How precise do precision tints have to be and how many are necessary?

Among those individuals who habitually wear precision tints, reading speed can vary as a function of the chromaticity of the illuminating light. The reading speed decreases as the chromaticity departs from optimum, whether in saturation or in hue. When the distance in the CIE 1976 UCS diagram between the chromaticity of the illuminating light and that optimal for reading exceeds about 0.08, the colour has little residual benefit. We attempt to answer two questions numerically: (1) how is reading speed affected by the variation in the colour provided by the tint under different lighting conditions? (2) how many different tints does an ophthalmic tinting system need to be able to provide? Analysis of 1000 recent prescriptions suggests that, for most, the variation in colour with illumination is sufficient to reduce, but not eliminate, the beneficial effects of the tints. The number of trial tints required by an ophthalmic tinting system is a power function of the degree of efficacy desired, and for an average efficacy >95% of optimum, the number of trial tints needs to exceed 1000. In practice this requirement can readily be achieved by combining trial lenses and by appropriate dyeing techniques.