Then we take a red light and also adjust it to be equally bright to this same reference white light. We might expect that it will be twice as bright as the reference white. However, we would be wrong. We can see this graphically. The significance of additivity is grounded in the luminance equation .
Integration is really just a fancy mathematical term referring to addition.
As can be seen in the luminance equation the radiance as a function of wavelength is multiplied by the sensitivity of the visual system as a function of wavelength.