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Radiant intensity calculation

  1. Jul 2, 2009 #1
    I’m hoping some genius can help me out here. I’m trying to determine how many joules of energy per square centimeter (from an LED) are radiated onto a body given the following parameters.

    LED radiant intensity = 80mW/Sr
    LED diameter = 5mm
    LED beam +/- 20 degrees

    Wavelength = 880nM
    Total radiant flux = 25mW
    Distance from LED to body = 5mm

  2. jcsd
  3. Jul 3, 2009 #2

    Andy Resnick

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    Education Advisor

    Simple geometry (and unit conversions) suffices, genius has nothing to do with it.
  4. Jul 3, 2009 #3
    Hi Andy,
    Well thanks for the help on this :-0.......well I get the genius bit but unfortunately the information to my original question is a little on the ‘light’ side (pardon the pun). I was only looking for some pointers and a little help.

  5. Jul 3, 2009 #4


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    Well, since you know the angle of the emitted cone of light, you can calculate the size of the spot if the LED is 5 mm from your surface to be illuminated. Assume the LED is a point source, for simplicity; this will result in a good first-order approximation. Once you know that, basic geometry will give you the area of the spot. And since you're given the total radiant flux, you can divide one by the other to find the radiant power per unit area.

    One Joule is equal to one Watt for one second.
  6. Jul 3, 2009 #5
    OK, sorry may math is not good.
    I calculated the beam radius of the point source based on a right angled triangle.

    LED height = 5mm
    Radiant flux = 25mW
    Divergence = 20 degrees

    So beam radius = (5 / tan70) = 1.82

    25mW/1.82mm = 13.73 mW/mm2


  7. Jul 3, 2009 #6
    25/(PI R2)
    =25mW/10.4 = 2.4mW/mm2
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