What Is an Electrochromatic Rear-View Mirror?

by Lexa W. Lee

Electrochromism refers to a color change in a substance that results when an electric current is applied. The change is reversible. An electrochromic rear-view mirror is one that automatically dims when it senses light from behind the car, thereby reducing glare that can create vision problems for those driving at night. The mirror is electronically operated; it uses light sensors and a special gel that changes color when an electric current causes it to either gain or lose electrons.

Sensing Light

An electrochromic rear-view mirror typically consists of an electrochromic gel placed between two pieces of glass, both of which have a transparent coating that conducts electricity. In addition, one piece has a reflector. According to Gentex Corporation, the mirror system includes: a sensor that faces towards the front of the car, sensitive to low light levels, which signals the mirror to detect signs of glare; and a second sensor that faces towards the rear of the car and picks up glare from vehicles behind the car.


Glare from behind causes the second sensor to send current to the electrochromic gel in the mirror, producing a voltage that is proportional to the amount of glare detected. The current causes the mirror to quickly dim. When the sensor no longer detects glare, the mirror reverts to its original state. An electrochromic mirror system can dim external, as well as internal mirrors.

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