Three visually demanding tasks: proof reading, search and signal detection, were devised and produced in a range of contrast from 0.09 to 0.87. Each subject performed all three tasks at one contrast for a set time at an illuminance of 200 lux. In order to investigate the visual fatigue effects, ocular responses before and after performance of the tasks were assessed and measurements were taken of muscle balance for esophoria and exophoria, and convergence and divergence break points; changes in accommodation time; and eye movements. A questionnaire was completed by all the subjects at the end of their experimental session, to determine their impressions of the work they had done. All the tasks showed a trend of increasing performance with higher contrast, but for one task the performance showed a significant decrease at the highest contrast and the other tasks indicated a possible decrease there also. The measurements of binocular co-ordination showed a tendency to esophoria after performance of the tasks, but no significant relationship with contrast was found. The accommodation time results were generally unreliable. The eye movement studies showed no relationship with contrast. Some of the questionnaire responses showed trends with contrast similar to the performance results. It is concluded that (a) the visual system experiences difficulty with tasks at very low contrasts, as would be expected and also (b) that very high contrast may inhibit task performance. The latter would not be expected from previous studies and warrants further research, which is at present being undertaken.
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