Transcript Silver Halide (AgX) Photography - RIT CIS

Silver Halide (AgX)
Imaging
Imaging Science Fundamentals
Imaging Science Fundamentals
Chester F. Carlson Center for Imaging Science
What is “Silver Halide”?
 Silver
Halide is a compound made of
silver (Ag) atoms and atoms from the
halide group (Cl,Br,I) of the periodic
table.
 The material property of a silver halide
crystal changes after absorbing photons.
Imaging Science Fundamentals
Chester F. Carlson Center for Imaging Science
Silver and halide atoms within the periodic
table of the elements
Silver (Ag)
Halide group
Imaging Science Fundamentals
Chester F. Carlson Center for Imaging Science
Origins of Modern Photography
 Silver
halide was first used as an
imaging material in the 1830’s by W. H.
Fox Talbot, J. N. Niepce, and J. M.
Daguerre.
 Today’s black and white film derives
from Talbot’s negative-positive process.
Imaging Science Fundamentals
Chester F. Carlson Center for Imaging Science
Structure of a Typical B&W Film
 Film
base
 Plastic

Antihalation backing
 Prevents
light from
reflecting back.

Emulsion
Silver Halide Crystals
Suspended in gelatin, like
fruits in Jell-O™!
Imaging Science Fundamentals
Chester F. Carlson Center for Imaging Science
Exposed AgX Crystals
 When
hn
Exposure
a silver halide
crystal is exposed to
light, some of the
AgX molecules
break up into their
constituents, one of
which is metallic
silver (“pure” Ag).
After
Exposure
Imaging Science Fundamentals
Chester F. Carlson Center for Imaging Science
Latent Image Formation (Ex.: shadowgram)
• Group of AgX Crystals
• Mask (object) prevents AgX
crystals underneath to be
exposed.
• “Unmasked” AgX is
exposed to photons.
• Exposed crystals have
different material property
(I.e. some AgX bonds have
been broken).
Imaging Science Fundamentals
Chester F. Carlson Center for Imaging Science
Latent Image Formation (with optics)
• Group of AgX Crystals
• Optics used to image
object onto the film.
•Illuminated AgX is exposed
to photons; but not all
AgX is illuminated.
• Exposed crystals have a
different material property.
Imaging Science Fundamentals
Chester F. Carlson Center for Imaging Science
Processing Photographic Film
 Turns
latent image into visible image.
 Basic steps:
 Developer
 Stop
bath
 Fix bath
 All
chemicals used in the process are in liquid
form.
Imaging Science Fundamentals
Chester F. Carlson Center for Imaging Science
Silver Halide Process Chain
Exposure
Latent
Image
Processing
Develop
Stop
Fix
Visible
(Stable)
Image
A latent image is formed after exposure
(invisible to human eye).
 After processing, the latent image is turned
into a visible, stable image.

Imaging Science Fundamentals
Chester F. Carlson Center for Imaging Science
Processing Photographic Film
Imaging Science Fundamentals

Developer “amplifies” the
atomic silver to visible
silver strands.

Stop Bath stops the
development process.

Fix dissolves the
unexposed AgX crystals,
making the film safe to
expose to light.

Wash with water to rinse fix
chemicals away.
Chester F. Carlson Center for Imaging Science
“Grain” of Film and Paper
 Electron
Photomicrographs of
Emulsion Grains
 (n.b.
Measurement Bars
indicate scale)
Imaging Science Fundamentals
Chester F. Carlson Center for Imaging Science
Why does processed film look
“negative”?
 Silver
strands formed by exposure of
photographic film to light actually appear
dark (they are NOT shiny).
 So, where light hits the film during exposure,
it turns darker.
Imaging Science Fundamentals
Chester F. Carlson Center for Imaging Science
What determines how dark film
becomes?
Darker
D
Lighter
Log H
Less Exposure

More Exposure
Consider the so-called “D-Log H” curve.



Describes how film responds to light:
Density (D) is how dark the film is.
Log H is the exposure (H) in logarithmic scale.
Imaging Science Fundamentals
Chester F. Carlson Center for Imaging Science
D-Log H Curve and Contrast
More contrast
Less contrast
Image
D
Film
response
D
Log H
Imaging Science Fundamentals
Log H
Chester F. Carlson Center for Imaging Science
Photographic Finishing
In order to get a “positive” final print, the
negative must be projected onto photographic
paper.
 Negative * negative = positive!
 2 basic “finishing” methods: CONTACT and
ENLARGEMENT/REDUCTION

Imaging Science Fundamentals
Chester F. Carlson Center for Imaging Science
Contact Printing
Light
Negative
Photographic
Paper
A CONTACT print:
The negative is in direct contact with the photographic paper
--essentially creating a shadow-gram.
Imaging Science Fundamentals
Chester F. Carlson Center for Imaging Science
Enlarger / Reducer
Light
Negative
Photographic
Paper
Optics are used to produce an image of the negative on
photographic paper.
Imaging Science Fundamentals
Chester F. Carlson Center for Imaging Science
Contact Printing vs. Enlarger
Enlarger
Contact Printing
No loss of signal by
the optics.
 No reduction in
resolution.
 Simpler system.
 Fixed image size.

Imaging Science Fundamentals
Flexible image size.
 Some loss in
resolution due to
enlargement.
 Additional optics
may degrade final
image quality.
 Again, flexibility!

Chester F. Carlson Center for Imaging Science