WORKING WITH VIDEODISPLAY TERMINALS AND T HE RESTING STATE OF THE VERGENCE SYSTEM

Subjects with different characteristics of the vergence system resting state (close or distant resting vergence, large and small increase up on depression of the eyes) worked with a VDU in a relatively high and a relatively low position for 1 1/2 hours. The directions of gaze were about -5-degrees and -35-degrees below the horizontal. A shorter obse rvation distance was chosen with the lower VDU position than with the higher one, and it increased during the course of the work. In the hig h VDU position the increase was particularly strong for subjects with a distant resting vergence. The discrepancy between the angle of conve rgence during visual work and the resting vergence was larger in subje cts with a distant resting vergence than in subjects with close one, a nd it was larger with the higher than with the lower VDU position (exc ept in subjects with a minimal effect of eye depression on resting ver gence). However, corresponding differences in visual fatigue as assess ed by means of a questionnaire could not be found. Nevertheless the ma jority of subjects preferred the low VDU position. A number of additio nal consequences of a low VDU position support the conclusion that it would be advantageous for many users to tilt the display backward and to sink it below the level of the table surface.