The Chemistry of Photography

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The Chemistry of Photography
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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.
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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
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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.

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Converting Silver Halide Crystals to Metallic
Silver
AgBr- (crystal) hv (radiation) Ag Br
e-Ag e- Ag0
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Silver Crystals Sensitivity Centers

• The silver halide crystal contains imperfections
  called sensitivity centers.

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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-

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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

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Developing the image

• All of what weve discussed so far has gone on
  within your camera.
• Now well 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)

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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.

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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.

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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.

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Fixing .. Cont.

• The chemical reactions of the fixing process
  follow this general form
• AgBr S2O3-2 --gt AgS2O3- Br-
• which is followed by
• AgS2O3- S2O3-2 --gt Ag(S203)2-3
• Then
• Ag(S203)2-3 lt--gt AgS2O3- S2O3 2
• And the final step
• AgS2O3 lt--gt Ag S2O3-2

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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 its 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.

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Developing Color Film
The processing of color film is very similar to
processing black and white film with some
notable exceptions. Color film also starts
with silver halides. But added to the film are
dye couplers thatare sensitive to various
wavelengths the silver halides are not. The
absorbed energy is transferredto the silver
halides to form the latent image. So it is the
dyes that are absorbed onto the silver halide
crystal that need to absorb the radiation in the
desired wavelength. During development the
silver halide crystals are reduced by the
developer, resulting in oxidationof the
developer. The oxidized developer then reacts
with the dye couplers to form the colored
dye. Color processing has an added Bleach step
which converts all of the silver back to a silver
halidewhich is removed during the fixing stage.
Some processes combine these two steps into a
single bleach/fix. So there is no silver left
in color negatives.
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Film vs Print Paper
As we discussed above film has a transparent,
clear or in the case of color film colored base.
Thisis so that light can be shone through the
negative film and onto photographic papers to
produce prints. The primary difference is the
base material. For prints it is an opaque paper
base. With the general concepts of the emulsion
and processing steps carrying over from film to
paper. BW papers however are not sensitive to
red light so when printing BW prints you can
work under a red light. Color papers are
sensitive to all light and must be handled in
complete darkness. Mostcolor print processes
use the combined bleach-fix step rather than
separate bleaches and fixers.
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Conditions effecting rate of development

• User Controlled Dilution
  Temperature pH
• Non User Controlled
• Diffusion rate of developer
  solution in gelatin Adsorption
  of developing agent Oxidation products of
  developing agents
  Accumulation of by products Purity of
  manufacturing chemicals

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Sample processing time vs temperature chart for
BW films
Film Type Temp/Time Temp/Time Temp/Time
20oC(68oF) 22oC(71.5oF) 24oC(75oF)
125 PX 830 700 530
100 TMX 1000 800 630
400 TMY 1130 915 730
3200 TMZ 1700 1400 1115