I know the relation for a diffuse light (as well as how to derive it) `P=\pi A L`

where P is the power of light, A is the surface area of the light, and L is the emissive radiance. It works fine if L is a scalar. But we know radiance is actually wavelength dependent. What if L is given as a three-component vector (RGB), each component representing the radiance of that component? I guess in this situation, the three components of L should be linearly combined to derive scalar P. But what are coefficients before each radiance component? Thanks a lot.

PS: how to input Latex?