EMULSION
Photographic materials consist mainly of a sup-
port base coated with one or more layers of pho-
tosensitive material. In silver gelatin photography,
this photosensitive material is customarily called
an ‘‘emulsion.’’ This term is a misnomer, but is
commonly used to refer to the light-sensitive mate-
rial coated on various substrates including glass
plates, flexible films, or papers. However, it is also
applied broadly to film and paper products.
Manufacturers and photographers pay great
attention to the characteristics of emulsions and
sensitized materials because their choice has great
influence on the effect and quality of the photo-
graphs obtained. Some emulsions are designed to
form negative images, and others, positive images.
One kind of positive emulsion is designed for re-
versal processing, where, during processing, a ne-
gative image is formed first and then chemically
reversed to form a final positive image. The other
kind of positive emulsion is the direct positive emul-
sion, where conventional exposure and processing
yield a final positive image. Negative emulsions
form negative images, where highlights are rendered
dark and shadows light. Negative images are usually
printed on another negative material such as print-
ing paper to make final positive images. This is
called a negative–positive system.
Photographic speeds of emulsions, that is, sensi-
tivity to light and thus how long the material needs
to be exposed to create an acceptable image, vary
widely. Contact printing papers are very slow to
provide comfortable exposure time in contact print-
ing frames. Enlarging (projection printing) papers
are much faster than contact printing papers but
much slower than negative emulsions for in-camera
use. Document films and duplicating films usually
fall between enlarging emulsions and negative emul-
sions in speed.
Emulsions also vary in contrast. Pictorial pho-
tography produces what are called continuous
tone reproduction of the original image, consisting
of black, white, and many shades of gray; while
graphic art processes and reproduction of docu-
ments generally use extremely high contrast to
produce images of black and white with no gray.
In a pictorial negative–positive system, negative
films have low contrast to register the subject’s
wide gradation range, while print emulsions have
high contrast to make final prints of normal con-
trast images. This system provides wide exposure
latitude at the time of in-camera exposure, while
allowing precise control of image darkness and
contrast in the darkroom. In reversal and direct
positive systems, the films have higher contrast
than negative films because the in-camera film
forms the final positive image. Exposure accuracy
of these materials is more stringent and has nar-
row latitudes because of the inherently high con-
trast of the films and the lack of ability to adjust
the contrast in the darkroom.
Most emulsions are of ‘‘developing out’’ type,
where light exposure forms a latent image, which
is made visible through the development process.
The other emulsions are of ‘‘printing out’’ type,
where prolonged exposure to intense light forms a
visible image without the development process.
Printing out papers (POP) were in common use in
the early years of photography to make contact
prints from large-format glass negatives, but are
still used by photographers. Along with silver-gela-
tin-based POP, there are salted papers and other
non-silver-based printing techniques such as plati-
num printing. These materials have inherently low
contrast and therefore the original negatives were
made rather high contrast to make the final prints
have normal contrast.
Silver-gelatin emulsion consists of light sensitive
silver halide crystals in a binding medium. An emul-
sion is made from soluble silver (silver nitrate) and
soluble halides (e.g., potassium bromide) in the
presence of a protective colloid, such as aqueous
dispersion of gelatin. These agents are mixed in a
highly controlled way, using a process called pre-
cipitation. Crystals thus prepared are physically
ripened to desired sizes, chemically digested to in-
crease the light sensitivity, and stabilized to ensure
photographic performance of the film and paper
until its specified expiration date.
After film or paper is exposed to light, exposed
crystals bear tiny specks of metallic silver, called
latent image, which renders the entire crystal devel-
opable in the developer solution, while crystals
without a latent image are not developable. Thus
the emulsion registers light exposure by means of
latent image from as little as a few metallic silver
atoms among several billion silver halide moleculesEMULSION