIGHT:
The light we are commonly familiar with , from incandescent bulbs, is created when energy is
transferred from one source (household electricity for instance) to a particular material
(tungsten filament). When this occurs, the atoms which comprise the tungsten filament are
affected by the electrical current. Essentially the electrons which orbit the atoms in this
material absorb the extra electrical energy and are in turn excited to a higher level of activity.
In the atom itself these electrons may move to higher orbits at a further distance from the
nucleus of the atom. Over time however, the electron seeks to return to its previous (ground)
state and gives off the energy it acquired in the form of a photon (packet of light energy) just
prior to its return to ground state.
The release of the electron's acquired energy in the form of a photon is referred to a
spontaneous emission, because there is no regulation over how much energy the various
electrons absorb or emit spontaneously. Those photons which are released may vibrate at
varying frequencies, thus creating variable wavelengths and sundry colors. The simultaneous
exposure of the eye to these various frequencies of electromagnetic radiation (light) creates
the visual sensation of white light coming from the light bulb.
It is important to note here that the various waves are not only different in their wavelength,
but also in how their movements are synchronized with each other. Because white light from
sources such as the sun is random in both the frequencies it emits and their relations to one
another, these sources of light are referred to as INCOHERENT.
43
COHERENT AND INCOHERENT LIGHT:
As stated in the discussion of light, because photons are emitted spontaneously from sources
such as light bulbs, they are not emitted in an orderly or structured fashion. Firstly, the waves
which are emitted consist of a myriad of differing wavelengths. If we compared them in a
diagram:
LIGHT
SOURCE
we would say that such waves are not TEMPORALLY COHERENT with one another.
Essentially, we would perceive their wavelength variations as differing color sensations. White
light is temporally incoherent since it lacks a singular, specified wavelength.
It is the quality of coherent/incoherent light which helps to distinguish the LASER from common
light sources such as neon tubes, florescent tubes, and the like. Unlike these incoherent light
sources the laser generates (in theory at least) one very specific wavelenghth of light. In the
case of the helium-Neon lasers commonly used in holography the specified wavelength is 632.8
nanometers or .0000006328 meters. Because the wavelength is so specific the light generated
by a laser appears very pure and intense, in the case of the helium-neon laser the wavelength
for lasing creates a brilliant red. Finally, because all the wavelengths emitted are the same, we
say that laser light is temporally coherent.
LASER SOURCE
ALL WAVELENGTHS EQUAL
44
If we continue to examine the character of light generated by the laser we find that there is yet
another pattern to it. Not only are the waves of energy emitted all of the same length, but they
are also perfectly in step with one another. Thus, all the waves of light which the laser creates
are in perfect synchronization with each other - as one of the waves rises, they all rise and so
forth.
This second, highly ordered characteristic of laser light, is known as SPATIAL
COHERENCE and when waves of light are synchronized in such a manner we say that they are
IN-PHASE (conversely, if these wave movements are not synchronized they are said to be "out
of phase.")
ALL WAVELENGTHS OUT-OF-PHASE
The special coherent properties of laser light (both temporal and spatial) are very important
because it is these properties of laser light which make it ideal for creating the interference
effects which are the essence of the holographic recording process.
LASER SOURCE
ALL WAVELENGTHS IN-PHASE
45
CREATING COHERENT LIGHT:
L.A.S.E.R.
Laser is an acronym which stands for Light Amplification by the Stimulated Emission
of Radiation. Lasers produce a very pure source of light in which all the wavelengths
are uniform and move in step (in phase) with one another. Because the laser's light has
only one wavelength it is of one color or monochromatic (the beam of a Helium-Neon
laser is a very pure red). The synchronized relation of the waves adds to this highly
structured pattern of laser light and it is therefore referred to as coherent.
There are many types of lasers and these are usually classified by the material which
supports their lasing action. To produce laser light the electrons in the atoms of the
lasing material are excited by absorbing added energy from a special source (A) (in the
popular helium-neon laser the energy source is electricity). As certain of these
electrons are excited, they move to higher level orbits within the atom for brief periods
of time. In their higher level orbits these electrons are relatively unstable and are
inclined to return to their original orbit levels. Before returning to their lower level
orbits however, these electrons must release the energy acquired during their
excitation. The emission of this energy takes the form of light (B) the electron
emitting a photon as it returns to its lower energy orbit.
The emission of light energy and return of an electron to a lower orbit may occur
spontaneously and is therefore referred to as "spontaneous emission". The photon
released however may also stimulate emission of other photons from similarly excited
electrons in nearby atoms, causing the "stimulated emission" described in the laser
acronym. Those photons whose emission is
46
stimulated by other photons are of the same wavelength and "in phase" with the
photons which caused their emission originally. By placing a mirror at one end of the
laser cavity and a partially silvered mirror at the opposite end, a small portion of these
photons are formed into a highly directional beam. Thus in turn, by exciting a great
many electrons in the atoms of a lasing material and setting off a large chain reaction,
the laser creates a highly structured or coherent beam of light energy in which all the
wavelengths are identical and in phase with one another.
A more sophisticated diagram of a helium-neon laser is presented here. This diagram outlines
the critical working components of a low-power laser. Typically a low-power laser tube of this
design might actually be from 10” – 14” in length. This diagram reveals the Capillary Tube set
inside the center of the larger Plasma Tube which contains the ballast for the laser. In addition
the diagram indicates a typical positioning for the electrical connections to the laser tube itself
(the anode and cathode connections). These electrical connections provide the input of
electrical energy which is transformed into the emission of laser light. As the text of the
diagram indicates, the actual lasing action of the instrument occurs in the very small space
defined as the capillary tube. The special mirrors which allow for the oscillation of photons and
the consequent lasing of gases (stimulated emission of radiation) are seen on the ends of the
capillary tube.
Today holographers also have at their disposal a range of increasingly sophisticated Solid State
Lasers. These lasers are constructed using principles and materials similar to those employed in
the creation of solid state transistors and other modern solid state electronic circuitry. These
lasers are currently being employed in electronics and optical research and will certainly have a
great effect on the evolution, practice and application of holography in the future.
47
48
Cathode pins (-)
Ballast enclosure
Front (output) mirror 98.5 % reflective.
Photons travelling through the capillary
tube exit here thus forming the laser beam.
Actual tube from a low power Helium-Neon Laser.
This tube has been cut open to reveal interior components.
Rear mirror is 99.9 % reflective
Interior of capillary tube contains Helium-Neon
gas mixture. Light amplification occurs within this
tube as photons move back and forth between the
end mirrors.
Anode Pin (+)
CROSS SECTION OF A
LOW POWER HELIUM-NEON LASER
HOLOGRAPHY AND DIFFRACTION:
To understand the application of diffraction concepts to holography we may begin by
visualizing two coherent plane wave sources interfering with one another. If such
interference created by two sources (labeled A & B in the diagram below) were
recorded on a light sensitive emulsion placed at “C”, the processed film would contain
a series of thin striated lines (fringes) which simulate a mechanically generated
diffraction grating. The dark lines in this pattern would represent exposed areas of the
film where constructive interference occurred; the clear areas of the film would
represent those areas where destructive interference occurred and thus the emulsion
was not exposed.
WAVELENGTH
WAVE CREST
ZONES OF
CONSTRUCTIVE
INTERFERENCE
E
X
P
O
S
E
D
F
R
I
N
G
E
S
In such a holographic diffraction grating, the spaces between the black lines serve the
role of the slits described in Thomas Young’s early experiment into diffraction. If a
coherent plane wave source is now shown onto this holographically generated
diffraction grating, the slits, as in any other hand or machine made grating, will diffract
the incoming waves into their various orders of diffraction. Using these simple
diagrams and our knowledge of diffraction we can thereby understand that a hologram
(in this case a hologram of a light source) can be employed to bend light.
49
The holographic grating described above may be varied in a number of ways. Perhaps
the simplest modification is to change one of the sets of waves used to construct the
slit elements in the grating. In the diagram below, the set of waves generated from “B”
has been altered to the form of a spherical wave instead of a plane wave. For purposes
of demonstration, we will treat this wave as if it is the light being reflected from the
point on an object which we are recording holographically. For our purposes we will use
a point on the tip of a tree from which the waves are being reflected. This is in fact
much closer to the reality of a holographic recording in which spherical waves are
reflected off points on the surfaces of objects.
SOURCE "A"
PLANE WAVES
SLIT
SOURCE "B"
IMAGE OF A
POINT
FRINGE
The difference in effect in the grating which is formed by such an arrangement is
dramatic. Now, because of the introduction of the spherical wave pattern from the point
on the object, the grating which is formed by interference of the two wave sources
“A&B” is itself
50
spherical in character. If greatly enlarged and viewed straight on the pattern of the
spherical diffraction grating would look very much like a series of concentric rings. In
this way it emulates an optical device known as a zone plate and also appears similar
to a fresnel lens (the type you see in gift shop windows that often show a virtual
image of the interior space). In this spherical form grating, however, the fringes (or
slit spaces) are no longer equal in dimension. Instead the slits at the center of the
grating are larger than those at its perimeter.
DEVELOPED FRINGE PATTERN VIEWED STRAIGHT-ON
DEVELOPED FRINGE PATTERN CUT-AWAY VIEW FROM SIDE
51
Knowing that such a new form of grating may be created we might want to ask what
the effect would be if the source of plane waves only was projected onto the
processed holographic grating. The result is relatively simple to see. The wavefronts
of light are diffracted in such a fashion that one set of wavefronts emerging from the
grating is deviated upwards in the form of diverging rays of waves of light.
SOURCE "A"
PLANE WAVES
SLIT
SOURCE "B"
IMAGE OF A
POINT
FRINGE
RECONSTRUCTED OBJECT WAVES
To the viewer, peering into such a holographic diffraction grating from the side
opposite of its illumination, it would appear that these diverging rays of light originated
from the point on the object from which they were originally reflected, the object
however in our diagram has been removed and cannot account for these rays of light.
They are strictly recreated, or as holographers term it “reconstructed”. It is easy to
see from this that if the spherical waves from many points on the object were
recorded that all of these points would be reconstructed for our viewer and they would
see an image of the object..
52
In looking at the image reconstructed by the deviated rays of the holographic grating
in our example, it is important to note that in the image described, the rays of light are
diverging. For this reason we must note that the image is of a “virtual” nature.
Therefore, the image reconstructed by the diffraction grating appears beyond the
plane of the diffraction grating.
SOURCE "A"
PLANE WAVES
SLIT
PROJECTED IMAGE
POINT
FRINGE
IMAGE SOURCE / ORIGINAL OBJECT
REAL IMAGE
In examining the effects of the spherical diffraction grating upon a set of plane waves
it must also be observed that a set of waves is deflected downward by the grating, but
in such a fashion that the waves and the rays used to represent them are forced to
converge rather than diverge. The effect of this is that the converging rays form a
complementary image point in space on the same side of the holographic diffraction
grating as the viewer, but opposite from the side on which the original object was
placed. This point would represent a REAL image
53
and while difficult for the eye to view, could be seen by placing a piece of ground glass
(or waxed paper) in the position of the point. It is easy then to imagine, as with the
virtual image, that if many of the points from this object were originally recorded, the
image of the object could be reconstructed in space.
REFERENCE BEAM
PLANE WAVES
RECONSTRUCTED
IMAGE WAVES
REAL IMAGE (INVERTED AND
LATERALLY REVERSED)
From this we realize that holographically generated diffraction gratings may produce
both real and virtual images of point sources (or points on an object). If we then stop
to consider that the objects which we observe in our environment are a complex array
of such points (point sources) of light, we may imagine that a very complex image
such as our object, with its myriad of reflecting points, could be recorded
holographically. Now, for each point, the spherical waves coming from the object
(object beam) interfere with a plane wave from the reference source (reference beam)
to construct a spherical holographic diffraction grating on the recording film. In an
actual recording, millions of these holographic diffraction gratings are formed,
overlapping and integrating with one another, to form a very complex and unintelligible
pattern which is unrelated to the appearance of the object being recorded. These
gratings however, which are the essence of the hologram, when properly illuminated
allow us to reconstruct the waves of light as if they came from the object itself.
54
HOLOGRAPHY:
CONSTRUCTION AND RECONSTRUCTION STAGES
CONSTRUCTION STAGE: When an artist sets out to create or record holograms we
refer to this as the “construction stage” of holography. This is the stage in which the
interference patterns (involving the constructive and destructive interference
phenomena) of wavefronts of light will be initially created using the special properties
of laser light. It is also the stage in which these interference patterns will be recorded
on light sensitive photographic emulsions. The use of the interference mechanism is
crucial here because this pattern contains information about the subject elements
being recorded in the hologram.
The basic phenomenon of constructive and destructive interference are fairly universal
with light; however, the manner in which sources of light energy interfere with one
another may create differing forms of interference patterns. The classical example in
holography is the laser transmission hologram developed by Leith and Upatnieks at the
University of Michigan in Ann Arbor in the early 1960s. This type of hologram is
designated laser transmission because a laser is used both to construct (record) and
reconstruct (view/playback) this type of hologram.
In a laser transmission hologram, as diagrammed here, the beam of coherent light
generated by a laser (A) is split into two beams by an optical element called a “beam
splitter” (B) (this beam splitter device may be as simple as a piece of window glass or
as complex as a precision coated optic). Each of the two beams which results is then
diffused by a lens (C, C’). For this task negative lenses are frequently
55
employed, in certain instances spatial filters might be substituted for these lenses. At
this point both the sources remain coherent with all their wavefronts synchronized
with one another. Eventually, one of these diffused beams is reflected off a mirror and
toward an object. As the coherent light source strikes this object however, its
regularity (or spatial coherence) is partially disturbed. Although the wavelength of the
light (or temporal coherence) is maintained, the phase
RECORDING FILM
OBJECT WAVES REFLECTING
FROM A SINGLE POINT
OBJECT
relationship between the waves is modulated by the object, as is their amplitude (fig.
2). This beam, having been modulated by the object, is referred to as the OBJECT
BEAM. The light energy from this beam is now scattered in all directions from the
object, and a certain portion proceeds in the direction of the recording material (film)
placed near the top of your diagram. The remaining beam does not collide with any
objects; instead, after being diffused, it is reflected off another mirror and passes
directly to the recording material.
Because it is unaffected by an object the
REFERENCE BEAM maintains its coherent structure. It is referred to as a reference
beam because it is in relationship to this that the object beams patterns are known
and may be reconstructed.
56
When the waves of light from the OBJECT and REFERENCE beam sources collide with
one another they create varying patterns of constructive and destructive interference
depending on their amplitude and phase relationships. Those areas of constructive
interference which occur in coincidence with the photosensitive emulsion of the
recording material are then exposed, and conversely the areas coinciding with the
destructive commingling of waves are not. When traditional silver based photographic
emulsions are the holographic recording material, these areas of exposure and nonexposure create latent images which must be chemically “developed”. After this
development, certain areas of the recording material may have a greater density than
other areas in correspondence with their degree of exposure. Traditionally, these
denser areas of the emulsion are referred to as FRINGES (fig. 3). It is these fringe
areas which in turn manipulate or modulate the light passing by them to reconstruct a
holographic image.
RECORDING FILM
OBJECT WAVES REFLECTING
FROM A SINGLE POINT
REFERENCE BEAM WAVES (PLANE)
57
RECONSTRUCTION STAGE: After processing, the hologram may be placed before a
diffused beam of laser light for viewing. In order for the holographic image to be
reconstructed, however, the laser light must pass through the hologram from precisely
the same orientation as the original reference beam. For this reason, the hologram is
generally returned to its original recording position (Fig. 4), and the reference
beam is passed through it. The original object and object beam are no longer essential
for viewing the object and are removed from the diagram. Now, the laser light is
passed through the hologram from the same orientation as the original reference beam
and the object, though no longer present, may be seen. At this point, the fringes
recorded during the CONSTRUCTION stage serve to bend the light passing through the
hologram. In one manner of representation, these fringes act like very small prisms or
lenses, which when combined in a minute and complex pattern permit the hologram to
bend the reference beam’s light so that it replicates those waves of light which
originally constituted the object beam. The manner in which the light is bent is
dependent upon the size, shape, and orientation of these fringes which was in turn
determined by the modulated waves coming from the object during
the recording of the original hologram. Now the hologram simply reconstructs those
object waves, making it appear as though the object is still there.
NOTE: The wave interference patterns which create holograms may be generated by
many different wavelengths from the electromagnetic spectrum or from other wave
media. Thus X-ray, infrared, microwave and acoustical (sound wave) holography are all
possibilities.
REFLECTION HOLOGRAPHY:
The recording of reflection holograms relies on the same principles of constructive and
destructive interference described in the case of laser transmission holography. The
method employed to create the necessary interference is quite different however and
requires a radically different form of reconstruction in the final image. Reflection
holograms are illuminated from the same side as they are viewed, the light frequently
comes from overhead. That light which enters the hologram is partly reflected back
toward the viewer with information about the object(s) introduced into the reflected
waves by the fringes of the hologram.
In constructing a reflection hologram, the reference and object beams (A) are made to
interfere by passing through the recording film from opposite sides. For this purpose the
recording material being used must be clear, for exposure on both surfaces. The effect
of such a construction technique is the formation (B) of interference fringes within the
thickness of the recording material. These fringes are like flat reflecting surfaces or
planes and occur in multiple layers.
55
If, after processing, a white light is cast onto the hologram from the proper angle (C),
these thin planes will reflect light back to the viewer.
The relationship of those waves reflected back is dependent upon the spacing formed
between the fringes of the hologram during recording. For light of the correct
wavelength the reflected waves interfere constructively during illumination of the
hologram and thus create a visible image. Those wavelengths which are longer and
shorter are reflected in such a manner as to interfere destructively and therefore are
negated in the final image. As a result of this process, one primary wavelength is
selected from the many wavelengths (colors) in the white light source. The holographic
image which appears to the viewer is therefore of only one primary color, or
monochromatic. The multicolor reflection hologram is created by using several
separate exposures, between which the recording film is swollen so that the distance
between fringes varies for each exposure.
56
TRANSMISSION HOLOGRAPHY:
Laser transmission holograms of the off-axis variety were first created by Leith and
Upatnieks at the University of Michigan in Ann Arbor during the early 1960's. Laser
transmission and other transmission holograms are easily distinguishable, since for
reconstruction, they must be illuminated by passing light through the hologram from
the side opposite that from which they are to be viewed.
Early transmission holograms required lasers for viewing as well as recording the
holographic image. Because these holograms were viewed with the aid of lasers, their
images were of a single color (that of the laser). In the late 1960's Dr. Stephen Benton
of the Polaroid Corporation developed a technique for creating holograms which were
viewable under white light, thus permitting some limited use of color in holographic
images of the transmission type.
A white light transmission hologram begins with the creation of a laser transmission
image, often referred to as the "master" hologram. If this hologram is inverted for
replay, its image will project in front of it (A)
into space (this is referred to as a real image). By passing the beam of reconstructing
laser light through a special cylindrical lens, this image may be recreated from a thin
strip in the center of the hologram. Now, if a new recording plate is placed near this
real image and a reference beam made to interfere with it, a second image will be
57
formed - a hologram of a hologram! When, after processing, this second stage
hologram is viewed using a white light source, the thin slit used to create it will project
out an image as if coming from a prism (B). The various wavelengths of light will form
images for the viewer at different points in space. While moving up and down before
such an image these spectral colors demonstrate why this is often referred to as a
"rainbow hologram".
58
EMBOSSED HOLOGRAPHY:
Holographic images can be created in great quantity and at relatively low cost through
the embossing process. The flexibility of embossed holograms permits them to be used
in a wide variety of applications, from magazine displays to anti-fraud devices on
popular credit cards. Because holograms contain a great deal of information in a very
discrete form, they are difficult to forge. This enhanced level of information storage
also makes holograms potentially useful for educational, as well as data storage
purposes.
The making of an embossed hologram involves several distinct physical stages, some
of which are similar to those in the other categories of holography. To begin, a laser
transmission hologram is recorded (A) onto a photo-resist material. After exposure,
this hologram is processed leaving behind a distinct surface pattern (B) caused by the
interference effects and fringe formation. Although the photo-resist in which the
hologram is originally recorded is fairly durable, it alone could not stand up to the
strain of producing thousands of impressions. In order to create a more durable
surface a metallic mold of the photo-resist is made by plating (C) the original photoresist material with a fine layer of nickel. After this stage, the metal mold (D) is used
to stamp out impressions of the original interference pattern (E) in great numbers. The
material most popular for such impressions is a mirrorized plastic. The final hologram is
seen when the embossed image is properly illuminated (F).
59
INTEGRAL HOLOGRAPHY:
The integral, or "multiplex" hologram is created from cinema/computer imagery and is
a hybrid process that combines the visual technologies of holography and film/digital
imaging. In creating an integral hologram, original cine footage (A) is
made of the desired subject. After this film has been processed it is then placed in a
special camera mechanism which is linked to holographic recording equipment. Through
this camera each frame of the processed cine film is projected onto a holographic
recor-ding material. With the aid of cylindrical lenses, (B) these camera images and the
reference beam which interferes with them are formed into thin vertical lines on the
film (the striated vertical line pattern on the hologram is easily observed with the
unaided eye). By placing the separate frames of the film in consecutive proximity to
one another, they may be viewed simultaneously. As we move about the hologram,
each of our eyes sees separate frames but fuses them stereoscopically, thus creating
dramatic spatial sensations and effects. Because of its reliance on this stereo concept,
the integral hologram is sometimes referred to as a `'holographic stereogram."
Integral holograms are often easy to distinguish by the cylindrical displays used in their
reconstruction. Integral holograms are, however, also created in a flat format. The light
source in many cases is built into the integral display and often simply consists of a
100 watt clear light bulb. Because of the dispersion of white light caused by the
hologram, these images, like white light transmission holograms, also have a distinctive
rainbow effect.
60
MAKING IT - Single beam reflection hologram
Materials required:
5 mW Laser
Stabilization system
1 spatial filter
1 4 x 5 front surface mirror
glass plates or film support device
assorted rods and clamps
laser safety goggles
shutter mechanism
Optical set-up: The diagram on the following page indicates that single-beam reflection
holograms are produced by passing the diffused laser beam through the holographic recording
material onto the object below it. From here the laser light is reflected back up into the film
where it interferes with later portions of the wave train causing a fringe pattern to be recorded.
It is improtant to remember that the angle at which the laser beam strikes the glass determines
your reference angle. When you reconstruct your hologram it will need to be illuminated from
the same angle for proper reconstruction.
PROCESSING:
time
agitation
Developer Stop Fix Rinse Bleach Rinse -
NOTE: All processing should be completed at 70 F
47
48
Presentation pointers:
A. To recreate a clear sharp image of your hologram you will need a point source of point
source of illumination. Some useful examples are sunlight, slide projectors, some flashlights,
automobile headlights or clear light bulbs. Without a point source for illumination your image
will appear fuzzy and unclear - try it under a flourescent lamp or frosted light bulb to observe
this effect.
B. Your hologram's fringes may be used to reconstruct either a virtual or a real image. Try
flipping your hologram over from its virtual image position and observing its real image.
Elements which were close in the virtual image will be far away and vice-versa. The entire visual
field of your image has been turned inside out.
C. Your hologram will appear brighter if placed against a dark ground such as black matboard,
black plexiglass, or black fabric. Some holographers use black spray paint to coat the backs of
their holograms in a more fixed manner, others cover them with a black contact paper.
D. Because your hologram was processed using a rehalogenating bleach, it is still sensitive to
light. Overexposure to the sun may cause some darkening of the film. Photographers refer to
this as "printing out." The hologram may be rebleached if this effect is undesirable otherwise
you must be careful of where the hologram is displayed and stored. Avoid prolonged exposure
to the sun.
E. Because of the character of reflection friges in your hologram, the image which you have
created here is sensitive to humidity as well as sunlight. Although humidity will not necessarily
damage your image, it may cause subtle or dramatic color shifts in the image. In many instances
the image will return to its "original" color when the humidity conditions resemmble those under
which it was made.
F. As with most standard photographic materials being stored archivally, limit the exposure of
your hologram to contaminants of all types. Wrapping the film or plate in an acidic paper for
instance may cause a deterioration in the emulsion. Some holographers attempt to seal there
work into glass or plexiglas using various tapes, glues (epoxy) or optical cements.
48
HOLOGENICS:
When making photographs, people used to describe good subjects as being "photogenic."
Holography, as a new medium, is not yet so formalized.
There are however, for technical
reasons, some qualities which may be desirable in the subjects which you employ in your
holograms. Among these are the following:
1. Stability: In general, your subject matter should be of a stable nature, unlikely to move
(even slightly) during the time period during which you are recording it. Because you are
recording the relationships between waves of light, even slight movements may cause
distortions or "blackholes" in you images. Therefore, it may be helpful to secure moving or
unstable elements in the objects you use in your hologram by gluing them, mounting them to a
more sturdy surface or transferring their image to a more stable material (i.e. a plaster cast of
your subject). Because of the super sensitivity of holographic recording processes to even the
slightest movements, objects that might normally seem stable are in an active state of decay,
growth or some dynamic process which makes them difficult to record holographically (plants
are a good example).
2. Lightness or Darkness: Although holography is theoretically advanced, it still relies heavily
on the 19th Century technology of photosensitve emulsions for recording the interference
fringes which constitute the hologram. Therefore, just as in photography, if your subject is too
dark, it will not reflect enough of the laser light to be recorded on your holographic film. If the
materials you choose to record are not bright enough you paint them white, or a metallic silver
or gold, or lighten them in some other manner. Metallic materials, and translucent or
transparent subjects may create special glare problems whcih we will discusss in lab.
3. Size: When you look through a hologram, it's like looking through a window. If an object is
close to that window and larger than the window then you may not see all of it. Likewise, if its
small and far away, it may be hard to make out. The depth or thickness of your subjects will
depend on the type of hologram you are making. Single beam reflection holograms permit only
a few inches of depth to be recorded, but single beam (deep scene) transmission holograms
viewed with a laser may permit up to several feet of depth to be recorded. In preparing your
subject it may be useful to its study in relationship to some framework which simulates the
window size opening through which you will be viewing your final hologram.
49
THE BIG "T" SYSTEM
In order to record the interference of wavefronts of light with a continuous wave laser it is
necessary to stabilize all the components used in the recording process (lenses, mirrors, film,
etc.). This is often termed "vibration isolation" by holographer, and in many laboratories this is
achieved by using state of the art equipment costing many thousands of dollars. In response to
this high cost, the vibration isolation system described here was developed by Chicago
artist/holographer Edward Wesly in the early 1980's and is intended to provide an economical
means by which to make hologram, thus aiding artists and educators who are unable to invest
large sums of money into equipment.
Essentially, the BIG "T" operates like an optical rail, but because of its size and physical features
it provides for flexible adaptation to numerous holographic geometries. Building a BIG "T" is
quiet simple and the following directions may be modified in any manner whatsoever to
accomodate this equipment to your working environment, existing materials, or special needs.
MATERIALS NEEDED
1. A beam of wood, length and thickness negotiable, but you'll probably want at least a three
foot length of 4 x 4 lumber. When buying you beam, try to select dry wood which is straight
and has few cracks (try to get a good grade of a harder wood if it's affordable).
2. One or two eight foot 2 x 4's.
3. Black paint (spray and flat latex, and some brushes) and some type of sealer such as a
polyurthane varnish.
4. Three inner-tubes. For smaller systems, go-cart inner tubes are fine, though automobile
inner tubes are usually cheaper.
5. Long carriage bolts (size depending on size of wook stock).
6. Concrete blocks (optional).
8. About 5 standard laboratory clamps, capable of holding up to 1/2" rods.
ASSEMBLY INSTRUCTION
A.If the wood you buy is not dry, let it dry out for a few weeks then fill in any splits or cracks it
might have with some type of wood filler. After these materials have dryed, paint the beam
with liberal quantities of flat black paint. After this paint has thoroughly dried, coat the wood
components with a sealer such as a polyurethane varnish.
B.The 2 x 4's should be cut so that they may form an "X" which extends over the top of the
inflated inner tubes. After cutting these to size, they should be notched so that they lie
together flush. Another piece of 2 x 4 or heavier wood should be cut ot form the top of your
BIG "T". This section of wood will stretch from the center of one of the inner tube X's to the
center of the other. At the point at which the top of the BIG "T" meets the X's the three pieces
of wood may be drilled, and a carriage bolt inserted to secure them to one another. For stability
the length of the top of the BIG "T" should be about 4/10ths as long as the main beam.
50
C. Your beam may be assembled on the floor or, to reduce back strain it may be elevated off
the floor on columns of concrete blocks. Try to locate your system in an environment where
there is not excessive movement, but it is not necessary to work there. Remember that the
ultimate test of the correctness of your systemn is the results it achieves - no the way it looks.
D. Holes may be bored along the length of the main beam, or on sections of the crossmember
at the top. These holes are used to support the 1/2" rods which are inserted into the beam
and are used for holding optics and other devices. Because their is no phycical mechanism to
secure these rods, the holes hould be bored slightly smaller than the diametr of the rod so that
the rods will fit snugly and securely int the wood hole. Both these rods and the lab clamps you
purchased should be spray painted flat black to reduce the possibility of unwanted reflections
when you are recording your holograms.
1. A slightly neater system but not much costlier, involves setting 1/4 x 20 T-nuts into your
beam and crossmember where you wish to place rods. T-nuts are available at most hardware
stores and the beam must be pre-drilled to accept them. In order for the rod to be set into the
T-nut, it must be bored and tapped to accept a 1/4 x 20 set screw. This set screw should be
expoxyed
into
the
threaded
hole
in
the
rod
so
that
approximately
1/2 its length projects out.
The rod may then be screwed into the T-nuts on your beam.
2. A more elaborate and costlier adaptation of this idea is to have a 1" x 1 1/2" piece of
aluminum bar stock bored and tapped on 1 1/2" intervals to accept the 1/4 x 20 threaded rods
described above. This bar or rail may then be screwed onto the side of your beam so that it
allows you to secure rods at any location along the side of the beam.
51
PROCESSING HOLOGRAMS
HANDLING TECHNIQUES:
EXPOSURE CONSIDERATIONS - The high resolution films employed in holgraphy have some very
unique aspects although they remain similar to many common B/W photographic films. In
general, because of the very fine grain structure required for recording the ultra-fine fringes of
laser light interference patterns, most holographic films are relatively insensitive to light. In
regard to the traditonal ISO/ASA film speed ratings it is probable that films from most
manufacturers would have ISO/ASA ratings of approximately 1/2 or lower. Thus, even in
situations with considerable laser power levels and small film areas, it is not uncommon to find
holographers using exposures of 2, 5 and 10 seconds or longer.
Although most manufacturers of holographic films can provide information on the amount of
energy required to produce adequate exposure of their photo-sensitive emulsions, in many
cases such information cannot be easily translated into an exposure guideline. In certain
instances holographers rely on creating exposures of a given density to insure proper exposure
and related development of their emulsion. By exposing and developing your holographic film so
that it matches the density of certain neutral density filters, you may be able to achieve
agreeable results.
For the image oriented holographer the choice of exposure and development may require finer
adjustment than that simply described by matching certain density characteristics in the
processed emulsion. Essentially this means that the desired holographic exposure and
processing be determined by simple test exposure techniques. The most rigorous testing
involves a "ring-around test" wherein exposures are made on individual pieces of film and then
processed for varying times. The selection of proper exposure and development are then based
upon the visual result much as in traditional photography. In a simplified version of this test, a
single piece of film is exposed for varying ime by covering all but the desired area with an
opaque material such as a dark mat board or the like. The several exposures may all then be
processed simultaneously.
INTERMODULATION NOISE - One of the reasons that it is difficult to rely entiely on energy
readings or density matching to determine exposure and development of holographic materials
is the presence of noise components in the holographic recording process. One of the most
critical of these noise components is known as "Intermodulation Noise" and results when light
from various points on the objects being recorded interferes with itself rather than the
reference beam. The result of this is that fringes are formed which will diffract light but do not
contain information about the object being recorded. Often this noise appears as a haze
throughout the image and while it does not obstruct the presence of the image this noise is
distracting in that it reduces the overall brightness of the image and limits the usefulness of the
hologram for generating other secondary images.
LATENT IMAGE DECAY - As with all photographic materials, holographic emulsions suffer from an
effect known as latent image decay. After the exposure of a photo-sensitive emuslions to light
the chemical structure containing the image information begins to disintegrate or decay causing
an eventual degrading of image quality. This effect begins to occur immediately after exposure
and thereforre it is best to process your exposed film as quickly as possible.
SAFELIGHT - Holographic films are generally monochromatic and sensitive to light from a
relatively small portion of the visible spectrum. This spectral sensitivity has been discussed and
it is important to relate to the selection of safelights. Generally, the holographic films fall into
one of two categories of sensitivity (A) red/orange sensitive or (B) blue/green sensitive. The
proper safelight must be opposite in color to the film's color sensitivity. Thus if using a redsensitive film such as Agfa 8E75, then a green or blue/green safelight is indicated.
FILM STORAGE - Unlike many commercial photographic materials, holographic films traditionally
have relatively short shelf lives, especially when stored in room temperature conditions.
Generally speaking holographic films should be stored in a cool, dry environment. If properly
packaged a film may be refigerated or frozen to promote logevity. In instances where the film is
frozen or refigerated it is important to warm the film slowly prior to usage.
MAKER
EMULSION
THICKNESS
RESOLUTION
SPECTRAL
(MICRONS)
SENSITIVITY
(ERGS/CM 2 )
TYPE
(LINE/mm)
(SENSI)
KODAK
120
173
649F
125
131
253
6
6
(17P)(7f)
3
7
9
400
400
900
50
4-8
4-8
2000+
2000+
2000+
1250
1500
1500
600-750
550-750
450-650
442-515
600-650
600-650
AGFA
8E56
8E75 *
10E56
10E75
7
7
250
100-200
10
20
5000
5000
3000
3000
350-560
600-750
350-560
600-750
ILFORD
SP672
SP673
7
7
750
800
7000
7000
350-560
525-750
PROCESSING GUIDELINES:
( All processing at 70 / photo-flo and squeegee after final rinse)
AGFA 8E75 NAH
REFLECTION HOLOGRAMS :
PYRO-DEVELOPER
1-2 MINUTES
RINSE
5'
DICHRO-BLEACH
clear + 1 minute
RINSE
10'
D-19 DEVELOPER
1-2 minutes
5'
clear + 1 minute
10'
CWC2
1-2 minutes
5'
clear + 1 minute
10'
EDTA bleach may be
substituted for the
Dichro bleach
TRANSMISSION HOLOGRAMS: (without an anti-halo backing, transmission holograms may
contain considerable noise)
PYRO-DEVELOPER
4 MINUTES
RINSE
5'
DICHRO-BLEACH
clear + 1 minute
RINSE
10'
D-19 DEVELOPER
4 minutes
5'
clear + 1 minute
10'
CWC2
4 minutes
5'
clear + 1 minute
10'
EDTA bleach may be
substituted for the
Dichro bleach
KODAK SO-173
REFLECTION HOLOGRAMS - because SO-173 is only manufactured with a anti-halo backing it
may not be used for reflection holograms (unless this backing is removed).
TRANSMISSION HOLOGRAMS
PYRO-DEVELOPER
(Pyro developer is not recommended for use with so-173)
D-19 DEVELOPER
2-5 minutes
Stop
30 sec.
D-8 DEVELOPER
2-5 minutes
Fix
5 minutes
Rinse
5'
Bleach (Ferric-Nitrate)
1-2 minutes
Rinse
10'
Stop
30 sec.
Rinse
5'
Fix
5 minutes
Bleach (Ferric-Nitrate)
1-2 minutes
Rinse
10'
DRYING TECHNIQUES:
Holograms may be dried using several techniques:
Squeegee technique - In this process, holograms are given a final rinse with photo-flo and then
squeegee the residual water off both sides of the hologram. At this point the hologram may be
air-dried or dried with a forced air.
Air drying - In this technique the freshly rinsed print is not squeegeed but simply leaned against
a surface to dry by allowing the surface water to run off an evaporate.
Alcohol drying - Using this technique the hologram is progressively dryed in solutions of water
and alcohol.