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A description is given of a computer controlled rotating drum film scanner suitable for the measurement of X-ray diffraction photographs. The advantages of such a densitometer for protein crystallography are discussed. By measuring integrated densities 'on line' and taking advantage of the high positional accuracy of the scanner, a very satisfactory combination of speed, accuracy and flexibility can be achieved. It is found that commonly used X-ray films are significantly nonlinear even at optical densities less than 1.2 but by using a parabolic modification of the integrated densities the usable density range can be extended to optical densities of at least 2.5.
Matthews et al. (1972) studied this question.