Visual performance is defined as the speed and accuracy of processing visual information. Existing models of visual performance evaluate illuminated tasks in terms of luminance contrast, retinal illuminance and visual size but do not consider the chromatic properties of the task. In consequence, there is no recommendation for the chromatic contrast needed to reach a high level of visual performance. This paper shows that chromatic information is crucial to visual performance when achromatic information is weak or missing altogether. When the luminance contrast is less than approximately 0.20, some task colours with excitation purities greater than 40% can be used to achieve a level of visual performance close to 90%. For luminance contrasts higher than 0.60, visual performance is determined only by luminance information, and tends to the value for an achromatic stimulus. For luminance contrasts in the range 0.20 to 0.60, both luminance and chromatic information are important for visual performance.
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