Luminance versus Luminous Intensity as a Metric for Discomfort Glare

Photometric performance specifications for vehicle headlamp specifications in North America are given in terms of luminous intensity values at various angular locations with the objective of providing sufficient illumination for forward visibility while controlling for glare toward oncoming and preceding vehicle drivers. Abundant evidence suggests that luminous intensity is an appropriate metric for characterizing the degree to which a headlamp can produce disability glare through veiling luminances under a wide range of viewing conditions. Notwithstanding that discomfort glare exhibits a differential spectral sensitivity from the photopic luminous efficiency function used to characterize light, luminous intensity does not always predict discomfort glare. For example, the luminance of the luminous element(s) can be more predictive of discomfort when headlamps are viewed from relative close distances. Since technologies such as light-emitting diodes might utilize arrays of small, high-luminance sources to generate a headlamp distribution, such headlamps might produce different levels of discomfort glare than conventional headlamp optics. The present paper describes an experiment to study this effect and to explore concepts for mitigating discomfort glare when high-luminance source arrays are used.