Accommodation performance for chromatic displays

The recent paper by Lovasik and Kergoat (1988) presents valuable data on the accommodation to various sizes of coloured letters on the screen of a visual display unit (VDU), a topic of considerable interest in relation to the increasing use of such displays. The letters were either ‘monochromatic’ (white, red, green or blue on a black background) or ‘multichromatic’ (blue-red, blue-green, redgreen, letter background combinations), at distances of 0.4 and 0.8 m (i.e. vergences of -2.5 and 1.25 D). The brightness of each multichromatic letter was subjectively matched to that of its surround to form a nominally isoluminant accommodation stimulus. The mean responses for a group of 20 subjects showed over-accommodation (accommodative lead) under all stimulus conditions, the response levels always being referred to the 633 nm wavelength of the laser optometer used to measure them. Although the ‘monochromatic’ colours were, in fact, of relatively broad spectral bandwidth, the responses to the monochromatic stimuli followed the form that might be expected from the longitudinal chromatic aberration of the eye, i.e. a higher level of accommodation was exercised for a red stimulus than for a blue stimulus, to compensate for the lower power of the eye at longer wavelengths. The accommodation for the white stimulus was at an intermediate level similar to that for the green stimulus (cf. Charman and Tucker, 1978). Strikingly, however, the mean responses to the multichromatic letters varied very little between the different colour combinations: in all cases there was substantial over-accommodation, the apparent lead exceeding even that observed with the red monochromatic letters. Lovasik and Kergoat (1988) state that the cause of this increased lead is currently under investigation. One obvious possible interpretation for the similarity between the multichromatic results is that such combinations may, in fact, represent poor accommodative stimuli. This would be predicted from the work of Wolfe and Owens ( 1981 ) who measured responses to red/green borders as a function of the luminance contrast between the two halves of the field. They found that the slope of the accornmodatiori response/stimulus curve was minimal when the colours matched in subjective brightness, suggesting that isoluminant stimuli elicit inaccurate accommodation responses: similar results were found with isoluminant redorange and blue-green borders. Any weak accommodative stimulus results in the accommodation response tending to drift to its resting state or tonic level. Thus if all multichromatic letters in Lovasik and Kergoat’s study were weak stimuli it would be expected that the final response levels would be closely similar and would approximate to the mean tonic level of the group of subjects. The observed tendency for over-accommodation would, then, merely reflect the fact that the tonic level was higher than that required for accurate focus on the targets. Unfortunately this simple explanation does not appear to be adequate. Although the mean responses to all the multichromatic stimuli at a fixed distance were closely similar, the level of response appears to be about 3.3 D when the target vergence is 2.5 D and only about 2.1 D when the target vergence is 1.25 D (see Lovasik and Kergoat, 1988, Figure 3). Clearly, the mean tonic level cannot be both 3.3 and 2.1 D and the vergence (distance) of the targets must be having some influence on the response. In fact, Lovasik and Kergoat state that the mean resting or tonic accommodation for their subjects was 1.9f0. 6 D, so that although a drift towards the resting state could account for the over-accommodation for the multichromatic targets at 80 cm distance (vergence 1.25 D), it would predict an under-accommodation for the 40 cm distance (-2.5 D) targets, rather than the observed over-accommodation. Data obtained at Manchester some years ago, which were reported verbally at an Applied Vision Association meeting but never published, suggest that rather than attempting to interpret meun data for the response to the two-colour targets, it might be helpful to look at the results for individual subjects and to attempt to understand them in terms of the focus-dependent changes in the retinal image. In our rather crude pilot study, four targets, each consisting of an array of four 6/24 Landolt C’s, were viewed at an effective optical distance of 35 cm (vergence -2.87 D). The targets were made from matt red, blue and black coloured paper, the red and blue being selected to be as saturated as possible and to have almost identical subjective brightness when illuminated with white light of effective colour temperature 6500K, their luminance then being about 7 cd m ’. Figure 1 ( m ) shows the diffuse spectral reflectance R , of the coloured paper samples and Figure I ( h ) the product E , V,R,, which effectively represents the spectral variation in reflected luminous flux when allowance is made for the radiation emittance, E,, of the light source and the assumed photopic luminous efficiency, V,, of the observer. Our ‘monochromatic’ stimuli, like those of Lovasik and Kergoat, were of broad spectral bandwidth. The