is there a way to output the power in watts from an R, G and B value? I know there are many formulae to get perceived brightness but is there one to get an objective amount (relative or otherwise)?

This is for an experiment I am working on which basically requires radiance to stay constant between images but I am not sure how to calculate radiance fro an image.

  • $\begingroup$ Are you looking for the actual output power that a specific display device will be producing, or a theoretical ideal result given the definition of the color space in use? (In the former case, there are enough non-idealities that you might need to measure the actual output to build a model.) $\endgroup$
    – Kevin Reid
    Jul 4 at 14:30
  • $\begingroup$ @KevinReid theoretical ideal result given the definition of the color space in use $\endgroup$
    – Dom DJ
    Jul 4 at 16:26
  • $\begingroup$ You would certainly need to do it as a relative value rather than a value in watts. If you say that a fully white image produces a power output of 100%, then you could estimate what percentage of that each image generates. I think it could only be an estimate, as different screens could output different amounts of energy to display the same colour, depending on the specific wavelengths being generated. $\endgroup$ Jul 4 at 17:08

1 Answer 1


If all you need is to check constancy, I would rely on $R+G+B$, as the energies are additive. Make sure that none of the components saturate and don't change anything in the geometry.

  • $\begingroup$ dont differet colours have different amounts of energy? $\endgroup$
    – Dom DJ
    Jul 4 at 19:57
  • $\begingroup$ @DomDJ: actually I don't know. This depends on the spectral response of the color channels of the camera. $\endgroup$
    – user1703
    Jul 4 at 21:10
  • $\begingroup$ The energies would be additive, but the RGB values are not usually linear with regard to the energy, so you would need to remove the gamma encoding to get linear RGB values first. The wikipedia article on sRGB gives the formula en.m.wikipedia.org/wiki/SRGB#Transformation. Shorter wavelengths have more energy, so B would have more energy than G, which would have more energy than R. The specific values would depend on the monitor, I don’t even have ballpark figures. $\endgroup$ Jul 5 at 16:08
  • $\begingroup$ @TimothyMaguire: this depends on the acquisition device. Some don't do gamma compression. A simple way to check this is to turn on several identical lamps progressively. $\endgroup$
    – user1703
    Jul 5 at 16:30

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