Effect of Color Temperature on Apparent Brightness

Two screens viewed side by side were illuminated by radiations approximating those of blackbodies of different color temperatures and matched in brightness at photopic levels. A group of 73 observers found that less light of the higher color temperature was needed to produce equality of brightness than would be required by the CIE normal observer. The average gain in apparent brightness was 1 percent per 100°K color temperature difference in the range 5400–6300°K. The gain decreases with the age of the observer.When 27 of the observers re-matched the screen colors using a “flicker” method, no gain in apparent brightness was obtained. Thus the CIE normal observer properly predicts equality of brightness under the experimental conditions used if the observer is allowed to respond only to brightness (flicker method). If the observer has time to color adapt, as in continuous side by side screen viewing, an increase in the apparent brightness of the higher color temperature screen results.