There have been several research projects during recent years on the effects of the spectral distribution of the light from roadway lighting on visibility. This paper illustrates how the spectrum of lamps affects vision at low light levels, and a later paper will examine the effects of higher light levels. Recent research has shown that the spectral distribution, or colour, of light sources has a significant effect of how well we see at night. The previous general assumption of lamp manufacturers and lighting practitioners, that all lumens (units of light intensity) create equal visibility, has been found false. A true assessment of the lumen output of a lamp should be based on the eye's response under the conditions under which the lamp is to be used, as the eye responds differently to different wavelengths of light under differing conditions. There are different types of eye response when light has scotopic, mesopic, and photopic levels, of which starlight, roadway lighting, and interior lighting, respectively, are typical examples, but the transitions between these types of vision are complicated, because the eye has two types of vision cell, rods and cones, with different properties. Sodium lamps, metal halides, street lighting levels, and practical vision experiments are discussed in relation to these factors.
Abstract