The effect of optical defocus on the accommodative accuracy for chromatic displays

Blur, probably arising from focusing inaccuracies, has been cited in ergonomic and vision literature as one of the causes of visual stress and decreased task efficiency in operation of video display units (VDUs). With the advent of colour coding in electro-optical displays, the need for a detailed quantification of focusing responses to chromatic stimuli is particularly important because of the influence of the chromatic aberration present in ocular optics on the focusing response of the eye. In this regard, we used a computer-aided laser speckle optometer system to measure the accommodative responses of 20 visually normal subjects, to brightness-matched monochromatic and multichromatic stimuli displayed on a high-resolution RGB monitor. We also investigated the effect of target size, target clarity, and viewing distance on the response accuracy of the focusing system of the eye. Our results show that while the accommodative responses are systematically influenced by target chromaticity, they are relatively independent of target size. There was no evidence of any anomalous focusing responses resulting from either target chromaticity or defocus that could account for the asthenopia frequently reported by VDU users. Furthermore, moderate levels of optical defocus did not drive accommodation into any visible hunting pattern for optimal focus or towards its tonic resting position. Implications of these findings in display designs are presented.