White light brightness–luminance relationship:

Brightness perception of near-white surfaces is influenced by the spectral power distribution (SPD) of the illuminant; at the same time, this has an effect on task performance, either directly or via influencing the mood of the worker. With the introduction of modern solid-state lighting sources, the SPD of the illuminant can be varied almost at will. Investigations were conducted to test the SPD-dependence of the brightness perception of equal luminance and correlated colour temperature white lights, to find out whether receptors other than cones, such as rods and intrinsically photosensitive retinal ganglion cells, have an influence on brightness. Observers could be clustered into groups for finding brightness equal to, or different from, luminance, depending on the SPD of the illuminant.