A Mesopic Vision Approach for a Better Design of Road Lighting

This paper approaches the design of road lighting from the point of view of mesopic vision. Up to date all lighting calculations are performed for photopic visual conditions. Although this is true for interior lighting, it is not always the case for road lighting. Usually, the luminance level on roads of low or medium traffic, fall below the lower limits of photopic vision. In that case, the vision becomes mesopic. Recent researches have proved that in mesopic visual conditions the sensitivity of the human eye moves to lower wavelengths. Therefore, some types of lamps which are widely used for road illumination (e.g. high-pressure sodium) are not as efficient as they use to be in the photopic vision. It seems that the efficiency of lighting installations could be improved by using lamps with light spectrum richer in shorter wavelengths (e.g. metal halide). This paper shows that the road lighting quality parameters may be improved if the high-pressure sodium lamps are replaced by metal halide lamps in identical lighting installations, given that the visual performance of the eye is considered under mesopic vision. The calculations shown in this paper prove that metal halide lamps are more efficient than is usually believed and, moreover, energy savings could be achieved. Key-words: mesopic vision, road lighting, high intensity discharge lamps.