Abstract
1. Differences of threshold contrast are predicted from optical theory for a grating acuity target in monochromatic and white light. The greatest differences, up to 65%, are predicted for gratings of lower contrast and pitch than those normally used in measurements of visual acuity.2. Using three subjects, we measured contrast thresholds with 1.5 and 2.5 mm diameter artificial pupils for natural and paralysed accommodation, using a tungsten lamp and wave-lengths of 546 and 578 mm.3. Excellent agreement is obtained between predicted and measured differences.4. Results confirm that observed acuity and sensitivity differences between white and monochromatic lights are largely optical in origin, but involve at least two independent colour mechanisms as spectral weighting functions. Stiles's pi(4) and pi(5) sensitivities afford a much better fit to observed differences than the C.I.E. visibility curve.
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