Abstract
P 062
Testing contrast vision under standard conditions – but how?
Wilhelm Durst, T. Peters, B. Wilhelm
STZ Autonomes Nervensystem und Sicherheitsstudien, Tübingen
Objective
Visual acuity and contrast testing using optotype charts are dependent upon the brightness and contrast conditions of testing. To test acuity, EN ISO 8596 recommends an ambient illumination (= luminance between optotypes) of between 80-320 cd/m² and high contrast acuity symbols. This background illumination should ideally be present throughout the whole of the chart with a permissible tolerance of ± 25%. With illuminated charts, using e. g. printed cardboard optotypes, it is not possible to achieve this specified illumination with either ceiling lights or halogen spotlights. The aim of the illumination equipment to be presented is to create standardised testing conditions as stipulated, with an illumination over the charts as homogeneous as possible.
Methods
Point sources and ceiling lighting must be avoided, to prevent an illumination gradient over the chart. We therefore use 2 fluorescent tubes with day light spectra, which are the same length as the chart, and placed on its left and right sides. The distance to the chart is about 40 cm. Each tube is fitted with an opaque filter to provide a diffuse incident light and is not visible to the subject. The luminance level is controlled by a high frequency ballast. This provides a flicker free luminance range of 7-700 cd/m².
Results
To obtain information about the homogeneity of the illuminance over the chart we measured the luminance between the optoypes at 21 different points. The variability was about ± 8% around a mean of 90 cd/m² (FDA recommendation 85 cd/m²) which is far below the allowed tolerance range.
Conclusions
An artificial lighting system is necessary to obtain a standardised and reproducible illumination of contrast charts. It is to be expected that studies which measure the contrast threshold and which depend on the comparability of results, should only be performed with homogenous chart illumination.
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