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1949 COLOR PROCESSING CONTROL 5
Solutions can be rejected and trouble located if pH measurements fall outside normal limits, but proper pH does not insure satisfactory performance.
B. Photographic Tests.
These can be divided into two parts: (1) photographic solution control tests, and (2) photographic tests made on the machine during operation.
1 . Photographic Solution Control Tests
Tests so termed are used for testing raw materials, solution mixes, and in locating possible sources of trouble with machine solutions. The photographic test consists simply in processing duplicate sensitometric strips of color film through the standard cycle of color-film processing except that the strips are separated at the solution to be tested and one strip run through the sample solution and one through the type solution. These sensitometric strips should be exposed on the same type of color emulsion the machine will process. A supply of film of a single emulsion number sufficient for several months' control operation should be set aside to avoid too frequent typing in of emulsions. Either time or intensity-scale sensitometry may be used, although intensity-scale instruments are recommended because they give a more accurate indication of a film's practical performance. The instrument, however, must be capable of highly reproducible results and should be adjusted to produce a color balance close to neutral. Both visual and densitometer measurements are more accurate when made with neutrally exposed film. Latent-image changes in exposed strips are normally of small magnitude. However, it is recommended for optimum consistency that no exposures more than two months old be used for control work.
It is essential that solution testing be done under carefully controlled conditions of agitation, time, and temperature so that the system itself has reproducibility greater than the solution tolerance to be tested. In practice it is possible to construct apparatus that will give results deviating by not more than 1/s stop speed or Vi6 stop color balance when identical solutions are used for type and sample. This degree of reproducibility requires mechanical agitation, water baths for solution temperature control, and methods of quickly changing film from solution to solution. In the Ansco laboratory, an apparatus has been constructed employing a series of stainless-steel