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The visual mechanism of a normal observer is so constructed that objects keep nearly their daylight colors even when the illuminant departs markedly from average daylight. The processes by means of which t he observer becomes adapted to the illuminant or discounts most of the effect of a nondaylight illuminant are complicated; they are known to be partly retinal and partly cortical. By taking into account the various fragments of both qualitative and quantitative information to be found in the literature, relations have been formulated by means of which it is possible to compute approximately the hue, saturation, and lightness (tint, value) of a surface color from the tristimulus specifications of the light reflected from the surface and of the light reflected from the background against which it is viewed. Preliminary observations of 15 surfaces under each of 5 different illuminants have demonstrated the adequacy of the formulation, and have led to an approximate evaluation of the constants appearing in it. More detailed and extensive observations have been carried out in the psychological laboratories of Bryn Mawr College, and these observations have resulted in an improved formulation. CONTENTS Page I . Introduction  ___   __  ____ ~ ___ ___ 294 II. Definitions   ___  ___ __   ___  295 1. Psychological t erms ____   295 2. Psychophysical terms_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 295 3. SpeCial symbols __   ____   __ 296 III. Statement and explanation of formulas __  ____   297 1. Assumptions ____ __ ___ ____  ___   297 (a) Color matching __  ___    297 (b) IIue ___   __ ____   __ ___ 298 (c) Saturation ____  ___ __ ____ ___ ___  _ 298 (d) Chromatic adaptation   __ 298 (e) Lightness   ___    298 2. Formulas ___ ____   ____   __ 299 (a) IIue    ___ ___ ___ __ ___  ___ 299 (b) Saturation  __ ___  __ __   300 (c) Achromatic point.      __ 300 Cd) Lightness ____ ____    ____ 303 (e) Observing situations ____     304 IV. Illuminants and samples used  __ __  ___  ___ ____ __ 307 V. Experimental method    ____   310 VI. Computed values of hue, saturation, and lightness ___ __  311 VII. Comparison of computation with experiment   ___ ___  316 1. South daylight  ____ ____ ____   __ 318 2. Strongly chromatic ilIuminants  ___ __  __ __ ___ 318 3. Substitution of a dark for a light background   324 VIII. Influence of light-scattering elements between sample and retina  326 IX. Bezold-Briicke phenomenon and the hue change by admixture of achromatic light   __   ___ 327 X. Relation to visual theory _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 328 XI. Summary ___    ___  ____  __ 329 XII. References ____ ____ ____  __ __  330

Hue, saturation, and lightness of surface colors with chromatic illumination