[Laser] Beacon to test a photodetector sensitivity limit

[F1AVYopto at aol.com]

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