mean values Lmean, with the appropriate L(ser180) or L(ala180) spectral sensitivity values. These are provided in the Appendix of Stockman, Jägle, Pirzer, and Sharpe [47].

Directional Sensitivity

Variation of the point of entry of light entering the pupil (or the obliquity of the light rays reaching the retina) produces, besides the effective change of retinal illumination level (the Stiles-Crawford effect of the first kind [150]), a modification of hue (the Stiles-Craw- ford effect of the second kind [151]). There is a hue shift throughout the spectrum and an increase in saturation in the blue-green [152] spectrum, both of which will have minor (and difficult to calculate) consequences for photopic luminous efficiency under normal (free) viewing conditions. For scotopic luminous efficiency, the effects of the angle of incidence can be disregarded. This is because the directional sensitivity of the rod photoreceptors is very small in magnitude and virtually negligible in wavelength dependency [153].

Variations in the Contribution of Chromatic Channels

An age-related reduction of efficiency has been observed at long wavelengths for photopic luminous efficiency functions based on HBM [104]. The decline has been attributed to a reduced contribution of the chromatic pathway signals to brightness with age. Interestingly, some evidence suggests that the infant’s luminosity function is mediated by luminance (largely without a chromatic pathway contribution) rather than by brightness (with a chromatic pathway contribution) signals, although this is difficult to test [154]. To date, there remains no evidence that the infant visual system computes a brightness signal. Moreover, such young infants also fail to make chromatic discriminations. This may mean that the chrominance pathways develop later than the luminance pathway, which seems to be intact even in 8-week-old infant subjects (see [154]).

CONCLUSIONS

Spectral luminous efficiency is well defined for almost all conditions of scotopic (rod) vision by the CIE V′(λ) function [32]. For photopic vision, a luminous efficiency function that obeys additivity has been defined for neutral daylight conditions, V*(λ) [44]. It is valid for small central viewing fields of 2° or less in diameter. For large viewing fields, 10° ore more in diameter, the V10*(λ) may be preferred instead. Moreover, both V*(λ) and V10*(λ) are representative of techniques that obey Abney’s law of photometric additivity. For some conditions, for instance, those requiring side-by-side brightness matching, a function based on brightness photometry may be more appropriate (such as the CIE brightness-matching function). However, luminous efficiencies estimated by that function will not obey additivity. For mesopic vision, uncertainties about how the rod and cone signals interact to determine luminous efficiency and how their interaction changes with increasing luminance make any recommendations about which luminous efficiency function to use impracticable. The standard luminous efficiency functions generally apply to young observers for fixed viewing and measuring techniques. However, formulas are available for correcting them so that they more closely approximate the luminous efficiencies of any individual observer for other viewing conditions.

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