[Effect of luminance on contrast sensitivity and glare in the mesopic range]

Elderly persons applying for driving licence frequently are not able to fulfil the recommendations given by DOG and relating to twilight vision without and with glare. One reason may be the low level of luminance used for this test.

We compared contrast threshold with and without glare in the mesopic range of vision measured by two types of Rodenstock Nyktometers. One instrument was the standard, commercially available instrument (500) and in the other instrument (500 S) the luminance of the viewing field was raised by a factor of three and the luminance of the glare source by a factor of 2.2. We tested 50 persons divided in three groups aged 20 to 49, 50 to 59 and over 60 years.

With the standard instrument, some of the younger persons and nearly all of the elder ones were not able to fulfil the requirements for driver licensing. With the modified instrument (500 S), the group of the elder was divided in a greater part now able to fulfil the recommendations and a smaller one unable even at this luminance level. The standard deviation of the measured contrast sensitivity was one step of the contrast scale of the instruments. We also tested the influence of adaptation time on the contrast sensitivity. Only very few persons showed slightly better results after 15 minutes of adaptation compared to 5 minutes.

The luminance of today's headlights of motor cars is significantly higher than those at the time when the DOG rules where acquired. Testing contrast sensitivity without and with glare on a higher luminance level will reduce the number of those persons not able to fulfil the requirements for drivers licensing. The revision of these rules is discussed.