[Laser] Beacon to test a photodetector sensitivity limit

[Terry Morris]

I spent many years (25) working in the night vision development field. 
During that time we tested many emitters at low level input currents. The 
old red LEDS do shift to IR emmission at low currents.
Have seen throught night vision systems LEDS emit down to sub uA levels. 
Also, it is well known that many forward biased PN junctions will emit IR. I 
think the emission of the old red LEDS at low current is just an example of 
this IR emission of a forwared bised PN junction. For example, an 1N914 will 
emit IR when forwarded biased as seen through an intensifier. The gain of 
the image intensifier in night vision can approch 1E6 on single MCP devices.

I have no data no how the new high brightness LEDS emit at very low current.

Since the emission at low current levels will depend on PN junction 
construction, if you are trying to construct a low level calibrated source, 
I would suggest the following:

Run the LED at normal current levels and measure output there. Then use 
optical methods to control intensity. Best methods would be either very 
small pin holes or ND filters, or both. Pin holes can control down to decade 
level, then fine tune level with ND filters. This is method we used to test 
image intensifiers. However we were not using LED as source, but lamps 
operating at 2856K.

You also need ND filter curve, as density falls quickly in the IR.

Hope this helps,

73
Terry W5TDM




>From: F1AVYopto at aol.com
>Reply-To: Free Space LASER Communications <laser at mailman.qth.net>
>To: laser at mailman.qth.net, Optical_DX at yahoogroups.com
>Subject: [Laser] Beacon to test a photodetector sensitivity limit
>Date: Sun, 20 May 2007 09:25:19 EDT
>
>Hello to all.
>Is there somebody who knows the properties of LEDs at  very low current (a
>few microamperes range)?
>The constructors datasheets  never give light curves at very low current.
>With the new bright LEDs the  light is not proportional to the current near 
>a
>threshold point (Above a few mA  level).
>The very old red LEDs seem to have a very linear relation between  light 
>and
>current even at ultra low level.
>Our goal is to build a modulated  beacon able to generate a calibrated very
>low flux.
>Is it possible to convert  the electrons flux deducted from the current to
>photons flux with a fixed return  quantum ?
>The very simple diagram of the experimental beacon is  at:
>http://pageperso.aol.fr/F1AVYopto/opto/400+Hz+beacon.jpg
>The peak  current is deducted from the 1K resistor leads voltage with the
>oscilloscope.
>The potentiometer value can be increased to 1 Mohm.
>With our  last photodetectors, we noticed it is possible to hear the signal
>to 1 meter  without lens while it is absolutely not possible to see the LED
>with  eyes.
>With Spectran it is possible to extract signal with a current reduce  again
>by 10 !
>Is the old red LEDs do not conserve their peak frequency with  the current
>decreasing and then go to infrared or are our receivers really well  better 
>the
>eyes sensitivity ?
>Thank you for help.
>73 Yves F1AVY
>
>
>
>
>
>_______________________________________________
>Laser mailing list
>Laser at mailman.qth.net
>http://mailman.qth.net/mailman/listinfo/laser

_________________________________________________________________
More photos, more messages, more storage—get 2GB with Windows Live Hotmail. 
http://imagine-windowslive.com/hotmail/?locale=en-us&ocid=TXT_TAGHM_migration_HM_mini_2G_0507