[Laser] Beacon to test a photodetector sensitivity limit

[Robo]

Hi Terry

good infos. It is like my mind. My goal is a 
build easy reproducibility calibration low 
optical power source. We need for ease compare our RX's
My priority is reproducibility in all country (easy, low cost).

I want build current controlled small lamp (few 
mA and 2-3V) with BPW34 feedback and mechanical chopper.
Popular BPW34 is a Si photodetector with a 
standard Si spectral characteristics. It is ideal 
for compare any other Si PD and Si APD

for reproducibility We need defined current of 
bpw34! (not a lamp), feedback distance and measure distance, only.
all can be defined like a (HAM)standard. Like 
options for very low light detections we can use 
pin hole or ND filters (who have a money :-) ).

What You mean, about?

Robo
om1arl
www.foton.sk/default.html



At 04:49 PM 5/20/2007, you wrote:
>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
>>
>>
>>
>>
>>
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