Size and color do matter in the prediction of brightness

Abstract

Introduction In addition to luminance, other (secondary) factors such as stimulus size (larger is brighter) and saturation (higher is brighter: Helmholtz-Kohlrausch effect) have an impact on perceived brightness. In this study, the former was systematically investigated for self-luminous stimuli. In particular, the accuracy of the CIE 2° and 10° luminance to predict perceived brightness was examined. In addition, the magnitude of the stimulus size effect was compared to that of the Helmholtz-Kohlrausch effect. Methods A gender balanced group of ten observers (average age: 23 years) with normal color vision were asked to match the brightness of a 2° or 10° stimulus to respectively a 10° or 2° achromatic reference stimulus presented simultaneously. The initial luminance, the test and reference stimulus sizes, as well as their respective locations (either above or below) were counter balanced to reduce bias. Data on thirty stimuli, covering a wide chromaticity gamut and with a luminance of approximately 60 cd/m², was collected. Stimuli were generated by size-adjustable circular openings in a box in which two RGB LED modules were mounted. Results After matching, the luminance (either 2° or 10°, depending on stimulus size) of each stimulus was measured and averaged over all observers, giving the following results for counter balanced stimulus sizes: - For equal chromaticity stimuli, the 2° luminance is on match consistently higher than the 10° luminance. - Saturated stimuli matched to an achromatic stimulus show a consistently lower luminance, illustrating the H-K effect. Conclusion When perceiving brightness the saturation or Helmholtz-Kohlrausch effect was found to be stronger than the size effect, which was insufficiently corrected by a switch from the CIE 2° to the 10° luminance. The magnitudes of the observed effects suggest they are non-negligible and should be accounted for in any Colour Appearance Model.

Meeting abstract presented at VSS 2014