Since it would be impossible to judge whether
densities are reproduced accurately by viewing your
normal production products, special test strips, or
control strips, are prepared. The test strips are
exposed accurately with varying amounts of light.
These test strips are developed in your process, and
the resulting densities are read on a densitometer and
averaged together (fig. 2-1). The densities are then
plotted on a graph. The plot of the data establishes
the standard. Similar test strips are then processed at
regular intervals and compared with the standard to
ensure that the processing is under control. This,
basically, is sensitometry.
Another term used in conjunction with
sensitometry is densitometry. Densitometry is an
integral part of sensitometry. These two terms are
often used together or interchangeably. Technically,
however, there are differences between them. The
differences are as follows:
Sensitometry, or measurement of photographic
sensitivity, is the science of determining the
photographic characteristics of light-sensitive
materials.
Densitometry, or measurement of densities, is
the method whereby data are obtained for
sensitometric calculations.
Measurements of densities are done on a
logarithmic scale. To understand sensitometry, you
must become acquainted with logarithms.
COMMON LOGARITHMS
Complex problems can be calculated easily and
accurately by means of logarithms. You can add
logarithms to achieve multiplication, subtract them to
achieve division, and divide them to derive square
roots.
In photographic quality assurance, logarithms are
used for the following:
Determining density
Plotting characteristic curves
Determining contrast
Determining log H
Reading the densitometer scale
A common logarithm (log 10) is an exponent to a
base number of 10. The base 10 is used because our
numerical system is based on units of 10. This can be
demonstrated easily by using scientific notation, or
"powers of 10" For example, the logarithm of 100 is
2, because 102 equals 10 times 10, or 100. The
logarithm of 1,000 is 3, because 103 equals 10 times
10 times 10, or 1,000. Table 2-1 shows how some
common logarithms are computed. Notice the
relationship between the exponent (superscript) and
the common log.
Table 2-1.—Examples of Some Common Logarithms
2-3
