The rms granularity values normally produced by an optical scanner are in terms of semispecular instrument density. It is shown that these semispecular values can be converted to a diffuse density basis by dividing by a quantity q that is defined as the ratio of the instrument density of the sample to the diffuse density. The q factor has been measured by two methods, one of which is convenient for use in the routine measurement of diffuse granularity with existing equipment. It has been found that, except at very low density levels, the q of a given material varies only slightly as the diffuse density changes. It does, however, increase significantly as the numerical aperture of the optical system decreases. When a range of samples is measured, maximum values of q occur at moderate values of sample granularity.
H. C. Schmitt, Jr. and J. H. Altman, "Method of Measuring Diffuse RMS Granularity," Appl. Opt. 9, 871-874 (1970)