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Old 08-25-2006, 07:25 AM
Don't Call Me Shirley Don't Call Me Shirley is offline
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IR through glass

On Mythbusters last week they defeated an infrared motion sensor alarm by placing a piece of glass over it. The glass blocked the IR radiation from Carrie and she walked right past it without setting it off. She was also invisible to an IR camera. Glass blocking IR radiation is also the reason cars heat up so fast in the sun.

My DVD player is in a cabinet behind glass. But the IR remote works through the glass. How?
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Old 08-25-2006, 08:16 AM
spingears spingears is offline
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Quote:
Originally Posted by Don't Call Me Shirley
On Mythbusters last week they defeated an infrared motion sensor alarm by placing a piece of glass over it. The glass blocked the IR radiation from Carrie and she walked right past it without setting it off. She was also invisible to an IR camera. Glass blocking IR radiation is also the reason cars heat up so fast in the sun.
My DVD player is in a cabinet behind glass. But the IR remote works through the glass. How?
Ordinary glass passes the IR which is why cars heat up in the sun in addition to the metal roof etc.
IR blocking fileters are expensive.
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Old 08-25-2006, 08:28 AM
hajario hajario is offline
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Quote:
Originally Posted by spingears
Ordinary glass passes the IR which is why cars heat up in the sun in addition to the metal roof etc. IR blocking fileters are expensive.
IR cameras cannot see through ordinary glass. If you point any IR camrea at glass you will see a reflected image. Anything that is at any temperature glows. If it is very hot, the glowing is visible to the human eye. A hot coal is an example of this. An IR detector/camera can see the glow at ambient or lower temperatures.

The IR remote is sending a signal in the infrared range but the CD player or whatever is detecting the signal, not the heat signature.
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Old 08-25-2006, 08:55 AM
scr4 scr4 is offline
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The infrared beam used by remote controls is "near infrared," usually in the 900-1000nm wavelength range. It's just beyond the range of human vision. It passes through most types of glass. It's generated by an LED or laser.

Infrared motion detectors detect radiation generated by body heat. That would be "far infrared", around 9000nm (9um) wavelength. It does not penetrate glass.
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Old 08-25-2006, 09:20 AM
muldoonthief muldoonthief is offline
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Yeah, the IR coming from your remote isn't close to the blackbody radiation wavelength of body temperature. In fact, it's so close to visual wavelengths that most digital cameras can detect it. Fun science: point your remote at the lens of your digital camera, and watch it through the screen on the back of the camera. Press some buttons - you can actually see the LED flashing on the camera screen.
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Old 08-25-2006, 11:34 AM
spingears spingears is offline
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Quote:
Originally Posted by spingears
Ordinary glass passes the IR which is why cars heat up in the sun in addition to the metal roof etc.
IR blocking fileters are expensive.
I knew better than that!
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Old 08-25-2006, 11:36 AM
scr4 scr4 is offline
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By the way, I'm still not sure why they (Mythbusters) used glass in particular. I think any solid opaque plate would work equally well.
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Old 08-25-2006, 11:40 AM
Lightnin' Lightnin' is offline
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Quote:
Originally Posted by scr4
By the way, I'm still not sure why they (Mythbusters) used glass in particular. I think any solid opaque plate would work equally well.
Probably because it was so dramatic- people would expect a metal plate to block IR, but glass is, you know, transparent. Besides, glass wouldn't heat up as quickly as, say, metal. Wood would work fairly well, I'd bet.

It's pretty obvious why they had Kari walk through the hallway. She's so hot that if anyone set off the alarm, she'd be the one.
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