An auto-dimming rearview mirror works on the principle of electrochromism, where an object changes color according to different levels of the electrical stimulus. An auto-dimming rearview mirror has five layers of material sandwiched between a piece of glass and a mirror. The glass is just clear with an electrolytic film between the outer two conductive layers followed by the ion storage layer, an electrolyte and the electoral chromatic layer. With no voltage applied, this would be transparent, and light would pass through the glass and bounce back from the mirror and into the driver's face.
By applying a very low voltage to the conductors, ions from the ion storage layer move over into the electrochromic layer, causing it to darken. This means that a very limited amount of light is allowed to bounce back through the mirror and into the driver's face, reducing glare. When low voltage is applied to the electrolytic film between the two pieces of glass in the control circuit, sensors sense that the vehicle is experiencing glare, sending a low-voltage signal through wires to the electrolytic layer in between the glass, causing the mirror to dim until the sensors indicate that the glare has subsided, and it will clear.
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