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1950 INTERPRETING RESULTS 713
A typical set of printing density curves for a color negative material has already been shown in Fig. 1. In evaluating such curves the three members of the set must be considered both individually and as a group.
The interpretation of the individual curves is but little different from the interpretation of a characteristic curve of a black-and-white negative material. Each curve represents, in fact, a negative which will control the printing of one of the components of a positive material. The "speed" of the negative can be determined by a gradient method25 or a fixed-density method, just as in black-and-white sensitometry. Methods in use vary with the product and with the individual sensitometrist. Any method must meet two requirements:
(1) Variations from sample to sample in the "speed" thus determined should directly measure the required or permissible variations in camera exposure of the product.
(2) Disagreement among the "speed" values from the three curves of the negative material should correlate well with any changes of camera lens filter or illuminant quality required to make best normal use of the product sample. Measurements made in the toe portions of the negative curves have generally been found most satisfactory for these purposes.
The absolute density values of the individual curves are, in principle, much less important than the density relationships. The overall density of a negative curve determines the amount of printing light required to print it; as long as the printer intensities can be made sufficiently high to permit printer operation at economical film speeds, it matters little what the absolute density levels are. Even the variations encountered (from product to product and sample to sample) in the differences in over-all density level of the separate printing density curves of negative materials can be offset by changes in printer setup. Of the qualities in the negative that can be shown by a gray scale test, the really important ones are those that are dependent on the curve gradients.
In black-and-white sensitometry, the most familiar expression of gradient is gamma, the slope of the straight-line portion of the sensitometric curve. In color photography the curves so rarely have straightline portions that the term "gamma" is seldom used. It is replaced by average gradient, sometimes called contrast, although the term is rather ambiguous. Average gradient is measured by either of two methods: (a) by selecting a minimum and a maximum value of log E