Radiance (optics) Units?

  • #1

Main Question or Discussion Point

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?
 

Answers and Replies

  • #2
Andy Resnick
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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.
 

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