[Laser] Re: Laser Digest, Vol 19, Issue 1

[TWOSIG at aol.com]

I would like to clarify two points on the recent exchanges.  Please  forgive 
me if I seem to be nit-picking.  I just think these need  comment.
 
Dave, wa4qal  said:
 
      "Finally, don't forget that optics can be  included to gather more light
on the  receiving end.  ...  If you can gather more light  
on the receiving end, you'll have a  better signal to noise ratio......"
 
I do agree with the idea of gathering more light to get more signal.   And 
more signal will give you a better ratio of signal to your system generated  
noise.  That will certainly be important, but the simple statement does not  
recognize the noise that will come into the system with the signal.  There  is 
light out there that is not the desired signal and if you bring it into the  
system it is part of the noise you need to deal with.
 
The design of the optical system should also consider the "other" light  
noise, and as any amateur astronomer can tell you it may not be just in the  field 
of view.
 
My point is that a bigger aperature is not always the answer.  It may  be a 
simple way to get more light in if there is no problem with light noise  
sources.  Particularly for use in daylight, you may want a narrower field  of view 
to improve your signal to light noise ratio instead of your signal plus  light 
noise to system (electronic) noise ratio.
 
I have seen a lot of suggestions for fresnel lens systems to increase light  
gathering.  If that works, I think that is great.  I just want to  suggest 
that there may be circumstances where a longer focal  ratio  lens should be 
considered.  Most fresnel lenses have  a focal ratio less than 2.  If you do the 
math the field of view is close  to 90 degrees.
 
 
 
 
 
 
Also there was a question about interchanging a photodiode for a  
phototransistor and if the bias would be similar.  My comment is  this:
 
Regardless of what bias changes need to be made, remember that a  
phototransistor is also an amplifier as well as a photodetector.  If you  substitute a 
photodiode in place of a phototransistor, do not expect the same  amount of 
signal at the terminals.  You may need to add gain somewhere  else.  Maybe more 
light in (bigger aperture).  Maybe more electrons  (increased amplifier gain or 
another amplifier stage).
 
 
I hope my comments are helpful.
 
73
 
James
N5GUI