Field measurements of direct and rearview-mirror glare from low-beam headlamps

This study measured direct and rearview-mirror glare illuminances produced by low-beam headlamps in a sample of 22 passenger vehicles. The glare illuminances were measured for 12 common glare situations that were defined by a full factorial combination of three scenarios (oncoming driver, center rearview mirror of a preceding driver, or driver-side mirror of a preceding driver one lane to the right), two longitudinal distances (25 m or 50 m), and two vertical locations (illuminated vehicle being either a car or a light truck/van/sport utility vehicle). The measurements were made outdoors at night on asphalt pavement. The median illuminances (not taking into account window transmittance or mirror reflectance) ranged from 0.5 lux for an oncoming driver of a light truck/van/sport utility vehicle at a distance of 50 m, to 3.4 lux at the driver-side mirror of a preceding car at 25 m one lane to the right. The ratios of the maxima and the minima measured for each of the 12 glare situations were large, ranging from about 5:1 to 36:1. The median actual illuminances were compared to the median expected illuminances based on a recent, laboratory-measured, representative sample of US low-beam patterns, taking into account the possible effects of dirt, voltage, misaim, and pavement reflectance. This analysis indicates that the actual illuminances could be very well modeled using the laboratory-measured beam patterns and assuming a linear relationship between the light output of clean and dirty headlamps. Additional analyses evaluated the relationships between headlamp mounting height and glare illuminance.