Transcript Chemistry of Photography PPT

The Chemistry of Photography
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.
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.
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 ….. Cont.
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The chemical reactions of the fixing
process follow this general form:
AgBr + S2O3-2 --> AgS2O3- + Brwhich is followed by
AgS2O3- + S2O3-2 --> Ag(S203)2-3
Then:
Ag(S203)2-3 <--> AgS2O3- + S2O3 –2
And the final step
AgS2O3 <--> Ag+ + S2O3-2
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.
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” that
are sensitive to various wavelengths the silver halides are not. The absorbed energy is transferred
to 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 “oxidation”
of 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 halide
which 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.
Film vs Print Paper
As we discussed above film has a transparent, clear or in the case of color film colored base. This
is 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.
B&W papers however are not sensitive to red light so when printing B&W prints you can work under
a red light. Color papers are sensitive to all light and must be handled in complete darkness. Most
color print processes use the combined bleach-fix step rather than separate bleaches and fixers.
Conditions effecting rate of development
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User Controlled
Dilution
Temperature
pH
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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
Sample processing time vs temperature chart
for B&W films
Film Type
Temp/Time
20oC(68oF)
22oC(71.5oF)
24oC(75oF)
125 PX
8:30
7:00
5:30
100 TMX
10:00
8:00
6:30
400 TMY
11:30
9:15
7:30
3200 TMZ
17:00
14:00
11:15