A method of obtaining spectrophotometric data plotted directly, relative approximately to magnesium oxide yet using glass samples, is described. The method consists of measuring a glass filter in series with the reflectance sample and using an opaque white glass as the reference sample. The transmitting glass is chosen so that over the wave-length range of interest its transmittance is approximately equal to the ratio of the reflectance of the opaque white glass to the reflectance of magnesium oxide.
JOSEPH W. GABEL and E. I. STEARNS, "An Auxiliary Working Standard Equivalent to Magnesium Oxide on a General Electric Spectrophotometer," J. Opt. Soc. Am. 39, 481-483 (1949)