Cite as:
Walkey H C, Barbur J L, 2003, "A model for effective contrast in the mesopic range" Perception 32 ECVP Abstract Supplement

A model for effective contrast in the mesopic range

H C Walkey, J L Barbur

Our aim was to model effective contrast in the mesopic range (C_mes) as a function of target photopic contrast (C_p), scotopic contrast (C_s), and chromatic difference (CD), plus background luminance (L_b). We defined our measure C_mes as the luminance contrast L_t - L_b, where L_t is the luminance of the target, of an achromatic target perceived to have the same effective contrast as the test target. The targets were 3 deg Landolt rings presented at 7° eccentricity on a uniform background with fixed background chromaticity (x = 0.305, y = 0.323). Using a staircase procedure, five subjects obtained equivalent achromatic contrasts for a large number of test target combinations of C_p, C_s, and CD across the colour gamut of the display. Measurements were made for six values of L_b in the range 10 to 0.004 phot. cd m^-2. The dependence of C_mes on the test-target variables was investigated with ANOVAs, and an empirical model was developed based on the statistically significant parameters. The results highlighted individual differences in the processing of luminance increments and decrements and in the contribution of colour to C_mes. The results also showed that the dependence of C_mes on C_p, C_s, and CD varied markedly with light level. The predominant factor at the higher luminances was C_p, and C_s was the main factor at the low luminances. Surprisingly, CD made a significant contribution to C_mes even in the mid-to-low mesopic range. Our model for C_mes was a complex function of C_p, sign-dependent C_s, CD, and L_b; neither C_p, C_s, nor CD alone were good indicators of C_mes. Our model shows that to describe the visual effectiveness of a target under mesopic conditions it is necessary to take account of the contribution of both the rod and the cone systems to luminance contrast, combined with a contribution from chromatic mechanisms.

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