The data sheet usually indicates the power consumed and the luminous flux in lumens. The luminous flux per watt can be calculated from this, but this does not allow any direct statement about the degree of efficiency.

Unfortunately, the Radiant Flux is only given for special LED emitters.

Since the luminous flux is a measure perceptual to the human eye, it cannot easily be converted into power [W]. Isn't there still an approximation with which this is possible taking into account the light temperature? The spectra of the white LED do not differ in principle, since a blue LED is usually used and part of the blue light is converted into other spectral colors.

This information would be useful for the purpose of correctly dimensioning the cooling capacity.

  • 1
    \$\begingroup\$ DamienD nailed it in his answer. Though such a precise calculation is not neccessary to do thermal calculations. If you dimension your thermal solution to dissipate the whole power consumed you're on the save side and have a margin. Cooling of LEDs can never be good enough ;) \$\endgroup\$
    – Sim Son
    Nov 1, 2021 at 11:43
  • \$\begingroup\$ @Slim Son that's right, you're right, that was the approach I used. but LEDs become more efficient e.g. a jelly bean UVA LED emits almost 50% as UV light. So it would be waste to put more effort into cooling than necessary. \$\endgroup\$
    – arnisz
    Nov 1, 2021 at 16:54

1 Answer 1


It is possible to compute it with some assumptions and the help of Python/Matlab/R etc.

Sometimes the datasheet will give you a plot of the spectrum with a proper scale in W/nm on the Y axis. Then you can 'simply' tabulate the spectrum into your favourite numerical computing package and integrate to get the radiant power in W.

If the LED spectrum is shown without a scale or normalised, you need a bit more work.

To go from radiometric units (W) to photometric units (lumen) you would typically plot or tabulate the spectrum of the LED (in W/nm), multiply with a standard luminous efficiency function, and integrate to get lumens.

Here you would do the reverse: start with the known value in lumen, and work backwards to give the LED spectrum a scale in W/nm.


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