[Laser] How to measure beam angle???

[MICHAEL COUTURE]

Hi All,

I should know this but just when I think I have it, puff! So tell me again.

How do you determine/measure the beam angle for a laser or LED?  I know how to 
do the math but I'm looking for beam diameter/spread at any distance. More to 
the point, if a beam 'strikes' a retro-reflector, albeit a long distance away, 
(like a satellite) is the beam spread dependent on the source laser or the 
retro's physics?

Knowing the beam power (laser or LED) what is the signal loss (in db) with 
distance?

Mike C.

(NW FL)