Abstract
The effect of various degrees of obscuration by dirt on the performance of headlamps removed from vehicles in use has been investigated. Reductions in intensity of over 90 per cent have been measured. The loss of light is accompanied by a change in the beam distribution which causes the amount of light reaching the important parts of the road to be reduced more than the unwanted stray light; under some conditions the light emitted above the horizontal is actually increased by the presence of dirt.