Sources of error in measuring opacity of paper by the contrast-ratio method

Contrast ratio of a paper sample is defined as the brightness of the illuminated sample when backed by a black-lined cavity divided by its brightness when backed by a surface of magnesium oxide. In practice other less fragile surfaces are substituted for the MgO surface, and to protect these substitute surfaces as well as the paper samples themselves the samples are kept from touching the surfaces. Both departures from the definition are potential sources of error. These errors have been investigated experimentally and a theoretical expression representing them is given.