The mean color difference from the mean (MCDM) is a standard measure of spectrophotometric repeatability.
This article proposes a supplementary measure:
the coefficient of variation (CoV), which is the ratio of the standard deviation of the measured reflectances to their mean. The CoV is calculated from the same repeated sample measurements as the MCDM. Unlike the MCDM, the CoV depends only on physical quantities, and not on perceptual quantities; furthermore, a CoV is defined for each wavelength. This article analyzes data from six different spectrophotometer–sample combinations.
An important empirical result is that the CoV is nearly constant across wavelengths, except when the reflectance at a particular wavelength is less than about 5%, in which case, measurement variability is markedly greater. Because the MCDM tends to lose this fact through averaging, the CoV is recommended as an adjunct to the MCDM for spectrophotometer analysis and development. The CoV analysis also provides evidence that samples’ surface geometry is a major factor in measurement variability.
© 2015 Wiley Periodicals, Inc. Col Res Appl, 41, 571–579, 2016