1264 L E T T E R S TO T H E E D I T O R Vol. 56

Temporal Factor in Color-Difference Judgments HILTON WRIGHT

Division of Applied Physics, National Research Council, Ottawa, Canada (Received 31 March 1966)

INDEX HEADINGS: Color; Color vision.

ACCORDING to certain reports in the literature,1,2 there is reason to expect that color-difference judgments may be

influenced by the duration of the observation times of the color samples. In color-difference experiments, the observer views pairs of colored samples and is required to judge the sizes of the color differences exhibited by the color pairs. The observing time is generally not controlled; consequently the judgments may be the result of observations lasting 1 or 2 seconds or, perhaps, only a fraction of a second. An experiment was performed to determine how stimulus duration affects observed size of color difference. Color-difference judgments were not found to be significantly affected for exposure times of one second or longer, but a depend­ence on both exposure time and hue was found for shorter exposure times.

Eight pairs of colors were viewed in two-pair combinations through a shutter mechanism. One pair of samples consisted of a lightness difference only; the samples in the seven other pairs were selected so that the colors making up each pair were of approxi­mately the same hue, but the pairs were of different hues. The CIE (x,y,Y) coordinates of the color pairs are given in Table I. The color pairs were observed against a black surround under a Macbeth artificial daylight source. Except for the shutter me­chanism, the observing conditions were identical with those described in Ref. 3.

The shutter mechanism provided seven exposure times ranging from 33 msec to unlimited time.4 When the shutter was open, the observer could compare the two color pairs with both eyes. The sizes of the color differences were judged by the method of ratio comparisons. For each exposure time all pair combinations including the reverse order of the combinations were observed. The order of the exposure times was changed for different runs of the experiment. The judgments were made by one observer (H.W.); the experiment was run 4 times over a period of 3 weeks.

Scale values representing the observed sizes of the color differ­ences were computed from the ratio judgments.3 The average results of the 4 experimental runs are shown in Fig. 1. This figure

TABLE I. CIE (x,y,Y) coordinates of eight color pairs.

FIG. 1. Scale values ti representing observed color differences of eight color pairs obtained for seven exposure times: green + , pink *, gray ●, blue ■ , yellow □ , red ▲. violet O, olive ∆ .

illustrates the ranking of the color differences obtained for the 7 exposure times. The scale values are denoted by k and are given by the abscissa; the time scale is represented by the vertical axis. The vertical axis has been broken between each exposure time because the ti values derived for the different exposure times are not related; that is, in no case were the two pairs in a given observation exposed for different durations. The observer sensed that the absolute sizes of the color differences diminished as the exposure time was decreased, the loss of sensitivity being due to an apparent dimming of the stimuli for the shorter exposure times. The actual quantitative reduction in the absolute sizes of the color differences for decreasing exposure time, however, was not determined.

The eight ti values for each exposure time have been normalized by setting their geometric mean equal to unity. We can see that the rankings of the observed color differences do not change between 33 and 40 msec, nor do the rankings change significantly between 500 msec and unlimited time. As the exposure time is decreased from unlimited time to the shorter exposure times, systematic changes occur in the ranking of the color differences. The gray color difference (represented by the solid circle) deterior­ates the most and changes the most rapidly. The blue, violet, and yellow color differences steadily increase relative to all the others except the red until the exposure time reaches 77 msec. There is no change in the ranking of the red color difference between un­limited time and 230 msec, but as the exposure time is decreased below 230 msec the red pair begins to evoke a relatively stronger sensation. These results suggest that observed color difference is a function of both the exposure time and the hues of the color pairs, provided that the exposure time is sufficiently short, that is, less than 1 sec.

We conclude that for practical purposes—in most color-difference experiments where the observing times are about 1 sec—color-difference judgments are not influenced by the exposure time. For shorter exposure times, however, color-difference judgments appear to be functions of both the exposure time and the hues of the color pairs. Although numerous observations were made in this experiment, they are not considered sufficient to draw definite conclusions. More observers and more sophisticated apparatus would be required to determine the complete effect of the temporal factor on color-difference judgments.

1 D. Farnsworth, in Visual Problems of Colour (Her Majesty's Stationery Office, London, 1958), p. 429. 2 S. M. Luria and S. Weissman. J. Opt. Soc. Am. 55, 1068 (1965). 3 Y. Sugiyama and H. Wright, J. Opt. Soc. Am. 54, 75 (1964). 4 The exposure times in this experiment are effective exposure times, measured by the method described in American Standards Association Specification PH3.4-1959.

September 1966 1265