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My RAM truck has a feature that intrigues me and I want to know how it works. In auto-dimmer mode, the truck will automatically put on low beams headlights when there is on-coming traffic at night, as well as when it is behind another vehicle moving in the same direction at night. However, it is smart enough to not go to low beams when my truck's headlights are reflecting back light off of signs. My guess is that my headlights are putting out a unique digital signature, so when the sensor registers that digital signature from a reflection, it knows not to dim. When the light is coming from another vehicle, it knows that that light is not its own, because that light does not have the digital signature. Does anyone know?

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  • $\begingroup$ What does the Google say? $\endgroup$ Oct 15 '19 at 21:58
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    $\begingroup$ FYI these systems often react way too late - on a country lane you can see undipped headlights coming around a corner, and should then dip yours before you get around that corner and blind the other driver. If the other driver does this for you, your headlights may not switch at all. It’s a gimmick that I would not rely on, as dazzling other drivers can be a serious safety risk. $\endgroup$ Nov 14 '19 at 6:55
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It's a question of relative intensity. An oncoming headlight is beaming lots of photons into the light sensor in your truck, whereas the light reflected off of road signs (which might seem bright to your eyes) is a lot less intense.

Bear in mind that the thermal time constant for a headlight filament is of order ~100's of milliseconds, which rules out digitally encoding the headlight output in a manner which would allow the sensor in your truck to tell the difference between a reflected beam and one originating from another vehicle.

Such techniques are however routinely used in the radars used in air-to-air missile targeting, where the search beam contains a time-stamped digital signature which gets compared to the return signal to verify it is "real" and not a spoofed version transmitted by the target plane which is intended to mislead the attacking plane's missiles.

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  • $\begingroup$ LED headlights do exist, my Mazda 6 has them. However bright white LEDs use phosphores that have slower time constants too. That said, I'm sure there is just an optical sensor measuring the oncoming light and controlling the dimming. My car also has the ability to detect lane markers so perhaps the system is actually doing image processing to determine the presence of oncoming headlights. $\endgroup$
    – Eric S
    Jul 11 '20 at 16:45

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