Chromatic Function of the Cones

CIE, Commission Internationale de l’Eclairage – The CIE is an independent, nonprofit organization responsible for the international coordination of lighting-related technical standards, including colorimetry standards. Color-matching functions – Functions of wavelength l that describe the amounts of three fixed primary lights which, when mixed, match a monochromatic light of wavelength l of constant radiant power. The amounts may be negative. The color-matching functions obtained with any two different sets of primaries are related by a linear transformation. Particular sets of color-matching functions have been standardized by the CIE. Fundamental spectral sensitivities – The colormatching functions corresponding to the spectral sensitivities of the three cone types, measured at the cornea. The spectral sensitivities may be normalized so that the maximum is unity or according to the nominal population densities of the cone types. Heterochromatic flicker photometry – The adjustment of the radiant power of one of two spatially coextensive lights presented in alternating sequence at a temporal frequency such that there is a unique value of the radiant power where the sensation of flicker is minimum; that is, with a higher or lower radiant power, the sensation of flicker becomes greater. Luminous efficiency function Vl – The inverse of the radiant power of a monochromatic stimulus of wavelength l that produces a luminous sensation equivalent to that of a monochromatic stimulus of fixed wavelength l0. The units and l0 may be chosen so that the maximum of this function is unity. It is also known as the relative luminous efficiency or relative luminosity function. Optical density – Absorbance; see spectral absorbance. Radiant power and quantum units – Radiant power is measured in watts but it is sometimes more appropriate to measure it in quanta s . Sensitivities may be expressed as the logarithm to the base 10 of the reciprocal of the radiant power required to reach a criterion level of performance. Spectral absorbance A(l) – Logarithm to the base 10 of the reciprocal of the spectral transmittance, t(l); that is, A(l) = log10 t(l). It depends on the path length. If l is the path length and a(l) is the spectral absorptivity, then, for a homogeneous isotropic absorbing medium, A(l) = la(l) (Lambert’s law). Spectral absorptance a(l) – Ratio of the spectral radiant flux absorbed by a layer to the spectral radiant flux entering the layer. If t(l) is the spectral transmittance, then a(l) = 1 t(l). The value of a(l) depends on the length or thickness of the layer. For a homogeneous isotropic absorbing medium, a(l) = 1 t(l) = 1 10 , where l is the path length and a(l) is the spectral absorptivity. Changes in the concentration of a photopigment have the same effect as changes in path length. Spectral absorptivity a(l) – Spectral absorbance of a layer of unit thickness. Absorptivity is a characteristic of the medium, that is, the photopigment. Its numerical value depends on the unit of length. Spectral sensitivity – The inverse of the radiant power of a monochromatic stimulus of wavelength l that produces a criterion response equal to that of a monochromatic stimulus of fixed wavelength l0. The units and l0 may be chosen so that the maximum of this function is unity. In specifications by the CIE, the lower limit of the wavelength range is generally 360–400 nm and the upper limit is generally 760–830 nm, but smaller ranges may be used. The spectral sensitivity of a cone is essentially the normalized spectral absorptance of its photopigment (discounting any geometrical factors, e.g., cone waveguide properties). Spectral transmittance t(l) – Ratio of the spectral radiant flux leaving a layer to the spectral radiant flux entering the layer. The value of t(l) depends on the path length. Unique hues – Hues that are judged to be unmixed. They form mutually exclusive pairs, so that no light can appear to contain both red and green or both blue and yellow. Wavenumber – The reciprocal of wavelength, usually in cm .