# How can I get an Irradiance or power value from RGB?

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.

• 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.) Jul 4 at 14:30
• @KevinReid theoretical ideal result given the definition of the color space in use Jul 4 at 16:26
• 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. Jul 4 at 17:08

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.