The Luminous Intensity Requirements of Vehicle Front Lights for Use in Towns

Previous studies suggest that the use of the dipped headlights on urban traffio routes lit by street lighting is undesirable because of glare, yet it is often contended that present marker lights are not adequately conspicuous. An investigation was therefore carried out, using appraisal techniques, to determine the intensity of vehicle front lights which will be adequately conspicuous without being too bright. It was found that the intensity requirements were largely independent of observer attributes, the luminance of the road surface and its surrounds and the number and movement of the vehicles. However, the results suggest that conspicuity and brightness are different attributes of a light. While both increased with increasing luminous intensity, the observers found that, for a given intensity, a larger source was the more conspicuous but that a smaller caused t he more discomfort. Optimum lighting appears to be a town beam, based on dimming the present dipped headlight, giving a straight ahead intensity of 80 ed. If the light is based on the small diameter marker light this value needs to be doubled to give adequate conspicuity, but then this light will be regarded as too bright in about one observation in twenty.