Basic Photography Course

Figure 2-2.Cross section of black-and-white film.

highlight area and a shadow area is not perfectly sharp.
This is because the high-density area tends to "bleed"
onto the low-density area due to the scattering or
diffusing of light in the emulsion during exposure.

CONSTRUCTION OF
BLACK-AND-WHITE FILM

Light-sensitive materials are composed of two basic

parts: the emulsion and the base. The emulsion is the
light-sensitive portion that records the image. The base

is the support to which the emulsion is coated. Also,
negative materials have additional layers that perform a
special purpose. They are as follows: overcoating,
antihalation backing, and noncurl coating (fig. 2-2). The
purpose of the five parts of photographic film are as
follows:

1. OvercoatingThe overcoating protects the film

from friction, scratches, or abrasions before develop-
ment. The overcoating is a clear, gelatin layer that is
sometimes called the antiabrasion layer.

2. Emulsionthin layer of gelatin that suspends

and supports the light-sensitive silver halides.

3. Base-This supports or holds the emulsion in

place. The base may be transparent, translucent, or
opaque, depending upon how the recorded image is to
be used. The base is generally made of a cellulose
acetate.

4. Antihalation backing-The antihalation backing

prevents light from reflecting from the base back into
the emulsion. The antihalation dye is sometimes
incorporated in the anticurl backing. The dye used to
eliminate halation is a color to which the emulsion is
least sensitive. This dye is water soluble and is com-
pletely dissolved during processing.

5. Noncurl coating-The noncurl coating is a

hardened gelatin, about the same thickness as the

PROTECTIVE OVERCOATING

BLUE SENSITIVE EMULSION LAYER - PRODUCES

YELLOW DYES

YELLOW FILTER - ABSORBS BLUE LIGHT

GREEN SENSITIVE EMULSION LAYER -

PRODUCES MAGENTA DYES

RED SENSITIVE EMULSION LAYER - PRODUCES

CYAN DYES

BASE

ANTIHALATION BACKING

NONCURL COATING

Figure 2-3.-Cross section of ordinary color negative film.

Figure 2-4.Modern high-speed color film.

emulsion, and is applied to the back of the film. A film
emulsion swells when wet and shrinks when dry. This
contraction produces a strain on a film base because it
is highly flexible. The noncurl coating prevents the film
from curling during the drying process.

COLOR FILM

Modern color films are constructed much the same

as black-and-white film, but color films consist of three
separate emulsions on a single film base. Each of the
three emulsion layers records one of the three additive
primary colors-red, green, or blue. The top emulsion
layer is sensitive to blue light and produces yellow dyes.
Between the top emulsion layer and the middle emulsion
layer is a yellow filter (fig. 2-3). The yellow filter
absorbs the blue light that would otherwise affect the
middle and bottom emulsion layers. During processing,
this yellow filter is dissolved. The middle emulsion layer
records green light and produces magenta dyes. The
bottom emulsion records red light and produces cyan
dyes. Many of the modern high-speed color films have
fast and slow emulsion layers for each primary color

(fig. 2-4).

2-5

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Basic Photography Course

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