Radiance (optics) Units?

[springBreeze]

While reading a paper on optics, I came across "the radiance of the projector is 20 mW/cm2, expanding out onto an f/18 cone."

What does the "expand out onto an f/18 cone" part mean? I thought that radiance is just in units of power/area/steradians. What does f-number have to do with anything?

[Andy Resnick]

You just need to convert the f/# into solid angle:

http://calctool.org/CALC/phys/optics/NA_to_f

an f/18 is a numerical aperture of 0.028, which gives a cone half-angle of 0.028 radians, or 0.0024 steradians, if I've converted everything correctly.