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SENSITOMETRIC CHARACTERISTICS 31
Exposure is measured in terms of the meter-candle-second (that exposure produced by a standard candle for a period of one second at a distance of one meter). Standardized exposures for routine test purposes are usually obtained from test sensitometers such as the Eastman lib. Photometrically calibrated lamps are provided with this instrument for standardizing purposes. Other standardized exposures may be provided in operating equipment such as cameras, sound recorders, printers, etc. ; these are often working standards that are referred to the lib exposure as a reference. The Eastman lib, although a time-scale device that is obsolete and no longer manufactured, is used; no other instrument has such wide use and acceptance despite the real need for other sensitometers.
When film characteristics are graphically plotted, the result is an H&D curve — so called because it was first described by Hurter and Driffeld. An H&D curve (Fig. 1) is similar to a single sweep of a hysteresis loop (B-H magnetization curve) or of a stress-strain curve. It shows the relationship between density plotted as the ordinate, and log exposure, as the abscissa. The slope or gradient of the curve — called the gamma — indicates the relationship between the change in density and the change in log exposure. This may be more simply defined as :
~ A Densitv
Gamma = -x~= =
A logio Exposure
The value given for gamma is the value at the center of the straight-line portion of the H&D curve.
When used in connection with development, "gamma" refers only to development contrast. Development contrast is not related to subject contrast since it is dependent upon the character of the photosensitive material, upon an arbitrary series of exposures, and upon the development. As the numerical value of the gamma obtained depends upon the conditions under which it is measured, comparisons among similar materials may be made in terms of the gamma measured with a lib sensitometer. In this case, the color temperature of the light source is 3000 K., the exposure is changed from one step to another by changing the exposure time and maintaining the light intensity constant. There are 21 exposure steps; the maximum exposure time is 4.16 second, the minimum exposure time is 0.00406 second, and the midstep exposure time is 0.130 second. Each step is related to the next adjacent in the ratio of \/2 : 1. The gamma determined with this sensitometer is usually referred to as the lib gamma.
Within a limited range, gamma can be varied by changing developing