The Land Demonstrations in relation to Colour Theory and Colour Photography

Abstract

Colour effects observed in Land photographs suggest other well-known perceptual phenomena. An explanation of the Land colours must, however, be in terms of the basic mechanisms on which all these phenomena are dependent. Trichromatic theory postulates the nature of the initial stage in the colour reception process, but it has little applicability to the qualitative phenomena of colour vision. The opponents theory of Hering is here used as a basis for later stages in the visual process in order to explain qualitative phenomena in terms of neural mechanisms. Methods are described by means of which tristimulus specifications for the receptors can be selected which are appropriate to the trichromatic theory and to an opponents theory based on the trichromatic theory. The Land photographs can then be understood in terms of such a system, provided there occur changes in sensitivity of the receptors, or adaptive changes which simulate such changes in sensitivity at a later stage in the visual process. Such changes are partly dependent on interaction between outputs corresponding to a particular retinal area and those corresponding to a larger retinal area. These mechanisms are also discussed in relation to the suppressor mechanism for chromatic aberration. Colour photography is essentially satisfied by a trichromatic process as a photograph has, ideally, to produce similar activity at the first stage in the visual process as do the objects photographed. But in so far as modifications are necessary in a photograph to compensate for the limitations of photographic reproduction and the conditions in which the photograph is viewed, it may be useful to go beyond the trichromatic scheme both to understand the nature of such modifications and to simplify their implementation. The Land demonstrations are important as they highlight basic perceptual phenomena and provide a means of obtaining quantitative data which may reveal the nature of the underlying mechanisms.