Visual information is reduced in fog conditions, and in particular the contrast of objects when driving at night-time with car headlights. This impairment of vision may induce delay in the detection of obstacles and have negative impact on traffic safety. In order to investigate the visibility thresholds of targets in these conditions, experiments were conducted in a fog chamber under using car headlights in comparison to a theoretical approach by using a model of Vision developed by Adrian.

Some targets have been place at distances from 5 to 30 m from car with low beam, and an observer in driving position determined visibility thresholds. The luminance of the target and background were monitored by two photometers when changing fog density recorded by a transmissometer.

Visibility criteria as contrast or visibility level are determined for various angular size of the target related to the distance of observations. These experimental data are compared to the predicted values of luminance threshold determined by using the Adrian model, based upon Ricco’s and Weber’s laws. A comparison is made also with the experimental data from Aulhorn. Results are analysed considering the specific situation of low luminance range, improvements of the experimental procedure are proposed.