A new 0°45° blue- and green-light reflectometer has been built with S-4 vacuum phototubes in a ratio-measuring circuit. Where whiteness is of interest, materials are usually yellowish in hue. In these cases, precise reflectance measurements with just the blue and green tristimulus filters are adequate for whiteness determination. Investigators have found that, in general, yellowness detracts from perceived whiteness much more than does grayness. For best correlation with visual rankings, the green-minus-blue reflectance difference corresponding to yellowness should receive four-to-five times the weight of luminous (green) reflectance alone. Because inadequate blue reflectance detracts so strongly from perceived whiteness, widespread use is now made of blue fluorescing dyes for the whiteness enhancement of textiles, papers and plastics. These “fluorescent brighteners” absorb in the near ultraviolet and fluoresce in the blue. An ultraviolet-absorbing filter in the new instrument may be alternated between the sample-viewing and incident light beams to include, and then exclude the near-ultraviolet which excites fluorescence. It is thereby possible to obtain a measure of the contribution of these fluorescent brighteners to blue reflectance, and thence to whiteness.
RICHARD S. HUNTER, "New Reflectometer and Its Use for Whiteness Measurement," J. Opt. Soc. Am. 50, 44-47 (1960)