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Figure 2-2.—Step tablets |
allowed to act, the exposure may be varied by
changing either the intensity or the time.
Light sources for a sensitometer must be carefully
chosen, and their characteristics must be precisely
known. The intensity of the light must also be
known. The time of exposure should correspond
closely with actual photographic practice and must
remain constant over long periods of time. In addition
to this, the color temperature must be known, must
remain constant, and must correspond closely to the
quality of light that is likely to be used in practical
photography. These qualities allow the test strip to be
exposed under conditions as close as possible to those
that occur in practical use. When you base sensitivity
measurement on a consistent factor that is known, the
response of light-sensitive emulsions, under practical
conditions, is predictable. Thus various emulsions
may be compared to each other. You know that
emulsions often change in apparent sensitivity (speed)
with a change in the spectral composition (color) of
the exposing light. Processing also has an effect on
the speed of emulsions.
When incandescent lamps are used in
sensitometers, they operate at a color temperature of
about 3200 K. The color temperature of sunlight is
about 5400 K. Therefore, you may need a filter to
alter the color of the lamp, so it is equivalent to the
spectral energy of sunlight.
Differences in time can also lead to errors due to
reciprocity law failure. The EG&G sensitometer, used
commonly in Navy imaging facilities, permits you to
change the exposure times as well. The exposure can
be changed from 1/100 second to 1/10,000 second.
This allows you to test the reactions of emulsions to
various exposure times.
A sensitometer is used to produce a
logarithmically graded series of exposures with values
that are already known. These are generally arranged
in steps from low to high. A part of the strip is left
unexposed, so the gross density of the material itself
may be determined.
The device or method used to vary the amount of
exposure must also meet certain requirements. The
device should be able to produce an exposure range
that conforms closely to that found in actual practice,
to be accurate, to be consistent, and to have no
significant effect on the color quality of light. A step
tablet is used in a sensitometer for this purpose.
Step Tablet
The 11- and 21-step step tablets are most
commonly used in sensitometers. These step tablets
are comprised of a series of neutral-density filters with
densities that range approximately from 0.05 to 3.05.
On an 11-step step tablet, one f/stop exposure
difference (or 0.30 density difference) exists between
each step of the tablet. On a 21 -step step tablet, a 1/2
f/stop exposure difference (or 0.15 density difference)
exists between each step of the tablet. Selection of a
step tablet should be based on the emulsion latitudes
and contrast differences in pictorial and copy films
and papers. A 21-step step tablet is used normally for
long-scale films, and a 11-step step tablet is used for
short-scale films and printing papers (fig. 2-2).
Processing Sensitometric Strips
Sensitometric strips (scnsi-strips) are made by
processing under controlled conditions of time,
temperature, agitation, and chemical activity. This
holds true whether the material is black and white or
color. Black-and-white control strips are normally
made in the lab, while color control strips are obtained
from the manufacturer of the material being processed.
When you are machine processing control strips or
sensi-strips, they can be attached to either a roll of
film or a leader. To reduce the likelihood of bromide
drag, ensure the leading edge of the sensi-strip has
received the least amount of exposure in the
sensitometer. Following this procedure, the strip goes
through the processing machine tanks with the rest of
the roll and receives the same processing. Remember
that all steps in the processing of control strips and
film are important if the process is to maintain high
standards and uniformity of results. Each step in the
entire process must be carried out as carefully and as
systematically as possible with uniform times and
handling techniques in each step from exposure to
drying.
Refer to figure 2-3 for the relationship between
the sensitometer, step tablet, and sensi-strips.
CHARACTERISTIC CURVES
The characteristic curve shows the relationship
between the exposure and the density, resulting from
2-6
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