[Laser] Beacon to test a photodetector sensitivity limit

[F1AVYopto at aol.com]

Thank you all  for the remarks about my beacon experiment. 
I add some  notes to precise the experiment conditions. 
The receiver  is the photo electrometer from K3PGP with a S2386-18K Hamamatsu 
photodiode (NEP  6,8.10^-16 W / HZ^1/2) and a 2SK170 JFET. 
The test is  done in a wine cellar and there is absolutely no visible light 
during the  experiment. 
(I do not  test wines during experiments!) 
The   beacon is powered by a separated battery to avoid spurious coupling 
with the RX  and a manual shutter shows the signal loss when I interpose it in 
the  beam. 
I use no lens  in the RX to be able to compare the signal with different RX 
types versus their  sensitive area without optical gain. 
The LED used  is rather flat and with red coloured bulb. 
It has a very  low directivity that avoids the RX beacon alignment changing 
effects. 
>From your  informations I  notice that: 
- Red LEDs  have a spurious spectrum in infrared. 
- They have a  very poor light / current linearity. 
- Pin holes  and absorbing layers are very best ways to calibrate a LED 
biased at nominal  current. 
- A one axis  shielded frame is better to avoid indirect reflected light. 
It seems not  so easy to built a low cost calibrated light source... 
I will try to  use directly a very poor quantum yield 1N914 diode with a 
dielectric IR  filter. 
It could be  more stable than a very low current biased LED. 
Thank you  again for all the ideas. 
73 Yves  F1AVY