Normal colour vision is presently described in the framework of one or other trichromatic opponent model. The trichromacy finds its origin in three different pigments, these pigments having maximal absorption in different parts of the spectrum. The absorption spectra determine the spectral sensitivities of the three colour mediating systems. The latter have of course to be corrected for the absorption characteristics of the ocular media and the macular pigment. To know the spectral response curves of the foveal receptor systems is one of the basic problems of the theory of normal as well as of defective colour vision. The purpose of the paper is to describe some methods to determine the spectral sensitivities of the three or two colour mediating systems.