[Laser] focal length question

[F1AVYopto at aol.com]

"iam new to laser  coms and while i was searching in the net for fresnel 
lenses i found lenses for  17.99$ that have maximum dimensions: 15.5x15.5 inches 
(~390x390 mm)
and are  available in two versions:
a) 330mm focal length
b) 220mm focal  length
What version is better for laser receivers???? Is it better to have  the 
short focal length or it doesnt matter since the max dimentions in both  versions 
are the same?????"

George,
The focal length choice  depends on the photo detector you use.
At the focal point, the light flux  from the TX must converge without losses 
on the photosensitive area.
The  convergence angle must be the same the chose PIN photodiode sensitivity  
angle.
For flat and thin photodiodes the focal length can be short. 
For  small diameter photodiodes with the sensitive area distant from the 
glass window  or for the ones with integrate lenses or convex clear plastic case, 
the focal  must be long.
The 330mm focal length lenses seem easiest to match with  standard 
photodiodes.
73 Yves
http://pageperso.aol.fr/yvesf1avy/