Download Report Liquid Lens

Introduction
Nowadays cameras are one of the hottest selling items
in all of consumer electronics. But as anyone who has ever seen
them can attest, the images that come out of these camera phones
leave plenty to be desired. Part of the problem is their CMOS
imaging chips, which typically have a censor array of only about
300 kilo pixels-a quarter or less of the number in a low-end digital
camera. But the major problem is their tiny, fixed focus lenses.
These fixed focus lenses are very small but they have poor light
gathering power and resolving power.
Conventional auto focus systems used in high
quality digital cameras use motors and gears to shift the position of
the lenses. They have high quality, but are difficult to miniaturize
because of the gears and motors.
‘FLUID FOCUS LENSES’ can combine both these
qualities. It is a special type of lens developed by Philips Research
Laboratories. It uses the principle of a human eye. Like the lens of
a human eye it focuses on objects at different distances by varying
the shape of the lens rather than by varying the relative positions of
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multiple lenses. It uses electrostatic force to alter the shape of a
drop of slightly salty water inside a cylinder 3 millimeters and 2.2
mm long. So it can be made to be very small and the images taken
by using these lenses will be having very high quality.
These superior capabilities of ‘FLUID FOCUS
LENSES’ should make them ideal not only in camera phones but
also in products whose design constraints demand a tiny but
capable optical systems.
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Camera
A camera is a device that captures an image on a film for an optical
camera, or a CCD(charged coupled device) for a digital camera. A
simple camera consisting of a lens, a shutter, a media holder, and a
viewfinder. The main part of a camera is lens. A lens is an optical
device that focuses light rays. In cameras, the lens is the device on
the front face (or in a tube extending from the front face) that
gathers the incoming light and concentrates it so that it can be
directed toward the film (in an optical camera) or the imaging
device (in a digital camera).
The term focus means to move the lens or film/image sensor in
order to record a sharp image.Then the term focal length describes
the distance from the surface of the lens to the focal point or
center point at which light rays converge; the focal length
determines the length of the lens.
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Image Formation by A Lens
Image formation by a lens depends upon the wave property called
refraction. Refraction may be defined as the bending of waves
when they enter a medium where their speed is different. Since the
speed of light is slower in a glass lens than in air, a light ray will be
bent upon entering and upon exiting a lens in a way that depends
upon the shape and curvature of the lens. In the case of a
converging lens such as the double convex lens shown below,
parallel rays will be brought together at a point.
The distance from the lens to this principal focus point is called the
focal length of the lens and will be designated by the symbol f. A
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converging lens may be used to project an image of a lighted
object. For example, the converging lens in a slide projector is
used to project an image of a photographic slide on a screen, and
the converging lens in the eye of the viewer in turn projects an
image of the screen on the retina in the back of the eye.
There is a geometrical relationship between the focal length of a
lens (f), the distance from the lens to the bright object (o) and the
distance from the lens to the projected image (i). The relationship
between the distances illustrated in Figure 2 can be expressed as :
This relationship will be used to determine the focal length of a
glass lens, and will be used as the basis for a qualitative
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investigation of image formation by the eye with the use of a large
eye model. The calculation of focal length of a lens is described
below.
1. Position the lens and white screen on the optical bench and place
them so that the distance from the lighted "object" to the lens can
be measured on the bench scale. Adjust the screen to get a clear
image.
Determine the object distance and image distance, o and i, and
calculate the focal length from the lens relationship. Describe the
appearance of the image, compared to the object (e.g, larger,
smaller, erect, inverted). Adjust the object distance to a different
value and repeat the process with a different set of measurements.
Object distance
Image distance
Focal length
(1)
(2)
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Description of
image
2. What is the average of your focal length measurements,
expressed in meters?
3. The lens strength in diopters is defined as S = 1/f(in meters).
The unit is 1/m but this unit is commonly called a "diopter".
Then these lenses can be classified into two:
1. Fixed focus lens
2. variable focus lens
A fixed focus lens is a lens in which the focus is preset and is not
adjustable.
But the focal length of a variable focus lens can be changed for the
need of zooming.
Nowadays these fixed focus lenses are widely used in camera
phones, pocket size conventional digital cameras, webcams ,hidden
security cameras,DVD recorders.and endoscopes.But the images
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that come out of these equipments leave plenty to be desired. Part
of the problem is their CMOS imaging chips, which typically have
a sensor array of only about 300 kilopixels—a quarter or less of the
number in a low-end digital camera. Also these have poor lightgathering and resolving power. These fixed-focus lenses use a
small aperture and short focal length to keep most things in focus,
but at the sacrifice of light-gathering power and therefore of
picture quality.
Fluid focus lens is a solution for this. This functions like our eye. It
varies its focus by changing shape rather than by changing the
relative positions of multiple lenses, as high-quality camera lenses
do. Fluid Focus lens can be made nearly as small as a fixed-focus
lens.
Fluid focus lens delivered sharpness that is easily on a par with that
of variable-focus lenses.
Thus to study about fluid focus lens we have to analyze the
functioning of human eye.
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Eye Anatomy
When you look at an object, light rays are reflected from the object
to the cornea, which is where the miracle begins. The light rays
are bent, refracted and focused by the cornea, lens, and vitreous.
The lens' job is to make sure the rays come to a sharp focus on the
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retina. The resulting image on the retina is upside-down. Here at
the retina, the light rays are converted to electrical impulses which
are then transmitted through the optic nerve, to the brain, where
the image is translated and perceived in an upright position!
Think of the eye as a camera. A camera needs a lens and a film to
produce an image. In the same way, the eyeball needs a lens
(cornea, crystalline lens, vitreous) to refract, or focus the light and
a film (retina) on which to focus the rays. If any one or more of
these components is not functioning correctly, the result is a poor
picture. The retina represents the film in our camera. It captures
the image and sends it to the brain to be developed.
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Theory Behind Fluid Focus Lens
A fluid focus lens varies its focus by changing its shape. The
human eye focuses on objects at different distances by contracting
and expanding muscles attached to the lens. The muscles change
the shape of the lens and alter its focal length.
Fluid focus lens, on the other hand, uses electrostatic forces to alter
the shape of a drop of slightly salty water inside a glass cylinder 3
millimeters in diameter and 2.2 mm long. One end of the cylinder
points toward the image plane; the other is directed at the subject
being imaged.
The lens exploits surface-tension characteristics of fluids. The
surface of a column of water in a clean glass cylinder forms a
bowl-shaped meniscus. Because the molecules in the glass attract
water molecules, the liquid surface curves upward near the clean
cylinder wall. If the glass is greasy, the water surface curves
downward near the wall, because grease repels water.
Fluid focus lens uses the phenomenon called electrowetting .In
electrowetting electric fields are used change the shape of a water
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drop sitting on a metal surface. The drop wets, or contacts, the
surface better when it is attracted by an electric field.
Electrowetting
With electrowetting a voltage is used to modify the wetting
properties of a solid material. An example of such increased
wettability is illustrated in the photographs of figure . The left hand
side shows a water droplet on a hydrophobic surface. The water
droplet does not like to be in contact with the surface and therefore
minimizes the contact area.
In the photograph on the right hand side, a voltage difference is
applied between the electrode in the water and a sub-surface
electrode present underneath the hydrophobic insulator material.
As a result of the voltage, the droplet spreads, i.e. the wettability of
the surface increases strongly.
Figure – water droplets on hydrophobic surface without and with voltage
applied.
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When the voltage is removed, the droplet returns to the original
state indicated on the left hand side.
Structure Of A Fluid Focus Lens
The FluidFocus lens comprises a volume of water [blue] covered
by a volume of oil [tan] inside a glass cylinder [light blue]. At the
inner surface of the glass are cylindrical layers of an electrode, an
insulator, and, on the very inside, a water-repellent material.
With no voltage on the electrode, the water surface is convex [top
figure]. And because the refractive index of oil is greater than that
of water, parallel light rays passing through the meniscus—the
interface between the water and the oil spread out.
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A voltage on the electrode attracts water molecules toward the
cylinder's surface, making it act less repellent, and the water
surface becomes concave. Here, parallel light rays passing through
the meniscus converge at a focal point.
Working Of A Fluid Focus Lens
The cylinder containing the water drop is filled with oil. Around
the inside walls of the cylinder is a water-repellent Teflon-like
coating, and behind this coating is an electrode. Basically, the
water and the oil make up the lens, and the shape of the interface
between the two—the meniscus—determines its focal length.
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Changing the voltage on the electrode changes the shape of the
interface and alters the focal length of the lens.
The lens exploits surface-tension characteristics of fluids. The
surface of a column of water in a clean glass cylinder forms a
bowl-shaped meniscus. Because the molecules in the glass attract
water molecules, the liquid surface curves upward near the clean
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cylinder wall. If the glass is greasy, the water surface curves
downward near the wall, because grease repels water.
At the center of the meniscus, the water surface is nearly flat
because of gravity. Without gravity the water surface would be
spherical—the ideal shape for a focusing lens. In our lens, we
cancel the effect of gravity by keeping the drop small and covering
it with oil, which doesn't mix with the water. To completely cancel
the effect of gravity, the oil must have the same density as the
water, because only then does gravity attract the oil and the water
with equal force. In our lenses, we use a mixture of special silicone
oils (phenylmethylsiloxanes) with that identical density. The result
is a water-to-oil interface whose shape will hold with any
orientation of the cylinder but can be changed by a voltage on the
surrounding electrode.
The optical power of the lens that forms at the surface between the
oil and the water depends on two things: the curvature of the
meniscus and the difference between the refractive indices of the
oil and water. The refractive index—the ratio of the speed of light
in a vacuum to its speed in the medium—characterizes the amount
by which light bends when it passes from one medium to another.
The curvature of the meniscus depends on the diameter of the
cylinder and on how strongly the cylinder wall repels or attracts
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the water. That attraction or repulsion changes with the voltage on
the electrode.
In our lens, the coating on the inside walls of the cylinder repels
water so strongly that the water does not even touch it: there is a
very thin oil layer between the coating and the water. So the water
touches the cylinder only at the flat surface on one end, which has
no water-repellent coating. With no voltage on the electrode, the
meniscus is hemispherical, with the center bulging outward beyond
the ring where the water comes closest to the cylinder. However, a
voltage on the electrode attracts the water and produces a concave
meniscus, forcing the edges beyond the center.
Advantages Of A Fluid Focus Lens
One Important Advantage of liquid lens is that it can be very small.
Smallness is inherently advantageous, because it minimizes the
effects of gravitational pull on the liquid. In addition,
miniaturization makes liquid lenses more powerful, because the
electrostatic forces between the liquid and the inner surface of the
cylinder become stronger as the lens size shrinks.
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This property makes small electrowetting lenses very fast. Fluid
focus lens can refocus in 10 milliseconds, much faster than the
human eye, which can refocus in about 200 ms. Scaled to the size
of a human eye lens, the refocusing time would increase to 50 ms,
which would still be four times faster than that of the eye.
An actual variable-focus camera [bottom, right] built with a FluidFocus lens
[bottom, left] is only 5.5 millimeters high.
The optical power of a lens is specified in diopters, a measure of
how much the lens can bend light. The dioptric value of a lens is
proportional to the inverse of the radius of curvature of the lens in
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meters. The closer objects are to a lens, the more the lens must
bend the light to bring them into focus. So when an object is far
away, a lens needs less optical power to bring it into focus than it
does when the object is near. Our liquid lens changes its focus by
changing its optical power through the change of the water drop's
radius of curvature with voltage on the electrode.
The strength of eyeglasses is also expressed in diopters. So, for
example, eyeglasses of +2 increase the optical power of the eye by
2 diopters, allowing the wearer to see things that are close.
To demonstrate the advantages of liquid lens, we can consider a
digital camera just 5.5 mm high and 4 mm across. At the back of
the camera is a CMOS imager with a 640-by-480-pixel sensor
array. Directly in front of the CMOS imager is a plastic lens, which
allows the image to be projected sharply onto the flat CMOS
image sensor. The eye does not need such a lens because the image
sensor in the eye (the retina) is curved.
In front of this plastic lens is the liquid lens in its cylindrical glass
housing, with the cylinder's outer diameter measuring 4 mm and its
inner diameter 3 mm. The oil side of the liquid lens is close to the
imager. A glass plate seals the liquid lens on the side near the
imager, and a truncated glass sphere mounted on a flexible
membrane seals it on the opposite side.
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In the schematic drawing of the liquid lens [top], a plastic lens at the
aperture provides the main optical power, while the glass lens below it
makes the camera's focal length independent of wavelength. The camera
captures images with a CMOS sensor.
The truncated sphere allows the focal length of the camera to be
independent of wavelength—as with the human eye. This property
is important because it focuses all the wavelengths that make up
the image at the same point, leading to a sharp image. The
membrane allows the volume of the liquids to expand or contract
depending on the temperature. In front of the truncated glass
sphere is another plastic lens, which, like the cornea of the eye,
provides the main optical power. In front of this plastic lens is the
fixed aperture.
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By changing the voltage on the electrode of the liquid lens, we
were able to focus on objects at distances anywhere from 2
centimeters up to infinity. To do so, we varied the focal length
from 2.85 mm to 3.55 mm .
Two photos made with the liquid-lens camera show how the focal length
can change to bring each of two objects into focus. The genie is 50
centimeters from the camera, and the ladybug is 5 cm away.
In contrast to the human eye, which is embedded in a temperaturecontrolled system, our lens must operate over a range of
temperatures. For portable applications, the lens must work
between -30 degrees C and +60 degrees C and survive
temperatures between -40 degrees C and +85 degrees C. Because
such a wide range requires special liquids, we added large amounts
of salt or antifreeze to the water in our prototype camera lens to
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lower the freezing point sufficiently without adversely affecting
the image quality.
There is, however, one property for which this lens probably can't
beat the human body, and that is lifetime. But we can vary the
focus of this liquid lens—from one end of its range to the other—
more than a million times without any decrease in performance.
The lenses have other intriguing possibilities, too. Replacing the
electrode that encircles the inner wall of the glass cylinder with
multiple vertical electrodes and adjusting their voltages separately
allows tilting of the interface between the liquids, offering the
ability to image in directions that are at an angle to the lens axis. A
lens that can be tilted and focused could let engineers design video
cameras and binoculars that would compensate precisely for hand
movement and other undesired motions.
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Conclusion
It is sure that, the liquid lenses will overcome the problems
associated with today’s camera phones. It is even possible to apply
the liquid lens in high-quality optical recording systems like DVD
recorders, because its resolution can be controlled so it is not
limited by lens imperfections but only by diffraction, which
restricts the resolution of all lens systems.
So we can expect that within the next year or two these Fluid
Focus lenses will be enhancing the resolution of pictures taken
with cell phone and PDA cameras.
Because the liquid lens is based on materials that are, at least in
theory, biocompatible, and because refocusing the lens requires
very little energy, we can envision future applications to replace a
malfunctioning human eye lens. With a zooming feature, we might
even far surpass it.
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References
 IEEE SPECTRUM December 2004/volume 41/number
12/International edition
 The European Physical Journal E 3 (2000), p.159
 Applied physics Letters85 (2004), p. 1128
 http://www.umiacs.umd.edu/~ramani/cmsc828d/lecture3.pdf
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