The photometric separator here described, illuminated by monochromatic light, enables accurate visual measurements of the transmission factors of parallel plates to be made. The geometrical design has been developed to eliminate systematic errors. The scale of densities depends upon Talbot law. Within the uncertainties of visual settings, the results of measurements are in very good accordance with those obtained from the best available photoelectric methods. Improvements are possible with a larger monochromator.
J. TERRIEN and F. DESVIGNES, "A Photometric Separator for Precision Visual Spectrophotometry," J. Opt. Soc. Am. 40, 845-848 (1950)