Abstract: 20 subjects were tested with Landolt ring tasks under condition similar to equi-valent sphere illumination. The sizes of the rings were 0.5′-10.0′in eight steps of visual augle, illumination range was 1.7-2160 lux. It was found that visual angle forcorrect discrimination is a function of illumination and that with further increase inillumination there is a slowing down in improvement in visual acuity which followsthe "law of diminishing returns". This law may be applied to classification of illu-mination levels in specifying standards of illumination. The present study also compared visual effects of positive and negative contrasts.It proved that with the same illumination and same high contrast tasks the visual ef-fect of dark (black) background is better than that of light (white) background. Thisimplies that for lower reflectance areas a higher illumination should be prescribed.

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