Auto Dimming Mirror and Electrochromic Glass

An auto dimming rearview mirror solves the problem of headlight glare from vehicles behind you at night. Instead of reaching up and flipping the mirror to its dim position manually, the mirror does it automatically and progressively. It does not just flip between two positions — it smoothly darkens to match the intensity of the light hitting it. This technology is called electrochromic, and once you understand what that word means, the whole system makes sense.

Break the word apart. Electro means electricity. Chromic means color. Electrochromic means using electricity to change color. The mirror glass is not a single piece — it is two pieces of glass with a thin layer of electrochromic gel sandwiched between them. When no voltage is applied, the gel is transparent and the mirror reflects normally — full brightness. When voltage is applied, the gel darkens. More voltage means more darkening. The mirror dims proportionally to how much voltage the control circuit sends through the gel. Remove the voltage and the gel slowly returns to its clear transparent state.

Two light sensors do the work. A forward-facing ambient light sensor on the front of the mirror housing measures how bright it is ahead of the vehicle. A rear-facing glare sensor on the back of the mirror measures how bright the light is coming from behind the vehicle. The mirror's control circuit compares the two readings. If the light behind is significantly brighter than the light ahead — meaning headlights are shining into the mirror in dark conditions — the circuit applies voltage to darken the gel. During daytime driving, the ambient sensor sees so much light ahead that the mirror stays clear regardless of what is behind. This prevents the mirror from dimming during the day when it is not needed.

The most common failure is the mirror stops dimming entirely and stays in full-bright mode. This is usually a failed control circuit board inside the mirror housing. Less commonly, the electrochromic gel can develop a blue or brown discoloration that becomes permanent — the gel has degraded and the mirror must be replaced as a unit. The light sensors can also fail, causing the mirror to dim when it should not or fail to dim when it should. Auto dimming mirrors are not repairable — the entire mirror assembly is replaced.

Some vehicles use the same electrochromic technology in the sunroof glass or rear window. A large panel of electrochromic glass can switch from transparent to dark at the touch of a button, replacing a mechanical sunshade. The principle is identical to the mirror — voltage applied to the gel layer between two glass panels changes the tint. These panels are significantly more expensive to replace than a standard mirror because of the size of the glass and the complexity of the wiring and control module.