Re: Solid Angles

["Dale E. Boyce" <dale@radpro.uchicago.edu>]

Russ,

All lasers have some divergence and therefore, neglecting
scattering and attenuation, behave as 1/r^2.  As Dave
points out, one must extrapolate back to an effective point
source to determine from where to measure r. 

Now that I think about it, the units on the constant I was
describing would be only watts or watts per steradian.  

(Output of the laser in watts)  / (solid angle of beam)

Dividing this value by the r^2 from the effective point source
gives the intensity of light in watts / m^2.  Since the steradian
is a "unitless" unit its inclusion is optional in the constant and
probably wrong in the constant / r^2.  This may be the "source" of
confusion.  That is someone may be talking about intensity at a
point in watts/m^2/steradian, when it should be only in watts/m^2.

I'm just speculating based on the units presented.  Maybe more about
the context in which it is used would shed some light. (pun intended)

Dale Boyce
dale@radpro.uchicago.edu