I am struggling with what I think is actually a simple problem. I have an LED that emits infrared light (950 nm). It is part of a sensor system. The manufacturer claims it has a radiant power of 3000 mW/steradian. I want to calculate its total effective radiance as viewed by a human eye. To do so I need to calculate its spectral radiance (L with a subscript '950 nm'). I know that radiance has the units W/steradian/squaremeters (W sr^-1 m^-2), and that it can be expressed per unit wavelength (nm) or frequency (Hz).

I cannot find the algebra form of an equation to convert the information I have (radiant power, 950 nm, 3000 mW/steradian) into spectral radiance. Can someone please help me?



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  • $\begingroup$ Spectral radiance only makes sense when you have a continuous spectrum. The LED either comes with a spectral chart or you have to assume that it is essentially monochromatic (which would be a better assumption for a laser diode than an LED, though). In that case you can only convert into radiance without the spectral distribution, but the units will be correct. $\endgroup$ – CuriousOne Dec 23 '15 at 23:14
  • $\begingroup$ You have incomplete specifications for the LED. Looking at the LED head-on, it is reasonable to assume that LED output is symmetric about that axis. However if you look at the LED sideways, the the angular output will be a function of the geometry of the LED itself. $\endgroup$ – MaxW Dec 24 '15 at 0:34
  • $\begingroup$ Look at the last diagram on page 7 of the following datasheet. www1.futureelectronics.com/doc/EVERLIGHT%C2%A0/… $\endgroup$ – MaxW Dec 24 '15 at 0:42
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    $\begingroup$ You say "spectral radiance as viewed by the human eye". You can probably figure out a way to calculate the radiance, but 950 nm is outside of the range of human eyesight, so it will not be viewed. I'm guessing that you already know that, and it's just a wording problem, but just in case ... $\endgroup$ – garyp Dec 24 '15 at 0:54
  • $\begingroup$ This question/answers at Electrical Engineering Stack Exchange may be helpful: electronics.stackexchange.com/questions/123674/…. At 3000mW/steradian and 950 nm wavelength, I think you need to be very cautious with your LED. $\endgroup$ – Ernie Jan 4 '16 at 15:39

Yes, the wavelength is outside the visible range but the energy can still damage a human eye, so that's why I need to know spectral radiance.

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