Variability of color mixture data - II. The effect of viewing field size on the unit coordinates.

-Theoretical sources of variation of published color mixture data with variation in field size were considered. The differences in unit coordinates for t” and lo” fields may be explained by postulating that the effective optical density of the cone photopigments decreases as field size increases. 1.33’ tritanopic coefficients and small-field (artificial) tritanopic coefficients are shown to be consistent with the density hypothesis. Effective optical density decreases exponentially as the centrally fixated field of view increases. Similar changes occur for fields placed at eccentric positions. Color mixture functions depend on the size of the viewing field. This observation is the basis for the CIE’s decision to establish a 2” observer as representative of data for field sizes of 1” and 4” in extent, and a 10” observer for field sizes greater than 4” (Wyszecki. 1969). However, Fridrikh (1957) found 1” color mixture functions did not conform to CIE 2” data. Other reports (Hering, 1893; Homer and Purslow. 1947. 1948) suggest that color matching functions change as a function of field size. If the changes reflected solely differences in inert visual pigments, the unit coordinates in a WDW system would be identical. However, the coordinates are not identical (Stiles. 19%) and imply variation in the shape or spectral position of receptor sensitivities as a function of field size. Previously (Smith, Pokomy and Starr, 1976). we discussed some possible models of inter-observer receptoral variability. In this paper, we consider models of variation of receptor sensitivity for different sizes of the viewing field. The models of inter-observer variability are relevant to the variable of field size. In the peripheral retina. the cones are “cone-shaped” and of reduced length. We could expect optical density to decrease and waveguide effects to vary with increasing eccentricity. The visual photopigments themselves may perturbate in i,,,.,,, if the microenvironment of the chromophore is changing. Stiles (1955b) and Stiles and Wyszecki (1974) have noted that macular pigment affects the 10’ data in effective density about half that for the 2” data. Thus we may ask if the spectral filter required to explain the 2” inter-observer variability is required also to predict average unit coordinates and variance for the 10