AN INVESTIGATION OF HEADLAMP GLARE: INTENSITY, SPECTRUM AND SIZE

Headlamp glare is an issue that has grown in terms of public awareness over the past decade. Developments in light source technologies and optical design have resulted in headlamp systems with higher efficiency (and thus the ability to produce higher illuminances), as well as differing spectral power distributions and smaller sizes than conventional halogen headlamps. This report describes research to investigate and quantify the impact of glare illuminance, glare spectral power distribution, and glare source size on peripheral detection of small targets in the field. Peripheral visibility is an area that heretofore has not been extensively studied in the context of headlamp glare, although peripheral vision is important for driving. The impact of glare can be segregated into two areas: the reduction of visibility caused by scattered light in the eye (disability glare), and the sensations of discomfort caused by a glare source in the field of view (discomfort glare). These phenomena often, but do not necessarily always, occur simultaneously. With respect to disability glare, detection of peripheral targets worsened as the glare illuminance increased from 0.2 to 5 lx, as expected. Detection of high-reflectance targets (located 60 m ahead) was relatively unaffected by glare, however, except for targets very close to the glare source and targets furtherest from the line of sight. Neither the spectral power distribution (halogen, high intensity discharge or blue-filtered halogen) nor glare source size (from 9 to 77 sq cm in area) affected peripheral detection, once the glare illuminance was held constant. With respect to discomfort glare, higher glare illuminances elicited subjective ratings of greater discomfort and was the most important determinant of discomfort. Spectral power distribution also affected discomfort (even though it did not affect visual performance) with the high intensity discharge headlamps eliciting ratings of greater discomfort than the halogen and blue-filtered halogen headlamps, when the glare illuminance was held constant. Glare source size had no impact on ratings of discomfort. For the range of conditions used in the present study, conventional far-field photometry based on the photopic luminous efficiency function is appropriate in characterizing a glare source in terms of visual performance, but the photopic luminous efficiency function does not accurately characterize discomfort glare.