1. Anatomy and Detection Characteristics
The Human Eye
Anatomy and
Detection Characteristics

2. Anatomy of the Human Eye

3. 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

5. The Retina

6. Retina up-close
Tapetum lucidum
Light

7. 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

8. Rod / Cone sensitivity

9. Distribution of Rods and Cones
Night Sky: why are there more stars off-center?
© Stephen E. Palmer, 2002

10. …because that’s where the
Visible Light
Why do we see light of these wavelengths?
…because that’s where the
Sun radiates EM energy
© Stephen E. Palmer, 2002

11. The Physics of Light
Some examples of the reflectance spectra of surfaces
Red
Yellow
Blue
Purple
% Photons Reflected
Wavelength (nm)
© Stephen E. Palmer, 2002

12. Physiology of Color Vision
Three kinds of cones:
© Stephen E. Palmer, 2002

20. Visual Observations
Navigation
Calendars
Unusual Objects (comets etc.)

21. Chemistry of Photography
The light sensitive emulsion
The latent image
Developing the image
Fixing the image

22. 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.

23. 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

24. Exposure, Development of Black and White Film - Overview
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.

25. The emulsion 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?

26. The sensitivity of the grains are proportional to their sizes
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.

27. The latent image
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?

28. Converting Silver Halide Crystals to Metallic Silver
Ag+Br- (crystal) + hv (radiation) ® Ag+ + Br + e- Ag+ + e- ® Ag0

29. Silver Crystals – Sensitivity Centers
The silver halide crystal contains imperfections called sensitivity centers.

30. Effects of light on the film
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-

31. Latent Image Formation
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

32. Developing the image
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.
Black and white film is handled in complete darkness as the film is sensitive to all wavelengths of light.
The Steps of processing/developing film are:
Development
Stop
Fix
Wash
Hardening bath (optional)

33. Development
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.

34. The Mechanism of Development
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.

35. 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 silver bromide sodium hydroxide hydroquinone sulphonate sodium bromide water SILVER!
| |
stabilizer ya gotta do something for the bromine! (plus it adjusts the pH)
Chemical velocity: ΔT = 1o C  Δvchem = 10%.

36. Stop Bath
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.

37. Fixing
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.

38. 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….

39. Washing
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.

41. Reciprocity Failure of Photograhic Plates
Cross section of a photographic plate

43. 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.

44. 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.