LUMINANCE

Luminance was introduced by the CIE as a photometric analog of radianc e. This implies that an additive spectral-luminosity function characte rizes the human observer. In practice, many different spectral-sensiti vity functions characterize human vision, although few produce the add itive spectral-luminosity function V(lambda), which is suitable for us e in practical photometry. Methods that give rise to additive spectral -sensitivity functions that most resemble V(lambda) tend to have in co mmon the use of spatial or temporal frequencies that will discriminate against signals from the short-wavelength-sensitive cone pathways or against signals in other chromatic pathways. Some of the difference am ong results obtained with different techniques seems to reflect the ex tent to which the methods can bring about changes in the state of chro matic adaptation, but it also seems likely that not all tasks tap the same postreceptoral mechanisms. Psychophysical evidence is equivocal r egarding the nature of the postreceptoral mechanisms: some evidence su ggests just three mechanisms, one of which has a spectral sensitivity that is like V(lambda); other evidence suggests the existence of multi ple mechanisms with different spectral sensitivities. Physiological re cordings from neurons in the macaque's visual pathway suggest that the properties of the magnocellular system may be sufficient to account f or spectral-sensitivity functions measured with the techniques of hete rochromatic flicker photometry, minimally distinct border, and critica l flicker fusion. These are the psychophysical methods that yield spec tral sensitivities that are most like V(lambda). Other methods of meas uring spectral sensitivity seem more likely to depend on signals that travel through the parvocellular system.