Abstract: In the study, the probability of correctly identifying the directionof the gap of landolt was measured. The results were compared withthose of similar experiments. The main conclusions were as follows: (1) The increase of any one of the three variables: the size of thevisual task detail, task contrast and task background illuminance wouldresult in an increase of the level of visual performance, but theincreasing effect on the level of visual performance was reduced as thevalue of the variables increased. (2) When the level of visual performance wes kept constant, therewere compensatory relationships between the size of the visual taskdetail, task contrast and task background illuminance, but thiscompensatory effect was reduced as the compensatory factor wasincreased. (3) The relative contrast sensitivity function of luminance forChinese subjects was very much similar to that recommended by theCIE, but there were some discrepancies in quantity occurring mainlyat the low level of illuminance.

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