Abstract
Although standard practice in up-to-date photometric laboratories shows that the expert is well acquainted with the intricacies of visual heterochromatic photometry, several recent publications on the non-additivity of heterochromatic brightnesses seem to indicate that the basic problems involved are not always fully understood by non-specialist photometrists. In this paper the author reviews the results of three independent investigations which all show that there is a definite, although non-linear relationship between the apparent brightness of surfaces of different colour and the photometric assessment of their brightness in terms of luminance as defined by the C.I.E. The general trend of this non-linear relationship between brightness and luminance is revealed and discussed. Finally, some suggestions are made how heterochromatic brightnesses can best be assessed visually in terms of their luminances.
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