Abstract
Abstract: :
Purpose: The processing of coloured stimuli in the mesopic range remains poorly understood. Little is known about how colour signals and cone– and rod–based luminance signals contribute to perceived contrast. In this study we have matched the effective contrast of coloured stimuli in the mesopic range with that of spatially equivalent achromatic stimuli. The main objective was to develop a metric for mesopic effective contrast (Cmes) as a function of background luminance (Lb), stimulus photopic contrast (Cp), scotopic contrast (Cs) and chromatic difference to the background (CD). Methods: Cmes was defined as the Weber contrast of an achromatic stimulus perceived to have the same effective contrast as the test stimulus. The test and achromatic stimuli were 3o Landolt rings, presented simultaneously at 7o eccentricity on a uniform, achromatic background. Matches for effective contrast were made using a staircase procedure. Five colour normal observers obtained values of Cmes for a large number of combinations of Cp, Cs and CD across the colour gamut of the RGB display. These measurements were made for six values of Lb from 10 to 0.004 phot. cd m–2. ANOVAs were performed to investigate the relationships between Cmes and the test stimulus parameters. An empirical model for Cmes was developed using a multiple regression procedure. Results: The results showed that Cp, Cs and CD all contributed to effective contrast, but that their contributions varied markedly with light level. The statistical significance of interactions between Cp, Cs and CD indicated that this relationship was nonlinear. Cp had the greatest contribution at the highest light levels and Cs had the greatest contribution in the low mesopic range. Surprisingly, colour made a significant contribution to Cmes from 10 to approximately 0.01 phot. cd m–2. An investigation of inter–subject variability at the higher light levels revealed large individual differences in the contributions chromatic signals make to effective contrast. Cmes was modelled successfully as a complex function of sign–dependent Cp and Cs, CD and Lb. Conclusions:The effective contrast of stimuli in the mesopic range is related to a nonlinear combination of photopic luminance contrast, scotopic luminance contrast and colour contrast. The model derived for Cmes can be used to compute the effective contrast of coloured stimuli in the mesopic range. The findings of this study suggest that Cmes may be a valuable metric for assessing mesopic visual performance.
Keywords: perception • color vision • brightness and lightness