Visual strain during VDU work: the effect of viewing distance and dark focus

Abstract

Some standards for work at visual display units (VDU) recommend viewing distances of about 50 cm. The present study investigated whether or not visual strain is lower at viewing distances greater than 50 cm. Specifically, the question of whether the optimal viewing distance is determined by the individual's accommodation in a condition without fixational stimulus, the so-called dark focus, was studied. Two groups of subjects with mean dark focus of about 1 dioptre (D) and 2 D, respectively, performed a search and comparison task for 2 hours at a VDU at viewing distances of 50 cm and 100 cm. The characters on the VDU subtended the same visual angle in both conditions. Visual strain was evaluated by recording the subjects' performance during VDU work, their judgements of the comfort of the viewing distances, and their subjective estimates of visual strain; further, several oculomotor functions were measured throughout the sessions. In both groups, visual strain was higher in the 50 cm condition, and the longer distance of 100 cm was preferred. Thus, irrespective of the individual's dark focus, visual strain can be reduced by using viewing distances greater than those recommended in some standards.

When the VDU was near, visual strain in near-dark-focus subjects (2D) was lower than in far-dark-focus subjects (1 D) thus, the strain of the accommodative system alone seems to be minimal when the viewing distance is made to agree with the dark focus. However, since both groups had a far dark convergence and both showed less visual strain at 100 cm, the stress on the convergence system may be the crucial factor for visual strain.

Keywords:

• Visual work
• Accommodation
• Convergence