Therefore, rather than experimentally controlling for luminance intrusion, we instead allowed for the possibility that the stimuli are not perfectly isoluminant for each observer and included luminance intrusion in our model of chromatic channels. There are many possible sources of luminance intrusion, including interobserver variations in V(λ) ( Gibson & Tyndall, 1923), retinal illuminance ( Ikeda & Shimozono, 1981), chromatic aberration ( Flitcroft, 1989), and the variation of the isoluminance point across the visual field ( Bilodeau & Faubert, 1997).
Truly isoluminant stimuli are difficult to achieve even when using a heterochromatic flicker paradigm ( Wagner & Boynton, 1972). We present a computational luminance-dependent model that predicts the CSF for achromatic and chromatic stimuli of arbitrary size.ĭue to the aforementioned purpose, the current study approaches the characterization of chromatic contrast sensitivity slightly differently from Mullen (1985).
In contrast, the chromatic sensitivity curves do not show a significant sensitivity drop at higher luminance levels. The achromatic contrast sensitivity increases with higher background luminance up to 200 cd/m 2 and then shows a sharp decline when background luminance is increased further. Our main finding is that the background luminance has a differential effect on achromatic contrast sensitivity compared to chromatic contrast sensitivity. Within each session, observers were fully adapted to the fixed background luminance (0.02, 2, 20, 200, 2,000, or 7,000 cd/m 2). Contrast sensitivity was measured in three directions in color space, an achromatic direction, an isoluminant “red-green” direction, and an S-cone isolating “yellow-violet” direction, selected to isolate the luminance, L/M-cone opponent, and S-cone opponent pathways, respectively, of the early postreceptoral processing stages. Stimuli consisted of Gabor patches of different spatial frequencies and angular sizes, varying from 0.125 to 6 cpd, which were displayed on a custom high dynamic range (HDR) display with luminance levels up to 15,000 cd/m 2. The purpose of our study was to further characterize the CSF by measuring both achromatic and chromatic sensitivities for background luminance levels from 0.02 cd/m 2 to 7,000 cd/m 2. Contrast sensitivity functions (CSFs) characterize the sensitivity of the human visual system at different spatial scales, but little is known as to how contrast sensitivity for achromatic and chromatic stimuli changes from a mesopic to a highly photopic range reflecting outdoor illumination levels.