Yoshinobu Nayatani

Simple estimation methods for the Helmholtz—Kohlrausch effect

Four kinds of simple estimation equations are proposed for the Helmholtz—Kohlrausch effect. Two of them can be used for luminous colors, and the other two for object colors. In each of luminous and object colors, the two estimation equations are given to each of the Variable-Achromatic-Color (VAC) and the Variable-Chromatic-Color (VCC) methods. All the equations are similar in type to the Ware—Cowan equation. They give the ratio between luminance (or metric lightness) of test color stimulus and its equivalent luminance (or equivalent lightness) directly. Though their computations are simple, they can apply to various H—K effects including their adapting luminance dependency. The applicable fields of the proposed equations are wider than those of the Ware—Cowan equation. The proposed equations can be applied to predict the H—K effect within the whole chromaticity gamut including spectral colors, spectral luminosity functions based on direct color matching from 0.01 Td to 100 000 Td using the photopic and the scotopic spectral luminosity functions specified by CIE, equivalent lightness values of NCS colors, and others.

Relations between the two kinds of representation methods in the Helmholtz‐Kohlrausch effect

There are two kinds of representation methods of the Helmholtz-Kohlrausch effect; one is the Variable-Achromatic-Color method, and the other the Variable-Chromatic-Color method. The following three items are described in detail. (1) How to use and adapt the prediction equations of the two methods to their practical applications. (2) Theoretical derivations of the prediction equations in both methods, and clarification of the simple relation existing between the numerical coefficients used in the prediction equations on the two methods. (3) Logical consistency between the long series of studies on the H-K effect by the author and his colleagues.

Opponent-colors responses in the visually evoked potential in man

Abstract The responses of visually evoked cortical potential (VECP) of three observers with normal-color vision were measured for 10 different spectral stimuli ranging from 400 to 700 nm with the equal retinal illuminance of 300 td. These responses were analyzed by use of the principal component analysis, and the results obtained are as follows: (1) Opponent-colors responses were found for two of the three observers which were similar to those derived by Jameson-Hurvich on the basis of psychophysical methods. (2) The VECP responses for another observer were almost independent of the different spectral stimuli applied. This was interpreted as a noncolor-coding observer, as shown by Shipley. (3) The same analysis was applied to the VECP responses measured by Shipleyet al. (1968). Though the waveforms are significantly different from those by the present authors, similar opponent-colors responses were also derived.

A simple estimation method for effective adaptation coefficient

A method was proposed in a previous article (CRA, 22, 240–258, 1997) to estimate the state of incomplete adaptation by using the effective chromatic adaptation coefficient αmin. The method could be applied to any experiment on chromatic adaptation using object-color or luminous-color stimuli, but its computational procedure was rather tedious. For this reason, the two simple methods, Methods I and II, are proposed to give the approximate estimates of αmin. Method I uses the corresponding reference color under reference illuminant to a test achromatic color under test illuminant. Method II uses the two kinds of relation equations between test adapting luminance and αmin. The estimates of αmin by each of the two methods agree fairly well with those given in the previous article to the three experiments studied.

A COLORIMETRIC EXPLANATION OF THE HELMHOLTZ-KOHLRAUSCH EFFECT

The Helmholtz–Kohlrausch effect is related with the chromatic strengths of spectral colors suggested by R. M. Evans. An estimate of the chromatic strengths of spectral colors is given by using the color-matching functions and their combinations. The estimate of the chromatic strength corresponds to “chromaticness per unit luminance” at each spectral color. The derived chromatic-strength function is very similar to the Helmholtz–Kohlrausch effect and the zero-grayness function by Evans for spectral colors.

On attributes of achromatic and chromatic object-color perceptions

Adapting luminance dependencies of various color attributes of object colors (lightness, brightness, whiteness-blackness, whiteness-blackness strength, chroma, and colorfulness) were clarified under white illumination with various adapting illuminances. The correlation between the perceptions of lightness and brightness and those of whiteness-blackness and whiteness-blackness strength is also clarified for achromatic object colors. The difference between the increase of brightness and that of whiteness-blackness contrast (the effect studied by Stevens and Jameson—Hurvich) by raising their adapting illuminance is resolved without any contradiction. It is also shown that the nonlinear color-appearance model developed by the author and his colleagues is able to explain the complex characteristics of all the above color attributes of object colors by making minor modifications to it. In addition, two kinds of classifications of various color attributes are given; one is based on the similarity of perception level, and the other on the degree of adapting illuminance dependency.

Prediction of the Helmholtz-Kohlrausch effect (VCC method) using the Swedish NCS system

The existence of the two methods of representation of the Helmholtz-Kohlrausch (H-K) effect was clarified in the previous studies by one of the authors. They are the Variable Chromatic Color (VCC) and the Variable Achromatic Color (VAC) methods. The effect by the VCC method is significantly larger than that by the VAC method, and the former is more effective in visual photometry than the latter. The authors already derived a prediction equation of the H-K effect by the VCC method using the CIELUV formula. In this work, a VCC prediction equation was directly derived from the colorimetric values of the Swedish Natural Color System (NCS). The new VCC equation is almost the same as the VCC equation previously reported. This again clearly suggests the existence of the two representation methods, and the theoretical method used for deriving the previous VCC equation is proved to be correct.

Prediction of experimental results on additivity-law failure

Experimental results on additivity-law failure observed by direct brightness matching are estimated for various combinations of two spectrum colors. The estimations are made by using the prediction equation of the Brightness/Luminance (B/L) ratio effect on chromatic colors, which is based upon the Variable Chromatic Color (VCC) method for the effect. The predicted results confirm the existence of the two types of additivity-law failures called enhancement or cancellation, already reported by several researchers. The prediction equation also clarifies that the effect of additivity-law failure does not change for a wide change of adapting luminance used in observation. Both B/L effect and additivity-law failure can be estimated quite nicely by the same prediction equation without making any modification to it. The Helmholtz–Kohlrausch effect can imply both effects in its wide definition.

Prediction of the Helmholtz-Kohlrausch effect using the CIELUV formula

The Helmholtz-Kohlrausch effect was predicted by using the CIELUV formula. The predictions were done to the chromatic colors in the whole chromaticity gamut including spectral colors by the Variable Achromatic Color (VAC) and the Variable Chromatic Color (VCC) methods. The effect of adapting-illuminance change in the perceived lightness of chromatic object colors was also taken into account in the prediction. The prediction equations by the CIELUV formula gave results quite similar to those by the equation for predicting the Helmholtz-Kohlrausch effect using the nonlinear color-appearance model. Their computational procedures for estimating various effects are very simple and easy to use.

On the field trials of CIECAM97s and its model structure

The structure of the color-appearance model CIECAM97s is examined. The problems with its chromatic-adaptation transform, called the Bradford transform, are discussed in detail. The contradictions existing between the measures at various stages of CIECAM97s are described, which are ea-eb, saturation s, chroma C, and colorfulness M. The main contradictions are (1) the inversion of chromatic components between test and reference colors at different measures; and (2) the similarity between chroma and colorfulness found in the experiments done under different adapting illuminances. The structural problems in CIECAM97s are clarified by comparing its predictions with those using CIECAT94LAB, which consists of the CIE chromatic-adaptation transform published in 1994 and the CIELAB formula.