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Glare from the headlights of following vehicles which is reflected in rearview mirrors can be a significant problem. Glare can cause driver discomfort, it can diminish the driver's ability to see, and it can cause driver fatigue, particularly during prolonged night-time driving common for many professional drivers. Conventional rearview mirrors, such as silvered and chromed mirrors, offer only one reflectivity level. Variable reflectance mirrors, such as those utilizing electrochromic and liquid crystal technology, are capable of multiple reflectance levels. With variable reflectance, the driver can select a high reflectivity level during daytime driving or when reversing into loading docks, and can select a reduced dimmer reflectance level when driving during glaring conditions. This paper briefly reviews the principal technologies available for variable reflectance mirrors and outlines their performance as glare-reducing mirrors.

This paper presents a review of the work which has been done on determining appropriate automotive mirror reflectivity levels for good vision as well as glare protection. In addition, its presents the results of two recent studies on the effects of glare on driver vision. In the first, the level is determined at which glare from an interior mirror degrades the driver's forward vision. Four different ambient conditions are included. The second study looks at the effect of mirror transition time on discomfort and disability due to glare. Several current mirror systems are compared to the recommended parameters.

Glare reflected from rearview mirrors degrades the ability of the driver to detect objects in the forward field. Mirrors are now available which reduce this glare. This study looked at the effect of those mirrors on the driver's ability to detect a pedestrian-size target along the edge of the road; specifically, the distance at which drivers were able to detect the target. Detection distance was measured for two glare levels and two reflectivity levels: the standard reflectivity level and the reflectivity level designed for glare reduction. The results showed a loss in detection distance when glare was present. Higher glare levels resulted in larger losses. Use of the lower reflectivity level was able to reduce glare to the point where the loss in detection distance was almost eliminated.

When the brightness of a test stimulus which is colored is matched in luminance to a fixed white reference, it is usually found that the colored stimulus will appear brighter than the white stimulus. Reflectivity is determined by measuring the luminance of reflected light. Therefore, although a white object reflected from both a colored mirror and a neutral mirror with the same reflectivity will have the same luminance, the object reflected from the colored mirror will look brighter because color has been imparted to it by the mirror. This study examined the effect of this added brightness on driver performance. Three colors were studied; blue, green and amber, each at three purity levels; 0.0, 0.3, and 0.6. Brightness was measured as the luminance of a neutral stimulus which appeared equal in brightness to the colored stimulus. Performance was measured as the ability to detect the presence of a vehicle in a roadway scene typical of those viewed in a rearview mirror.