Re: gEDA-user: sparkfun 4 layer boards

[John Doty <jpd@xxxxxxxxx>]

On Aug 23, 2008, at 12:48 PM, John Griessen wrote:

> Eric Brombaugh wrote:
>> Dave McGuire wrote:
>
>>>    I'd be shocked and amazed if they could detect thermal IR.
>>
>> Agree - there's a big difference between the short-wave IR that's  
>> used
>> by common IR remote controls (which is easily seen on most any  
>> webcam)
>> and the long-wave IR used in thermal imaging.
>
> Ohh...  we can control the situation to get near enough IR.  We're  
> looking for shorts
> anyway, not normal operating temps of + 1 degree...   Seeing just  
> the really hot spots
> -- meaning  120 deg F is plenty good, and the amount of near IR in  
> that temperature surface is
> enough to see with a CCD I bet.

Nope. Quantum efficiency of silicon detectors drops like a rock just  
beyond 1 µm wavelength: the radiation just goes right through without  
interacting. Indeed, Si wafers make excellent entrance windows for  
for thermal IR detectors.

A 350 K blackbody emits ~271 µW/mm^2 of thermal radiation, but only  
~4.4 fW/mm^2 of that is short of 1 µm. With very fast optics,  
cryogenic temperatures, a state of the art scientific CCD, the  
extremely low noise video chains I design for astronomy, and rigorous  
exclusion of every optical photon, you might be able to see  
something. With commercial/industrial technology, not a chance.

On the other hand, a 350 K component is pretty easy to find with your  
finger...


John Doty              Noqsi Aerospace, Ltd.
http://www.noqsi.com/
jpd@xxxxxxxxx




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