Abstract: After prolonged observation of a visual task visual performance isaffected by sudden changes in luminance level of the visual field. Contrastsensitivity decreases with increase in the ratio of the two luminance levelsand is independent of their absolute values. The experiment used 5 ratiosof luminance changes of the visual field: 1:1, 1:10, 1:20, 1:50 and1:100.Contrast sensitivity is greatest when the ratio is 1:1, and falling to 50%of the original when the ratio reaches 1:20. If the former is arbitrarily setto be 100% seeing probability, then the latter would be 50% seeing proba-bility which is the threshold for visual discrimination. It is recommendedthat in designing actual lighting environments changes in luminance ratio ofthe visual field should not exceed 1:20.

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