in a single crystal, and this invisibly registered
latent image is chemically amplified by the devel-
oper to make a metallic silver grain, which forms
the visible image.
Most primitive emulsions are sensitive to blue and
ultraviolet light. This is because green and red light
do not have sufficient energy to create a latent image
on silver halide crystals. Many photographic appli-
cations require sensitivity to green or red light, and
therefore the spectral sensitivity is extended by sensi-
tizing dyes. Sensitizing dyes allow light of lower
energy (longer wavelengths than blue) to form latent
images. For example, a sensitizing dye for all visible
colors (panchromatic) is used for black-and-white
camera negative films so that a natural grayscale is
obtained from colored objects.
Enhanced sensitivity is obtained by various
means, including optimizing ripening and digestion
steps, incorporation of very small amounts of do-
pants, and use of suitable sensitizing dyes. Optimal
formulation improves the efficiency with which the
latent image forms upon exposure to light, the sta-
bility of the latent image, and the developability
of the crystals with latent images. In the past, one
could increase photographic speed of emulsion only
at the cost of shortening the emulsion’s shelf life.
This is because chemical reaction will slowly pro-
gress in coated, dried materials, until they are for all
intents and purposes processed. This appears as
elevated fog level, decreased speed, decreased con-
trast, or a combination of these. Today’s high-speed
emulsions have excellent photographic properties
and long shelf life due to improved technology in
sensitization as well as stabilization.
The silver-gelatin process was invented by ama-
teur photographer R. L. Maddox in 1871, in the
form of ‘‘silver gelatinobromide’’ plates to be used
as in-camera negatives. Maddox’s plates were
photographically inferior to then-existing wet col-
lodion plates, but within a few years of his publica-
tion of the process, other photographers made
important contributions to the field. The silver-
gelatin process is thus evolved through continued
experiments by scientists and application by engi-
neers and photographers. That is, the silver-gela-
tin process is a form of applied science that
enabled various branches of photography that
were previously impossible. Indeed, its influence
went beyond photography-related fields; the
photographic industry motivated and drove early
research on dye chemistry, gelatin chemistry, and
other branches of physics and chemistry.
RYUJISuzuki
Seealso:Contact Printing; Film; Non-Silver Pro-
cessesFurther Reading
Carroll, B. H. ‘‘The Preparation of Photographic Emul-
sions.’’Journal of Chemical Education8 (1931) 2341–
2367.
Duffin, G. F.Photographic Emulsion Chemistry. London:
Focal Press, 1966.
Hill, T. T. Laboratory-scale Photographic Emulsion Tech-
nique.Journal of Chemical Education43 (1966) 492–498.
Keller, K.Science and Technology of Photography. VCH,
1993.
Mueller, F. W. H. ‘‘The Photographic Emulsion.’’ In Nebl-
ette’sHandbook of Photography and Reprography: Mate-
rials, Processes, and Systems, New York: Van Nostrand
Reinhold, 1977.
Sturmer, D. M. and A. P. Marchetti. ‘‘Silver Halide Ima-
ging.’’ In J. Sturge, V. Walworth, and A. Shepp, eds.,
Imaging Processes and Materials: Neblette’s eighth edi-
tion, New York: Van Nostrand Reinhold, 1989.
Zelikman, V. L. and S. M. Levi.Making and Coating Photo-
graphic Emulsions. London: Focal Press, 1964.ENLARGER
An enlarger is a device used to project negatives
onto light-sensitive materials, most usually photo-
graphic paper, which are then processed in appro-
priate chemistry so that the latent image appears
visible. An enlarger can be used to make the image
either smaller or larger than the negative itself,
though it is most often used (as the name would
suggest) to enlarge negatives. Enlargers are gener-
ally set up so that they project the image downward
onto a base, though some enlargers can pivot so
that they project the image straight ahead. These
enlargers are often utilized when the size of theEMULSION