[Assessment of contrast vision by the Mars letter contrast sensitivity test - which illumination should be used?]

For expert opinions on mesopic and contrast vision as stipulated in the recent version of the German driving licence regulations (2011), a standardised implementation of the relevant test method is crucial. DIN 58220, part 7, regulates the assessment of mesopic vision and the required parameters of evaluation are guaranteed by the respective test instruments. Because no standard exists for photopic contrast vision, it is recommended that it conform to that of DIN EN ISO 8596 for visual acuity. For test equipment with integrated viewing fields, the application of such tests may not be difficult. But if visual charts are used the question of standardised illumination arises. The present paper investigates this question when using the Mars Letter Contrast Sensitivity Test in the light of the recommendations by the producer to use a desk lamp for illumination.

The Mars Chart was illuminated by three different commercially available desk lights from different directions at a distance of 50 cm, which is the test distance recommended by the producer. The luminance distribution was measured on the chart at twelve equally distributed test points.

For all conditions, the asymmetric illumination produced by a single light source leads to an extremely inhomogeneous luminance distribution over the chart. Adherence to the tolerance ranges for brightness, as well as the homogeneity of illumination from such light sources is left to chance and a subjective judgment of both parameters is not adequate.

Illumination of the Mars Chart with a desk light only does not necessarily satisfy the requirement of reproducible test conditions that is mandatory for expert evaluations. For comparable test outcomes for contrast vision, illumination devices that produce a uniform illumination should be employed.