NOTE 1 – These stimuli are either real colour stimuli or theoretical stimuli which are defined by linear combinations of real colour stimuli; the magnitude of each of these three reference colour stimuli is expressed in terms of either photometric or radiometric units, or more commonly by specifying the ratios of their magnitudes or by stating that a specified additive mixture of these three stimuli matches a specified achromatic stimulus.

NOTE 2 – In the CIE standard colorimetric systems, the reference colour stimuli are represented by the symbols [X], [Y], [Z] and [X₁₀], [Y₁₀], [Z₁₀].