Abstract

In order to establish a correlate for lightness in terms of CIE-tristimulus values, a simple experiment was performed using Munsell samples of value 5/. The observers compared each sample with a lightness scale and estimated the value of the neutral which would appear equally light. Analysis of the data by the least-squares method showed that a correlate for lightness may be calculated with good approximation from the tristimulus values by means of a second-degree equation, which, for constant lightness, may be interpreted geometrically as an hyperboloid in the CIE-color space. From this, it is concluded that the CIE method of calculating luminous reflectance and using it as a lightness index does not conform with the visual sensation.

It has been established that although the coefficients of the second-degree equation vary with field size and depend upon the individual observer, characteristic features of the results are maintained in each case.

With respect to color vision theory the nature of the results seems to indicate that the contribution of the violet receptor to the sensation of lightness decreases with field size.

© 1957 Optical Society of America

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