How Do Auto Dimming Mirrors Work?

by Shelley Moore

Glare Elimination

Auto dimming rear-view mirrors, both the interior mirror at the top of the windshield and the exterior side mirrors, increase driving safety by eliminating glare that can impair vision. Headlight glare in the mirrors from trailing vehicles can make it very difficult to see the road in front. This is particularly dangerous on dark rural highways with cross-traffic, and areas where deer and other animals may dart across the road.


In auto dimming mirror technology, a forward-looking sensor detects low ambient light from headlights behind the car and directs the rear-view sensor to look for glare. The mirrors darken automatically in proportion to how bright the glare is, then clear once the glare is no longer detected. The technology is called electrochromics, which applies electricity in order to tint glass through a low-voltage power supply. The electricity moves through an electrochromic gel placed between two pieces of glass during manufacturing, which have been treated with an electrically conductive coating. Electrochromic technology is also increasingly included in building design with the use of so-called "smart windows," as an energy-efficient response to light and heat.


Several types of auto dimming mirrors are available, including easily installed models that clip right over the interior rear-view mirror. Some include a compass and temperature display, while others also include audio driver interface, garage door openers and headlight control sensors. Engineers have found that the sensors, micro-controllers and circuitry used in automatic dimming can be effectively combined with additional options while saving cost and space. Additionally, research shows that drivers look at the mirrors more often than they do the instrument panels, according to auto dimming mirror manufacturer Gentex Corporation. Gentex pioneered the development of these mirrors at the commercial level, and as of 2009. has been the leading supplier of auto dimming mirrors to the automotive industry for over 20 years.

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