to record an image are silver salts and are called silver
sensitive to other colors of light and infrared radiation
by the addition of dyes. Depending on the amount of
light and the type of silver halide, the light produces a
visible or invisible change in the halides of a film or
printing paper. An invisible change is made visible by
emulsion is the light-sensitive portion of a film or paper
that records the image. The emulsion contains the silver
halides and any special sensitizing dyes suspended in a
binder of gelatin. The gelatin holds the silver halides
evenly dispersed and prevents action by a developer
until the silver halides have been made developable
either by exposure to light or chemical action. Also, the
place. The base, or support, may be transparent or
opaque, depending upon how the recorded image is to
be used. A transparent base is used for transparencies
that are viewed by transmitted light and for negatives
that are printed with transmitted light. An opaque base
is used for prints that are viewed by reflected light.
Video disks do not contain an emulsion or a base. When
electronic mediums are used, light is converted to
electrical impulses and these impulses are stored
magnetically on a tape or disk. Since it is the camera
itself that converts light to electrical impulses, the
recording medium and all stages of the photographic
process can be carried out in normal room light.
emulsion. These characteristics include the following:
exposure latitude, emulsion latitude, and emulsion
of films. This knowledge is helpful in selecting the film
most suitable for each photographic assignment.
This light causes invisible changes to the emulsion and
is called the latent image. The latent image can be
physically made visible by the chemical step of
development. The extent of the reaction to the light of
the emulsion is affected greatly by the size of the silver
halide grains and the amount of light reaching the film.
The inherent property of a film emulsion to respond to
light is termed film speed.
image. Emulsions are rated as slow, medium, or fast,
depending on the amount of light required to produce an
image satisfactorily. Fast emulsions require less light to
produce an acceptable image than slow emulsions.
manufacturer has developed a system of rating emulsion
speed. The rating system used is the IS0 film speed
system. ISO film speed is a numerical value assigned to
an emulsion used for determining exposures.
world. It has approved a uniform set of film speed
standards. The standards call for a universal expression
of both arithmetic and logarithmic speed values with the
ISO designation. The ISO designation generally looks
like the following:
Basic Photography Course