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Determining LED light intensity from its radiant power

  1. Aug 30, 2011 #1
    I have LEDs of 0.7mW, but for my experiments, I want my values to be in light intensity unit - W/cm2.
    I cannot figure out how to do the conversion.
  2. jcsd
  3. Aug 30, 2011 #2


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    Light intensity of your (not quite point-like) source, or light intensity at some object lit by your source?

    In both cases it is just a simple arithmetics (under assumption your LED emits uniformly in all directions)
  4. Aug 30, 2011 #3
    The best way to do this is to measure physical quantities in order to calculate the irradiance (IMO, a more appropriate and less confusing term than intensity*) in (watts/cm^2). Here's the procedure.

    1. Get a photo diode with calibrated responsivity (R, in amps/watt) at the wavelength of the LED and known detector area, Ad.
    2. Place the detector a known distance, z, away from the LED.
    3. Measure the current, I, passing through the LED. You may have to turn the room lights out. Or measure the current with LED on and then again with the LED off then take the difference.
    4. The LED irradiance at the detector, and in that direction, is
    E = I/(R*Ad)
    5. A more useful property of the LED is its radiance, an optical invariant. If you know the emitting area, As, of the LED, Then its radiance is
    L = I*z^2/(R*Ad*As*cos(theta)).

    where theta the angle of the line of sight from the normal of the LED to the normal of the detector.

    All this assumes that the size of the LED and detector are small compared to the distance, z.

    * Intensity is a confusing term because different authors have different meanings for the term. Some authors use it to mean irradiance (power received/receiving area, in watts/cm^2) but some authors use it to mean "intensity" (power emitted/solid angle, in watts/steradian).
    Last edited: Aug 30, 2011
  5. Aug 30, 2011 #4
    The LEDs are being used to experiment with photocatalysis.

    It's the light intensity of the source (LEDs)

    I was hoping I won't have to go down that road as I don't have a calibrated photodiode.
    I think from your asterisked comment, what I am looking for is irradiance but its more like power emitted/receiving area (W/cm^2).
    Wish there was a simple formula
  6. Aug 30, 2011 #5
    It is a simple formula (e.g. 4 or 5 in my post above). You just don't have the information you need to make the calculation.

    Is 0.7 mW the total power emitted by the LED?
    Do you know the emitting area of the LED?
    Do you have graphs showing the angular beam width of the LED or the angular beam width (Full Width Half Maximum- FWHM)?

    If the emitting area, As, and the angular beam width (FWHM), b, are small, the average radiance normal to the LED surface is
    L = P/(2*As*b^2)
    where P is the total power emitted by the LED.

    With the LED radiance, you can estimate the irradiance at any distance, z, (in the far field) from the LED.
    E = L*As*/z^2

    PS: The power emitted/receiving area (W/cm^2) is likley not very useful. If you need to know the Radiant Exitance, M, then calculate
    M = P/As
    Last edited: Aug 30, 2011
  7. Aug 31, 2011 #6
    I'll just find the datasheet of the LED, hopefully all that information should be there.
    Thanks aabottom.
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