Auto-Headlights: How do they work?

[Truckosaurus]

I have automatic headlights upon my E46 3-series. They normally work as you'd expect with the only 'overriding' normally needed when it is raining but quite bright.

However, this morning (approx 8am) it was blazing sunshine yet my car and several other cars had their lights on.

So, what does the sensor looks for, is it a certain amount of IR or what?

[anonymous-user]

Not sure how they work exactly but they use a photoelectric sensor to get the info they need to switch on and off.

[paulmoonraker]

It must also be related to the position of the sensors. Mine come on late afternoon when trucks pull up next to me. I think I am probably quite sad to have noticed this.

[JamieMatthews]

I would imagine they'd use the same sort of system as the lights you can get for your house that illuminance in Lux.

So, say that ambient light is below 100lux for 10 seconds, your lights would come on.

100 lux being a very dark overcast day.

[HellDiver]

The sensor is on the screen, behind the rear view mirror. If you're driving with the low sun behind you, the sensor will be in shadow, and lights come on.

My old Astra had auto lights and wipers, and it put the headlights on if you went above 85mph, or if the wipers were on at anything above normal speed - I think they had three wiping speeds above the normal intermittant ones.

[runboy]

I could be wrong, but using a bit of mad theory and practical experience of them in my Octavia, bright sunshine seems to make any shadows from trees etc that bit more defined, whereas if it was just a bit dull then you are not going from bright light to dark shade, which in my experience turned my auto lights on. It is the sudden drop of light that turns them on, the system thinking you've just gone into a tunnel.

I found my auto lights quicker to turn on than turn off too.