Two important sources of variability in the shapes of corneally measured cone spectral sensitivities are individual differences in the densities of lens and macular pigmentation. These prereceptoral filters absorb mostly at short wavelengths.
The large individual variations in lens and macular pigmentation can obscure whether cone isolation has been attained at short wavelengths, and they are the main source of observer sampling error in the estimation of cone sensitivities.
The ocular media, including the crystalline lens, absorbs light up to 650 nm (van Norren & Vos, 1974), though the lens itself, which contains a yellow, water-soluble pigment, shows a pronounced absorption maximum at a wavelength c. 365 nm with no sign of any specific absorption in the range 450 to 650 nm (Cooper & Robson, 1969). On the assumption that the distribution of yellow pigment is uniform in the lens, the density will change with effective path length. Thus the parafoveally imaged portions of the mixture fields may be less selectively filtered than the foveally imaged portions (owing to the lenticular shape of the lens).
Individual differences in lens pigment densities are large: densities have been found to vary by approximately +/-25% of the mean density implied by the CIE 1951 scotopic luminosity function (van Norren & Vos, 1974), as estimated from the individual scotopic luminosity data of Crawford (1949).
The spectral transmittance of the human lens can be estimated by: (i) measurements on excised human eyes; (ii) comparison of the visual spectral sensitivities of normals, with those of aphakics; (iii) comparison of the light reflected at the lens-vitreous surface (which has twice traversed the eye lens) with the light reflected at the aqueous-lens surface (which has not traversed the eye lens) (Said & Weale, 1959); and (iv) by reference to standard psychophysical functions such as the CIE 1951 scotopic luminosity function.
Lens pigmentation can be estimated psychophysically by measuring scotopic flicker thresholds for certain test wavelengths (say 413 and 545 nm) at a peripheral eccentricity and by assuming that individual variation in the ratio of the sensitivities reflects lens absorption in the violet (see Ruddock, 1965; Stockman et al., 1993). This assumption neglects possible individual differences in the pigment optical density of rhodopsin itself, but these can play only a minor role.
The cornea has definite absorption properties that differ from those of the lens. However, the corneal absorption is traditionally included in the lens transmittance estimates; since the two densities are typically confounded with one another in psychophysical measurements. The average spectral transmittance of the cornea is rather flat between 450 and 700 nm (about -0.1 to -0.2 log unit). However, below 450 nm, it gets progressively larger. It may be as large as -0.5 log unit at 390 nm (van den Berg & Tan, 1994).
The values given in the three tables in this section are relative to the optical density at lambda = 700 nm. To determine the optical density in absolute terms, intraocular scattering and pupil size must be taken into account. Van Norren and Vos (1974) suggest that a density value of 0.15 should be added to the relative values to convert them to absolute values. Additionally, the values in the three tables apply to the completely open pupil. For small pupil sizes, the density values will increase because of the varying thickness of the lens across the pupil aperture (Weale, 1961). Accordingly, van Norren and Vos (1974) estimate that for a small pupil, the open pupil values should be multiplied by a factor of 1.16. The lens density tabulations given here for Stockman, Sharpe & Fach (1999) and for Stockman, MacLeod & Johnson (1993) are for a small pupil. Those for van Norren & Vos (1974) and for Wyszecki & Stiles (1967;1982) are for an open pupil.
The shape of the lens density spectrum changes with age (e.g., Pokorny, Smith & Lutze, 1988; Weale, 1988). When unusually young or old groups of subjects or individuals are employed, such changes should be taken into account.
The macular pigment is a yellow carotenoid located in the fibres of Henle. It is believed to act as an optical filter counteracting the deleterious effects on foveolar resolution of chromatic aberration and scattering in the ocular media (Snodderly et al., 1984). It may also protect the inner retina from light damage. The macular pigment is very slight in the foveola, intense on the slopes and margin of the fovea, and gradually fades out beyond. The region of most dense pigmentation is about 3000 μm or 10 deg in diameter; the total extent of pigmentation is c. 5000 μm or 17 deg in diameter. Because it peaks in the central fovea and declines to a low, constant value within 5-deg eccentricity, it reduces the spectral sensitivity of the foveal cones, as compared with the parafoveal ones, to blue and violet light.
Macular pigment density is typically estimated from the differences between cone spectral sensitivities measured centrally and peripherally, yet both macular pigment density and photopigment optical density vary with eccentricity. The confounding effects of changes in photopigment optical density can potentially cause a serious overestimation or underestimation of the actual macular pigment density, or they could be misinterpreted as a novel macular pigment spectrum (see Sharpe et al., 1998; Stockman and Sharpe, 1999).
Individual differences in macular pigmentation are large: in studies using more than 10 subjects, macular pigment density has been found to vary from 0.0. to 1.2 log units at 460 nm.
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