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Radiance (optics) Units?

  1. May 15, 2009 #1
    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?
  2. jcsd
  3. May 15, 2009 #2

    Andy Resnick

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


    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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