The multipurpose reflectometer was developed primarily to measure apparent reflectance, specular gloss and trichromatic coefficients. These measurements are useful in the ceramic, paint, textile, paper and chemical industries to indicate lightness, gloss and color of finished articles. In the reflectometer, two light beams from a single source are directed along separate paths to two barrier-layer photo-cells. Various types of these photo-cells were studied to find which could be used most advantageously. The reflectometer employs a substitution null method and requires a galvanometer to indicate equality of the currents generated by the two photo-cells. For each sample tested, there is a photometric adjustment to restore equality of the currents. The amounts of photometric adjustment are measured on the direct-reading scales; one of which is used for apparent reflectance and the other for specular gloss. Because of its high precision, the instrument is well suited for measuring small differences in apparent reflectance, gloss or color of nearly identical samples. However, for greatest accuracy, it is necessary to correct the scale readings by calibration.
RICHARD S. HUNTER, "A Multipurpose Photoelectric Reflectometer," J. Opt. Soc. Am. 30, 536-557 (1940)