formula by the capital letter 0 and is the reciprocal of
the transmission (T). The formula for opacity is given
as follows:
O
= Amount of incident light
Amount of transmitted light
You can see that opacity is the transmission
formula inverted. Again, when a material has 10 mc
of light falling on it and 5 mc is being transmitted,
you can determine the opacity by the formula O =
10/5, or 2. Putting it a different way, opacity is the
reciprocal of transmission, or O = 1/0.50 = 2. Or,
when 2 mc is being transmitted, the formula is O =
1/0.20, or O = 10/2; in either case, the opacity is 5.
DENSITY
Density is indicated by the capital letter D and is
another way of expressing opacity or the
light-stopping ability of a medium. Density is nothing
more than the common logarithm of opacity. For
example, when opacity is 2.0, the density is log 2 =
0.30.
In sensitometry, density is the term with which
you are most concerned. However, density cannot be
disassociated from transmission and opacity, because
they all are dependent and directly related to each
other. When the value of any one of these factors is
known, you can calculate the others. When you know
the transmission of a film, you can easily determine
the density. Or conversely, you can measure the
density and then determine the amount of
transmission. While charts are available to provide
some conversions directly, you should be capable of
determining any of those figures.
SENSITOMETERS
Sensitometric-densitometric testing requires a
method of providing the exact same exposure to
different emulsions, or the same emulsion type that is
processed differently, and then comparing the resulting
densities.
Sensitometers make controlled exposures that are
suitable for sensitometric-densitometric testing
procedures. Densitometers measure density.
For sensitometric testing purposes, you must
provide a way of exposing sensitized material with a
known quantity and quality of light.
One of the first requirements for sensitometric
control is to have a sample of the light-sensitive
material that has been exposed properly under
measurable and reproducible conditions. For
sensitometric test purposes, it is common practice to
expose a strip of film, so a number of varying
exposures are made on the same strip. This series of
controlled exposures is made with a sensitometer
through a series of neutral-density filters.
Ideally, a sensitometer should be designed so you
can do the following:
Predetermine the total amount of exposure.
Determine the difference in exposures given
to various areas.
Control the color quality of the light.
Reproduce or duplicate the exposure
consistently.
Obtain a wide enough range of exposures to
produce densities ranging from very light to
dark.
Scenes that might be photographed include a wide
range of brightness values that are represented on a
negative as areas of varying amounts of density;
however, these different densities are scattered
throughout the picture area and are difficult to
measure. For the sake of simplicity, uniformity, and
reproduceability in the application of sensitometry, the
exposures produced by a sensitometer are arranged in
a series of gradually increasing steps. These steps
correspond to the relative brightness values of a
normal scene. The exposures are made on a length of
film or paper of the same type that you are
processing. This sample is called a sensitometric
strip.
"Photographic exposure" is defined as the product
of illumination and time. The two important parts of
a sensitometer are the light source and the device for
controlling the amount of light transmitted to the
emulsion. Since total exposure is the result of the
intensity of illumination and the length of time it is
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