Formulated by Hermann Grassmann, these laws state that human color perception is linear and additive. If two light mixtures match in color, they will continue to match if an identical third light is added to both, regardless of their underlying spectral compositions. This mathematical linearity forms the absolute foundation of standard colorimetry and additive color mixing.
We can calculate the exact XYZ coordinates for this blend because Grassmann's law guarantees the primaries will sum predictably.
Grassmann's law assumes photopic vision; it breaks down at very low light levels when rods begin contributing to color perception.
Related terms
Chromaticity
Where a light's color sits on a color map once brightness is set aside, given as coordinates such as x, y or u', v'.
Metamerism
The effect where two colors match under one light source but not under another, because their spectra differ.
SPD
Spectral Power Distribution: a graph of how much power a light source emits at each wavelength.