Something behind Rearview Mirror

I wonder how an auto-dimming mirror senses when it needs to dim?

 

It measures the ambient light to determine how dark the mirror needs to be when it senses headlights from behind. If you cover the sensor with your finger, you'll notice the mirror will darken. Some people cover the sensor with a film to force the mirror to darken more.

 

Was the sticker over the sensor?


As mentioned it tells the mirror the amount of light outside -- the mirror does not darken during daylight hours. The inside sensor would always be sensing light and darken the mirror if it did not have this.

 

Many Prius owners (including myself) don't think the mirror dims enough in urban environments (sensor gets too much ambient light). I believe it was John1701 who suggested adding a colored transparent film over the sensor to trick the mirror into dimming more easily. Perhaps this was the "sticker" you removed.

 

Oh. Sounds like incompetent dealer addition to me.

 

Its a small photodiode that detects light with wavelengths close to human eye response and sends a current signal proportional to the light intensity to the dimming control circuitry... covering it with a sticker would render it "off" all the time

MB860 ?

 

There are two sensors: one facing forward and one facing back. The back facing sensor detects headlamps from behind, prompting the mirror to dim. The forward facing sensor senses ambient light, which tells the mirror how much to dim when it does dim.

The sensor in question in this thread is the forward facing sensor, so covering it will not stop the mirror from dimming properly. Many Prius owners find that the auto-dimming mirror does not dim enough in the city, where there is a lot of ambient lighting. Covering the forward sensor will block some or all of the ambient lighting, causing the mirror to dim to a darker setting when activated by the rear facing sensor.

A white sticker will not completely block the transmission of light, unless it is lined with something opaque. Most likely the one described by the OP was being used to reduce sensitivity to ambient light, but not completely defeat ambient light adjustment.

If the OP was happy with the performance of his auto-dimming mirror, then he should have left the sticker in place. He will now find that the mirror dims less in city traffic. Fortunately it is easy to adjust. All you need is a roll of tape or a couple of stickers.

Tom