Influence of the target size on the deterction threshold for luminance and chromaticity contrast.

The effect of the target size on the detecton of luminance and chromaticity flicker and gratings was studied. Discrimination ellipses in a luminance-chromaticity plane were determined with square test areas subtending from 1/4 degree to 1 degree. Pure luminance modulation thresholds and pure chromaticity modualtion thresholds were obtained from square targets subtending from 1/16 degree to 2 degree. Square-wave stimuli were presented on a color television monitor; the mean color of the screen was yellow, the average retinal illuminance was 350 td. The main effect of enlarging the field size is that the threshold for any luminance-chromaticity combination decreases monotonically except when spatial frequencies are high. The summation area for detecting a fine bar pattern is at least 8 x 8 periods; the integration area for detecting flicker is more than 1 degree x 1 degree. This holds for any luminance-chromaticity mixture. For a fixed spatial or a fixed temporal frequency the change in sensitivity sometimes depends strongly on the ratio of luminance modulation to chromaticity modulation. The main conclusion of this study is that if a target is predominantly yellow the target size has similar influence on the sensitivity to both luminance and chromaticity contrast.