Many spectrophotometers utilize polarizing prisms and determine the reflection factor of the sample by plane polarized light. In general, the reflection factor of the sample is independent of the azimuth of the plane of polarization but there are some materials, notably textiles, for which the reflection factor depends upon the azimuth. It can be shown mathematically that, if the reflection factor is determined for two azimuths at right angles to one another, the arithmetic average of the two values is the reflection factor for ordinary, or unpolarized light. This is true regardless of the azimuths selected. Experimental data are presented which indicate the applicability of this principle to the measurement of the color of textile fabrics.
ROBERT D. NUTTING, "Interpretation of Data Obtained with Spectrophotometers of the Polarizing Type," J. Opt. Soc. Am. 25, 211-212 (1935)