Influence of Background Spectral Distribution on Perceptions of Discomfort Glare

The advent of light-emitting diode (LED) technology for automotive lighting allows flexibility of the spectral distribution of forward headlighting systems, while meeting current requirements for "white" illumination. As vehicle headlights have become whiter (with more short-wavelength light output) over the past several decades, their potential impacts on visual discomfort for oncoming and preceding drivers have been hotly debated. It is known that a greater proportion of short-wavelength energy increases discomfort glare, and that increasing the background light level (e.g., through roadway lighting) will decrease perceptions of discomfort. More recently it has been demonstrated that the visual system exhibits enhanced short-wavelength sensitivity for perceptions of scene brightness. As a result, roads illuminated by light sources with higher correlated color temperatures (CCTs) will be judged as appearing to be brighter than those illuminated to the same light level by sources with lower CCTs. The present laboratory study was conducted to identify whether the increased scene brightness of a road illuminated with greater short-wavelength light helps to mitigate discomfort glare more than the same scene illuminated to the same light level, but with less short-wavelength light. The results indicate that the spectral distribution of the background plays little role in the degree to which it lessens discomfort glare. The implications of these results for vehicle and road lighting practices are discussed.