In some previous work we have found that for SA-II and 103F emulsions on film the logarithm of the density (log D) preliminary curves are essentially straight lines. This experimental result leads to a rather simple and convenient expression for the equation of the characteristic curve. For SA-I emulsion on film, we found that the linearty of the log D preliminary curve depends considerable on the wave length region for which the measurements were made and the magnitude of the calibration intensity ratio employed, e.g. the step filter ratio. A more careful analysis of preliminary curves of SA-I emulsions shows the reasons for such a behavior. Actually, the log D preliminary curves are always curved and the degree of curvature varies roughly in proportion to the product of the step ratio of the filter and the gamma at the wavelength at which the measurements are made Probably this same condition exists for SA-II and 103F emulsions, but because their gammas are low, the curvature is not readily observable with a step-filter of moderate ratio. In spite of the curvature of the logD preliminary curve of SA-I emulsions, a straight line fits the data well for the useful range of density measurements for spectrochemical purposes, that is, for a range of densities corresponding to a range of transmittance values between 75% and 1%, the fit is nearly perfect. In attempting to fit various analytical functions, theoretically derived by other investigators, to the experimental data for SA-I emulsion for the wavelength range 4000A–4200A, it was found that the error function gives the best fit.

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