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Sensing fog on the windshield

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I'm not the first to think about it, since I see a bunch of patent filings related to it, but how hard would it be to have a sensor for windshield fog. Seems to me you could bounce light (UV perhaps which water scatters, though other colours might work) off the windshield to detect if there's fog on the inside and use that to control the defogger.

In particular, modern defoggers use the air conditioner which provides dehumidified air when you heat it up again, great for defogging. But also fuel inefficient. I swear that while today's AC based defoggers are better than the old pure heat ones, I think the AC-off mode of modern defoggers is not as good as the AC-less mode of the old ones.

Anyway, the sensor could at least control if the AC is used, if nothing else.

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My 2001 Jetta doesn't have anything to detect it, but it does have some logic to put hot air on the window automatically when you start the car and the temperature (and maybe humidity?) conditions are just right.

I'm not sure what their algorithm is like, but at least in Southern California it works out really great. It never comes on in the summer or when it's otherwise not needed, and it always comes on when it is needed.

The only real improvement to it I can see is that it could be better at turning off when it's not needed anymore.

- ask

Yeah, I'd like those systems to be smarter. In some cars all defogger use turns on the A/C, which isn't always needed.

Maybe a light beam bounced at a low angle off the inside of window. Plus inside/outside temps would make for a more intelligent system.

I really liked the idea I saw 10 years ago about shining light through the window to automatically modulate the frequency of intermittent wipers. At some amount of scattering, the wipers would wipe. So they'd continually adjust, perfectly. And respond immediately to the dangerous situation of a large splash from an adjacent car hitting a puddle. Any car ever offer that?

-David
Kenai, Alaska

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