Anatomy and Detection Characteristics 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

Retina up-close Tapetum lucidum 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 Night Sky: why are there more stars off-center? © Stephen E. Palmer, 2002

…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

The Physics of Light Some examples of the reflectance spectra of surfaces Red 400 700 Yellow 400 700 Blue 400 700 Purple 400 700 % Photons Reflected Wavelength (nm) © Stephen E. Palmer, 2002

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

Visual Observations Navigation Calendars Unusual Objects (comets etc.)

Chemistry of Photography 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 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 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?

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.

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?

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

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

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-

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

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)

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.

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.

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%.

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.

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.

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 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.