[Laser] Beacon to test a photodetector sensitivity limit

[Terry Morris]

Glenn,

Does your basement darkroom have fluorescent lighting, if so that is 
probably where your stray light is comeing from. Many fluorescent lamps 
continue to emit NIR glow long after they are turned off.
In some cases up to 20 min. Try waiting 1/2 hour and run your test to see if 
problem improves. I know you know that just because you can't see it with 
your eyes doesn't mean the detector at 900nm peek can't see it.  If yor are 
interested in the long story let me know.

73
Terry W5TDM


>From: Glenn Thomas <glennt at charter.net>
>Reply-To: Free Space LASER Communications <laser at mailman.qth.net>
>To: Free Space LASER Communications <laser at mailman.qth.net>
>Subject: RE: [Laser] Beacon to test a photodetector sensitivity limit
>Date: Sun, 20 May 2007 10:39:42 -0700
>
>At 10:29 AM 5/20/2007, Art wrote:
>>I couldn't agree more Terry!
>
><snip>
>
>
>>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?
>
>73 de Glenn wb6w
>
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