A microphotometer has been designed and built to measure the granularity of photographic deposits in terms of two-dimensional variations in their transmitting properties. These measurements can be made in terms of either density or transmittance, and the diameters of the circular areas on which the measurements are made can be varied from 1.5µ to 78µ on the sample. Either ΔD or ΔT, the conventional density or transmittance differences from the mean, or SΔD or SΔT, the syzygetic density or transmittance differences between immediately adjacent surface elements of equal area, are measured and recorded automatically. The instrument will make a reading, record this reading, and move the sample to a new area at the rate of approximately one reading every three seconds. When a sufficient number of readings, normally at least 300, have been made, the recorder plots the results as a frequency distribution curve.
L. A. JONES and G. C. HIGGINS, "Photographic Granularity and Graininess. VII. A Microphotometer for the Measurement of Granularity," J. Opt. Soc. Am. 41, 192-200 (1951)