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Ocular filtering of ultraviolet radiation and the spectral spacing of photoreceptors benefit Von Kries colour constancy

Adrian G. Dyer

Faculty of Science, Monash University, Victoria 3800, Australia

View larger version (32K): [in a new window]   Fig. 1. The spectral absorbance of ocular filters in the eyes of various vertebrates. (Redrafted from Muntz, 1972, with the permission of Springer-Verlag and W. R. A. Muntz.)

View larger version (32K): [in a new window]   Fig. 2. Reflectance spectra of the 24 stimuli (numbered 1–24) described in Table1. Division of the stimuli into A, B and C is purely to aid the reading of the reflectance curves.

View larger version (15K): [in a new window]   Fig. 3. Spectral distribution of typical daylight with the relative quantity of radiation normalised to 1.0 at 560nm. Curves are based on the data of Judd et al. (Judd et al., 1964) and represent correlated colour temperatures of (A) 10 000K and (B) 6500K.

View larger version (20K): [in a new window]   Fig. 4. Plots of 24 coloured stimuli (numbered as in Table1) in CIE 1931 colour space. The chromaticity coordinates (x, y) of the stimuli are plotted considering an illumination of correlated colour temperature=6500K, and the spectral locus is a plot of pure spectral radiation.

View larger version (24K): [in a new window]   Fig. 5. Spectral sensitivity of various hypothetical visual systems (HVSs) plotted using a vitamin A1 template described by Stavenga et al. (Stavenga et al., 1993) with the height of the photoreceptors adjusted to a maximum of 1.0. (A) HVS1 is based on human photoreceptor peaks at 420, 534 and 564nm (Bowmaker and Dartnall, 1980) and assumes that all radiations shorter than 400nm are absorbed by ocular filters. (B) HVS2 is based on the same photoreceptor peaks as HVS1, but assumes no ocular filtration and that ultraviolet radiations are absorbed by common ß-band peaks maximally sensitive at 340nm. (C) HVS3 is based on three symmetrically spaced photoreceptors maximally sensitive at 420, 492 and 564nm and assumes that all radiations shorter than 400nm are absorbed by ocular filters.

View larger version (17K): [in a new window]   Fig. 6. Plots of 24 coloured stimuli (numbered as in Table1) onto Cartesian coordinates to represent the colour space of various hypothetical visual systems (HVSs) considering an illumination of correlated colour temperature=6500K. Chromaticity coordinates (lws, mws) are the normalised stimulations of the long-wavelength-sensitive and medium-wavelength-sensitive photoreceptors. (A) Colour space for HVS1, which is a visual system based on human colour receptor peaks (420, 534 and 564nm; Bowmaker and Dartnall, 1980) and ocular filters absorbing radiations shorter than 400nm. (B) Colour space for HVS2, which is a visual system based on human colour receptor peaks and no ocular filters. Ultraviolet radiation is assumed to be absorbed by secondary ß-band peaks that are maximally sensitive to 340nm radiation. (C) Colour space for HVS3, which is a visual system based on symmetrically spaced photoreceptors maximally sensitive at 420, 492 and 564nm and assumes that all radiations shorter than 400nm are absorbed by ocular filters (see Fig.5). The spectral locus is a plot of pure spectral radiation.

View larger version (33K): [in a new window]   Fig. 7. The relative performance of two hypothetical visual systems (HVS2 and HVS3) compared with HVS1 for a variety of differently coloured stimuli (numbered 1–24; see Table1) when considering spectrally variable illumination and von Kries colour constancy. The change in illumination colour is from a correlated colour temperature of 6500K to 10000K. A positive number indicates a relative increase in the predicted colour shift of a stimulus in colour space (poorer performance), and a negative number indicates a reduced colour shift (better performance). For example, an increase of 100% means the predicted colour shift is twice the magnitude of that calculated for HVS1. If the performance is identical to that of HVS1, then the relative increase is zero. HVS1 is based on three receptors with maximum absorption of radiation at 420, 534 and 564nm, plotted with a vitamin A1 template (Stavenga et al., 1993) and assuming all radiations shorter than 400nm are absorbed by ocular filters. HVS2 has the same main receptor peaks as HVS1, but does not have ocular filters that prevent the ß-band absorbing short-wavelength radiations. HVS3 is based on symmetrically spaced receptors with maximum absorption of radiation at 420, 492 and 564nm and has ocular filters absorbing radiations shorter than 400nm (see Fig.5).