Glare on visual display terminals

Status:

completed 03/2004

Aims:

Incorrectly installed lighting may cause glare at visual display terminal workplaces. Glare may reduce or even prevent the recognition of displayed information. In addition, the increased physical and psychological stress may give rise to health hazards. In order for these hazards to be avoided, the conditions under which glare occurs and the means by which it can be prevented must be ascertained. The objective of the project was to review the suitability of existing recommendations for the avoidance of glare on visual display terminals, and where necessary to draft new recommendations for specific technical solutions and for organizational measures for the reduction of glare. The provisions of DIN ISO 9241-7, "Ergonomic requirements for office work with visual display terminals (VDTs) - Part 7: Requirements for display with reflections" were to be assessed for whether they still conform to the state of the art, or should be amended. Proposals were to be made if necessary for reformulation of the provisions of the standard and for reduction in glare at work.

Activities/Methods:

A literature survey was performed in order to identify the essential lighting parameters for the incidence of glare. Tests were also performed involving test subjects. A model VDT workplace was set up in the laboratory for this purpose. Glare at varying intensity was then generated on the screens by means of a variable glare source. In series of tests, in which all 60 test subjects were involved, the unwanted luminance on the screens and the resulting contrast between fields and characters were measured on six different monitor types, at five or six different glare intensities, and in two different display modes (normal and reverse video). The test subjects were then questioned regarding their subjective impression of the level of interference. The measured unwanted luminance and the subjective experience of glare were correlated graphically and mathematically. The correlations were used to derive recommendations for revision of the provisions in DIN ISO 9241-7 concerning glare, and also to identify general relationships between field/character contrast and subjective perception of glare and recognizability which are independent of the techniques employed.

Results:

The results showed that the requirements concerning the reflection classes set out in DIN ISO 9241-7 no longer conform to the state of the art. Monitors in the poorest reflection class (III) are no longer being distributed. CRT monitors fall within either Class I or Class II. All flat screens fall within the highest reflection class (I) in both normal and reverse video display mode. The standard does not permit further distinction, even though the studies have shown that the critical luminance, which is the essential criterion for the suppression of reflections, may differ by up to a factor of three. In consequence, either new reflection classes should be included in the standard, or the maximum luminance of monitors introduced as an further classification criterion. The general relationships identified between field/character contrasts and the resulting subjective glare enable glare situations on monitors to be evaluated more easily in future. Measurements of contrast should be sufficient. Intensive surveys of test subjects would appear to be no longer necessary.

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