Neutral densities from measurements made only with aspectrophotometer. A third type, first described by Schneider and Berger," synthesizes the test sample from separate deposits of individual process dyes the equivalent neutral densities of the sample are deter- 3 R. A modern version of the analytical method described by Heymer and Sundhoff 4 is represented by the Eastman Electronic Densitometer, Type 31, and its associated computer with this apparatus equivalent neutral densities are computed by a linear transformation of three integral densities. The type designed by Evans 3 requires forma- tion of a gray absorber by combining the test sample with the just-required additional amounts of process dyes the equivalent neutral densities of the sample are determined from the visual density of the gray thus formed and the amounts of the required additions. Color Technology Division, Eastman Kodak Company. t Testing Division, Eastman Kodak Company. Three types of color densitometers have been used extensively for determination of equivalent neutral * This is the third of three papers under the same main title' The authorship of the three papers is divided, but all authors were members of a committee, all members of which contributed to the work reported in each of the three papers. 2 The present paper discusses methods of calibrating den- sitometers used in routine testing and control to read equivalent neutral density values of color-film images. DEFINITIONS and basic concepts for the deter- mination of equivalent neutral densities were presented in the first paper of this series.' The problem of determining primary values of equivalent neutral densities was considered in the second paper. Using this system, it has been possible to secure essentially identical equivalent neutral density values with different types of densitometers. Degree to which routine densitometer measurements conform to the primary values. Accuracy of primary image calibration, and 2. The calibration of three types of densitometers suitable for routine use is described and the system is evaluated for 1. POWERS: Eastman Kodak Company, Rochester, New York (Received February 4, 1954) Color densitometers of certain types provide measurements which can be related to equivalent neutral densities of color film dye images. VOLUME 44, NUMBER 7 JULY, 1954 An Objective Method for Determination of Equivalent Neutral Densities of Color Film Images. Densitometers may also be calibrated by means of film patches whose equivalent neutral density values have been established on other densitometers (that is, by a secondary calibration) but the starting point in any such series of operations must be the film patches which have received the primary calibrations. As will be described in the succeeding paper, these patches can then be used to calibrate various types of densitometers so that equivalent neutral density readings may be obtained from them. Their equivalent neutral densities have been established with high accuracy. The procedure which has been described yields 42 patches of film which have received primary calibra- tions. Figure 5 gives the spectral densities of one patch of Ektachrome Film and of the three neutrals derived from this patch. The magenta and yellow equivalent neutral densities of the patch were similarly obtained. The visual density of this hypothetical JOURNAL OF THE OPTICAL SOCIETY OF AMERICA neutral was by definition the cyan equivalent neutral density of the film patch. The cyan equivalent neutral density of each Ekta- chrome Film patch to be calibrated was found as follows: Scalar multiples of the 31-component vectors f2i and f3i (i= 1, - 31), that is, scalar multiples of the spectral density distributions of the assumed magenta and yellow dyes, were found by a process of successive approximation such that by adding or subtracting them to the spectral densities of the patch a neutral was obtained. Application of this definition leads directly to the following procedure, which was followed in the calibra- tion of each of the 42 selected Ektachrome Film calibra- tion patches. The three equivalent neutral densi- ties of a film patch may be regarded as its three analyt- ical densities expressed in units such that if the patch were neutral they would be equal to each other and equal to the visual density of the patch. MORRISSEY neutral densities from measurements made only with a spectrophotometer.
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