Sensitometer
an instrument used in testing photographic materials. It subjects the material to a series of exposures that vary in accordance with a specified law. The instrument includes a light source and provides exposing light of a given constant relative spectral composition. After development, the material exhibits a series of different densities of blackening, which is called a sensitometric strip. The densities are measured with a densitometer, and the results are used to construct a characteristic curve for the material. This curve shows in graphic form the dependence of the density on the logarithm of the exposure. By means of the characteristic curve it is possible to determine the numerical values of the quantities expressing the principal properties of photographic materials: overall speed, effective speed, gamma, photographic latitude, and fog density.
In most present-day sensitometers, the exposure is varied by changing the illuminance of the sensitive layer while holding the exposure time constant. The exposure time is selected from the range of times typical of photographic practice—that is, between 0.02 and 0.1 sec—and is most often taken as 0.05 sec (an exception is in the testing of some special properties of photographic materials or the properties of special-purpose materials).
The light source in sensitometers is usually an incandescent lamp with a color temperature of 2850°K. The radiation from the lamp is transformed into artificial daylight with a higher color temperature (or some other radiation of required spectral composition) by means of glass or liquid light filters.
The variation of the illuminance in accordance with the specified law is provided by equipping the instrument with a neutral gray photometer wedge, which is placed in direct contact with the material being tested. The density increase per step in the wedge is 0.3, 0.15, or 0.10. As a result, with each step the illuminance of the sensitive layer changes by a factor of 2, 
2, or 
respectively. The material of the neutral gray wedge must absorb light nonselectively—that is, it must not change the relative spectral composition of the light. This condition is satisfied by, for example, colloidal graphite, platinum coatings on glass or quartz bases, and highly dispersed blackening silver in a gelatin layer.
In the USSR, the FSR-41 type of sensitometer (Figure 1) is used for general sensitometric testing of black-and-white photographic materials in accordance with GOST (All-Union State Standard) 2817–50. The TsS-2 instrument (GOST 9160–59) is recommended for the testing of multilayer color materials.
L. N. KAPORSKII
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