Mirror sensor?

[Anna999]

On the back of the interior rear view mirror is a small rectangular “sensor?” which faces forwards. What is it for? It is not the circular dimming mirror sensor which is on the front.

 

[st13phil]

It's the ambient light sensor that works in conjunction with the one on the glass side of the mirror.

The dimming function needs to know if it's dark outside.

 

[Anna999]

Why? If the sun is directly behind you it can shine very brightly in your eyes via the mirror during the day. Therefore you need the mirror to dim automatically. Previously I would dim it manually.

 

[Trickythemerc]

I think they sense the darkness rather than bright light.

 

[st13phil]

Why?

The dimming function works proportional to the difference in light levels on the front and back of the mirror. In your example of direct sunlight behind the car it will still dim.

 

[whitenemesis]

Sun directly behind would be the same as a car behind with it's headlights on.
If you cover the forward facing sensor the mirror will dim. The sensors work on difference in light level