[Laser] CONCLUSION TO >>> focal length question

[TWOSIG at aol.com]

George
 
Welcome to the interesting field of communicating with light.  I wish  you 
much success.
 
You have received good responses to your original question.  
 
I hope that what I have to say will help you.  
 
I would like to start with some comments on the field of view rating of 60  
degrees on the BPW34 sensor.  If you hold that sensor face toward a steady  
light source you can measure its output.  If you then tilt the sensor 60  degrees 
it will will measure half, not because the sensor is only half a  sensitive 
to photons from that direction, but because it is only intercepting  half as 
many photons from the source.  A crude analogy would be to hold a  piece of 
paper at arms length.  If you hold it perpendicular it will look  wider than if 
you tilt it. 
 
The sensor will still receive and convert photons from angles greater than  
60 degrees.  The field of view calculations on the fresnel lens on the  other 
hand are "hard" transitions from angles that have light passing through  the 
lens to angles that are beyond the edges of the lens.
 
The bottom line is that the BPW34 sensor will work with either of the  
fresnel lenses that you described.  What I think is more important is that  you 
realize that the lenses are not "perfect".  Depending on how well  you mount them, 
the light from your signal source will create a fuzzy spot  rather than a 
tiny point of light near the focal plane.  As long as that  fuzzy spot falls 
entirely on the sensor, it will do its job of converting  photons to electricity.  
The size of the BPW34 will make your job of focus  and alignment easer than 
if you used a smaller sensor.  
 
It will also allow the system to receive more noise.  The sensor  active area 
is, if I remember correctly, about 3.5mm square.  If you  use a 300mm focal 
length lens, it will "see" an area about 12 milliradians  across, or about half 
again the diameter of the full Moon.
 
The necessary field of view of the  sensor calculations:    Worst case is the 
corner.   Assuming that you are going to use the 180mmX260mm lens with focal 
length  300mm,   start by calculating the diagonal. Square root of ( 180X180 + 
 260X260) is about 316.23mm.  The tangent of the angle you want is half of  
that divided by the focal length.    Roughly 27.8  degrees.   Full field of 
view is less than 56 degrees.  Certainly  not a problem for the BPW34.
 
 
Best wishes.
 
 
James 
N5GUI