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The Human Eye
Anatomy and
Detection Characteristics
Anatomy of the Human Eye
The Eye
The human eye is a camera!
• Iris - colored annulus with radial muscles
• Pupil - the hole (aperture) whose size is controlled by the iris
• What’s the “film”?
– photoreceptor cells (rods and cones) in the retina
Slide by Steve Seitz
The Retina
Cross-section of eye
Cross section of retina
Pigmented
epithelium
Ganglion axons
Ganglion cell layer
Bipolar cell layer
Receptor layer
Retina up-close
Light
Two types of light-sensitive receptors
Cones
cone-shaped
less sensitive
operate in high light
color vision
Rods
rod-shaped
highly sensitive
operate at night
gray-scale vision
© Stephen E. Palmer, 2002
Rod / Cone sensitivity
Distribution of Rods and Cones
# Receptors/mm2
.
Fovea
150,000
Rods
Blind
Spot
Rods
100,000
50,000
0
Cones
Cones
80 60 40 20 0
20 40 60 80
Visual Angle (degrees from fovea)
Night Sky: why are there more stars off-center?
© Stephen E. Palmer, 2002
Visible Light
Why do we see light of these wavelengths?
…because that’s where the
Sun radiates EM energy
© Stephen E. Palmer, 2002
The Physics of Light
% Photons Reflected
Some examples of the reflectance spectra of surfaces
Red
400
Yellow
700 400
Blue
700 400
Wavelength (nm)
Purple
700 400
700
© Stephen E. Palmer, 2002
Physiology of Color Vision
Three kinds of cones:
440
RELATIVE ABSORBANCE (%)
.
530 560 nm.
100
S
M
L
50
400
450
500
550
600 650
WAVELENGTH (nm.)
© Stephen E. Palmer, 2002
The Human Eye as an Astronomical Instrument
The eye is a camera with:
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Focal length f = 18 mm
Aperture variable 2 – 7 mm
Fast scanning and focus adjustment
A high-resolution color sensitive center: the fovea with cone
cells
Lower resolution peripheral vision, both cones and rods
Separate day and night vision detectors:
o Cones for color vision during the day
o Rods for low-light monochromatic vision
Redundant system
Stereoscopic Rangefinding system
Powerful image processing and object identification system
connected
Empirical Starting point:
Experienced observers under ideal conditions
can just barely see stars of 6 th magnitude.
Calibration of the magnitude scale:
A star of 0 magnitude
in the visual band emits:
3.75 10-11 J m-2 s-1 nm-1
(Joules per square meter collecting area (0.38 10-4 m2),
per second collecting time (0.15 s),
and per nm filter bandpass (100nm) )
This makes 2.14 10-14 J of energy received
by the eye in one reaction time.
Each photon carries an energy of
hc/λ = 3.61 10-19 J
This means the eye receives 59000 photons
per second from the 0 magnitude star.
A 6 mag star receives a factor of 251 less
Photons, i.e. 235 photons
The eye also receives 1711 competing photons
From every square degree of the sky.
Assume light from 0.1 square degrees actually
interferes or competes with the detection of
The star, i.e. 171 competing photons.
Quantum efficiency of the eye is about 5% under optimal
conditions:
Healthy eye, perfectly dark adapted, using peripheral(rod) vision,
having enough oxygen, good nutrition (vitamin A), experienced in
the mental evaluation of faint signals.
Under these conditions, in the reaction time interval, we have:
12 photons detected from the star
competing with 9 photons from the sky.
The 12 photons have to be detected against
a total noise of sqrt (12+9) = 4.6 photons.
The 6 mag star is thus a 2.6 σ detection, which
is just a quantitiative way of saying: “barely able to see it”
Resolving Power of the Eye
Resolution (daylight viewing with fovea): 1 arcmin
Projected diameter of fovea: 100 arcmins
Sensor density:
30 106 rods / steradian = 2.7 rods/arcmin2
1.2 106 cones / steradian = 0.1 rods/arcmin2
In the fovea:
50 106 cones / steradian = 4.2 rods/arcmin2
Diameter of individual cones: 2 μm (25”)
Diameter of individual rods: 1 μm (12”)
Comparison to Diffraction Limit
Pupil diameter: 2.5 mm
Wavelength:
500 nm (green light)
Diffraction Limit:
1.22 λ/d = 0.000244 radian
= 0.84 arcmin
Under optimal bright daylight conditions, the eye
is capable of nearly diffraction-limited resolution.
At night, the pupil is larger (up to 7 mm) and the
resolution is limited by rod-cell density.
Visual Observations
• Navigation
• Calendars
• Unusual Objects (comets etc.)
Chemistry of Photography
1)
2)
3)
4)
The light sensitive emulsion
The latent image
Developing the image
Fixing the image
Black & White Film
Black and white film is composed of 4 layers.
*An upper protective coat.
*A layer of gelatin that contains silver halide (AgBr, AgCl, or AgI) crystals.
(The type and proportions of the different silver halides determining the
speed of the film)
*The film base, usually made from a flexible polymer.
*And the anti-halation backing to prevent light from reflecting back onto the
emulsion.
Color Film
The Color film “emulsion” is actually made up of 3 different layers of emulsion.
* Each is sensitive to different wavelengths of light.
*The emulsions still contain silver halide crystals but are now coupled with dyes.
*The dyes are the compliments to the colors too which that layer is sensitive.
*There is a yellow filter between the first and second emulsion layer to prevent blue light from
getting through to the lower layers because all silver halides are sensitive to blue light.
*The film base is an orange color to reduce the contrast of the negative and to correct for
sensitivities in the red and green layers.
*The anti-halation layer in color film serves the same purpose as in black and white film
Exposure, Development of Black and White Film - Overview
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A. Unused film in camera
B. Exposure of film to light
(photons)
C. Formation of silver ions
(latent image)
D. Development changes silver
ions to metallic silver
E. Fixing – removes unreacted
silver halides from the emulsion.
F. Wash – rinsing with clean
water. Removes all by-products
of development process.
The emulsion
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AgNO3 + KBr = AgBr + KNO3 in gelatin
AgBr precipitates (WHY??) and remain in the gelatin to form minute grains.
AgBr is light sensitive, forming a latent image that can be “developed”
But how?
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The sensitivity of the grains are proportional to their sizes. If all the grains
were the same size, there would be no shades of grey at all! Typical
densities of grains are about 5 x 108 grains per cm2. If you consider a grain
to be equivalent to a “pixel”, you see that photographic film (taken by itself)
it quite a bit more capable of “resolving” detail than our current digital
cameras.
The latent image
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For many years, it was thought that 2AgBr + light = Ag2Br + Br
(the “sub-haloid” hypothesis…). But there was never evidence of a chemical
change. Less than 5 silver atoms are involved at any site!!
X-ray spectroscopy finally showed that silver is liberated
Br - + light  Br + e –
The electron then migrates to a shallow “trap” (called a sensitivity site).
Ag + + e -  Ag
Species produced include: Ag2+, Ag2o, Ag3+, Ag3o, Ag4+, Ag4o
Why doesn’t it go the other way? i.e. why is it stable?
Converting Silver Halide Crystals to Metallic Silver
Ag+Br- (crystal) + hv (radiation) ® Ag+ + Br + eAg+ + e- ® Ag0
Silver Crystals – Sensitivity Centers
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The silver halide crystal contains
imperfections called sensitivity
centers.
Effects of light on the film
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Within a crystal the Silver atoms have a
positive charge and the halide atoms a
negative.
Light (photons) striking the halide atoms
within the grains causes excitation of
electrons which move within the
crystalline structure.
Those electrons are attracted to the
Sensitivity Centers.
Ag+ Br - (crystal) + hv (radiation)  Ag+ + Br + e-
Latent Image Formation
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The silver ions are attracted to the
negative charge of the electrons at the
sensitivity center.
As more light (photons) hit the halide
atoms silver ions build up on the
sensitivity centers.
The silver ions acquire and additional
electron and become metallic silver.
These sites form development centers
and make up what is called the “latent
image”.
Ag+ + e-  Ag0
Developing the image
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All of what we’ve discussed so far has gone on
within your camera.
Now we’ll go to the process of “developing” your
film.
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Black and white film is handled in complete
darkness as the film is sensitive to all
wavelengths of light.
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The Steps of processing/developing film are:
Development
Stop
Fix
Wash
Hardening bath (optional)
Development
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Photographic Developers are generally
Reducing agents. The silver ions are reduced to
silver metal. The developer donates electrons to
the positive silver ions.
The greater the number of silver nuclei attracted
to the sensitivity centers the faster the developer
will reduce the silver ions to silver metal. So the
more light a crystal is exposed to the faster it will
develop and the darker it will be.
Developers need to be somewhat selective so
as not to turn unexposed silver dark. A process
known as fogging.
Photographic developers contain carefully
balanced levels of the developing agents,
“accelerators” such as Sodium or Potassium
Hydroxide, and Sodium or Potassium
Carbonate. There are also restraining agents
built in such as Potassium Bromide. These
restrainers slow down development in areas that
received less exposure.
The Mechanism of Development
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The photographic process depends upon the fact that the reaction:
Ag + + e  Ag
(i.e. the reduction of silver ion to metallic silver by a developing
solution), proceeds much more easily for an exposed silver halide
grain than for an unexposed grain. The “gain” can be ~109.
Development- Continued…
The reduction potential of the developer must be such that it will develop those
exposed silver halide grains, but not large enough to develop them all. (A
“fogging” developer…)
What actually happens?
C6H4(OH)2 + Na2SO3 + 2AgBr +NaOH C6H3(OH)2SO3Na +2NaBr+H2O +2Ag
Hydroquinone
sodium sulphite
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stabilizer
Chemical velocity:
silver bromide sodium hydroxide
hydroquinone sulphonate
sodium bromide water
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ya gotta do something for the bromine! (plus it adjusts the pH)
ΔT = 1o C  Δvchem = 10%.
SILVER!
Stop Bath
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Photographic developers are generally of a pH
greater than 10.
A “Stop bath” usually made from a weak acid
such as acetic acid is used to stop the
development, and prevent fogging of the
unexposed silver.
Fixing
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Undeveloped silver halide crystals
remaining in your film will darken with
time if exposed to light.
To prevent this, film is “fixed” or has the
undeveloped silver halide crystals
removed from the film.
Sodium Thiosulfate, usually referred to as
“Hypo” is one of the most common fixing
agents though others are used depending
on the specific characteristics wanted in
the fixing solution.
The silver halides have a low solubility in
water. To remove them they need to be
turned into more soluble forms that can
be removed in the water wash.
Fixing the image
The biggest problem after the invention of photography in the 1830’s
was the lack of permanency. You have to get rid of that remaining
bromide, or eventually the photograph will go black. There are no true
solvents of AgBr. When sugar is dissolved in water, and then
evaporated, the sugar is recovered. This never happens with AgBr.
The residue left behind is always a transformed salt. So what we
need to do is make sure the transformed salt is soluble, so it can be
washed away.
AgBr + Na2S2O3 = AgNaS2O3 + NaBr (only slightly soluble)
But if we have a more liberal solution of sodium thiosulphate:
2AgBr + 3 Na2S2O3 = Ag2Na4(S2O3)3 + 2 NaBr (bingo!)
Does anything else work? KCN.
We won’t go there….
Washing
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The final wash of a photographic negative
needs to be lots of fresh clean water to
remove any residual developing agent,
fixative or silver complexes as these can
cause degradation of the image with time.
The ability of a film to withstand this
degradation is referred to as it’s Archival
Quality.
Depending on the film, and processing
methods film can remain unchanged for
many decades.
An optional hardening bath can be used
after the wash to try and minimize
scratches to the dried emulsion.
Reciprocity Failure of Photograhic Plates
Cross section of a photographic plate
Emulsion Hypersensitization
• Baking of plates drives out Water
• Cooling during exposure slows competing chemical
processes
• Soaking in Nitrogen drives out Oxygen and Water
• Soaking in Hydrogen reduces AgBr and produces a
few Ag atoms per grain
• Pre-Flashing generates a few Ag atom per grain
All this worked surprisingly well for a low-sensitivity, fine grained emulsion
originally developed for technical photography, making gas-hypered
Kodak Technical Pan 2415 competitive with the specialized
astrophotography emulsions of the late photographic era. It is no longer
produced today.
So why do we not still use plates?
Disadvantages of photographic plates:
• Low quantum efficiency. The best plates have a QE of about 3%
• Long exposure times, inefficient use of time
• Reciprocity failure. It becomes less effective as exposure time increases
• Non-linear color sensitivity. Plates are more sensitive to blue light
• Hypersensitising and Developing. Hypersensitising involves baking plates to
increase efficiency (up to 10%). You cannot see results until after developing usually
many hours later.
• Storage. They are fragile and take up space. They also decay with age.
• Digitisation. Must be scanned to put the data in digital form
• Cost and availability. In 1996 a single 30 cm x 30 cm plate costs $100 USD. Kodak
no longer makes plates.