The Way Auto-Dimming Rearview Mirrors Work Is Actually Fascinating

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An auto-dimming mirror may not sound like the most exciting car part, but call it by another name—electrochromic mirror—and you begin to suspect that there’s actually a bit of nerdiness behind that flat reflective surface. And indeed, there is.

Driving at night with a bright pair of headlights in your rearview can not only be blinding as it happens, but also after the trailing vehicle turns off, since your vision is often left with a blind spot. Gentex, a major manufacturer of auto-dimming mirrors, calls this the “Troxler Effect,” and describes its dangers, saying:

This phenomenon, known as the Troxler Effect, postpones driver reaction time by up to 1.4 seconds. At 60 mph, a car will travel over 123 feet in this amount of time.


An auto-dimming mirror fights glare that can cause the Troxler Effect by actually darkening the mirror—via a process called electrochromism—when an ambient light sensor detects glare from the tailing car’s headlights.

Auto-dimming mirrors usually have two sensors, which tend to be either cameras or photodiode-based photodetectors (a photodiode is a semiconductor that turns light into current). These sensors work with a microprocessor to detect glare from headlights, and to then send the appropriate charge through an electrochromic gel.


That charge causes materials within the gel to undergo oxidation-reduction reactions, which prompts them to darken from their initial transparent states in a process called electrochromism. This darker gel, which resides between the two layers of glass that comprise the mirror, restricts how much light from the car behind can reflect into the driver’s face, and voila, the driver can drive at night without crashing into parked cars.

It’s a fascinating technology that has become standard in a lot of vehicles since it started gaining traction in the 1980s. So the next time you’re in a rental Hyundai Elantra, and you check your rearview mirror, just try to imagine those sweet, sweet electrons jumping around in that electrochromic layer.