Cone spectral sensitivities and color matching

The eye’s optics form an inverted image of the world on the dense layer of light-sensitive photoreceptors that carpet its rear surface. There, the photoreceptors transduce arriving photons into the temporal and spatial patterns of electrical signals that eventually lead to perception. Four types of photoreceptors initiate vision: The rods, more effective at low light levels, provide our nighttime or scotopic vision, while the three classes of cones, more effective at moderate to high light levels, provide our daytime or photopic vision. The three cone types, each with different spectral sensitivity, are the foundations of our trichromatic color vision. They are referred to as long-, middle-, and short-wavelength–sensitive (L, M, and S), according to the relative spectral positions of their peak sensitivities. The alternative nomenclature red, green, and blue (R, G, and B) has fallen into disfavor because the three cones are most sensitive in the yellow-green, green, and violet parts of the spectrum and because the color sensations of pure red, green, and blue depend on the activity of more than one cone type. A precise knowledge of the L-, M-, and S-cone spectral sensitivities is essential to the understanding and modeling of normal color vision and “reduced” forms of color vision, in which one or more of the cone types is missing. In this chapter, we consider the derivation of the cone spectral sensitivities from sensitivity measurements and from color matching data.