[Laser] Beacon to test a photodetector sensitivity limit

[Art]

At 01:39 PM 5/20/2007, you wrote:
>At 10:29 AM 5/20/2007, Art wrote:
>>I couldn't agree more Terry!
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>
>>It's a very long story, and I won't bore you with ALL the details. 
>>The short version is that I eventually migrated to my landlords 
>>basement in the apartment building to do optical testing. It was in 
>>the lower basement, 20 feet below ground level and there were no 
>>windows or way for light to enter the room although it was a large 
>>open area with lots of junk in the room. I went to this area 
>>expecting it to be pitch black so I wouldn't have to worry about 
>>stray light. But, using a photodiode without a lens, it was 
>>painfully obvious that light was getting in somehow (the ambient 
>>light level was 5 to 15 millivolts while the dark level (with a 
>>cover over the PD) was well under 1 millivolt. I started using my 
>>receiver as a probe to find out where the light was coming from. 
>>Basically, light was coming from everywhere, even off my hand 
>>placed a few inches in front of the photodiode! There was 
>>absolutely no visible light anywhere in the room that I could 
>>detect with my eyes.
>
>Since you were seeing IR of some sort, perhaps you were seeing the 
>"black body" radiation (a'la Boltzmann) of the room itself?

Maybe.......I wasn't sure what I was seeing.

But, if it was due to the temperature of the surroundings, putting a 
blocking cap over the photodiode wouldn't change the output as it 
would be at the same temperature as the walls, ceiling and floor. So, 
I never investigated any further.

We know that photodiodes get much quieter when we cool them, so 
Boltzman is defiantly a factor. It might be interesting to point a 
photodiode at a hot plate and then point it at some dry ice-and 
noting the noise output each time (while holding the photodiodes 
temperature constant).

Regards,

Art