# 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.) Commented Jul 4, 2023 at 14:30
• @KevinReid theoretical ideal result given the definition of the color space in use Commented Jul 4, 2023 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. Commented Jul 4, 2023 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.