Another important factor in defining a standard function is that the spectral luminous efficiency function is strongly dependent on the viewing conditions. The function will change as the viewing field increases in size (e.g., “small” 2° as opposed to “large” 10° fields) or as it is moved from the fovea (optimal for photopic luminous efficiency) to the periphery (optimal for scotopic luminous efficiency). As discussed in the section on individual differences influencing luminous efficiency, this is in part due to the decline of the optical densities of the macular pigment and the photopigment with eccentricity. But, under mesopic conditions, the relative contributions of the rods and cones may change as well. A further complication is that the S cones are absent in approximately the central 25-min diameter of vision (e.g., [26–29]). However, S cones make a minimal contribution to photopic photometry if additive techniques are used (see section on psychophysical measures of luminous efficiency). For practical reasons, photopic luminous efficiency functions (see section on photopic (cone) luminous efficiency functions) have been standardized for 2° central viewing fields, which are assumed to be a largely rodfree area and thus exclusively cone dominated, and for 10° central viewing fields because the effective field of view in many real-world situations is much larger than 2°. The application of these standard functions to other viewing conditions may require them to be appropriately adjusted.

SCOTOPIC (ROD) LUMINOUS EFFICIENCY FUNCTION

Introduction

Defining scotopic luminous efficiency is comparatively straightforward because it depends solely on the activity of a single univariant photoreceptor type, the rods. Only at the highest rod operating levels, at luminances above about 10−3 cd m−2 (see Fig. 1), do the cones intrude to complicate things. Even then, their intrusion most conspicuously occurs when small or foveally fixated stimuli are viewed, particularly long-wavelength ones, because for such stimuli the absolute sensitivities of the foveal and parafoveal cones may surpass those of the rods [30]. Under such conditions, the luminous efficiency function then becomes a nonlinear combination of rod and cone responses that varies with the observing, measuring, and adapting conditions (see the section on mesopic (rod-cone) luminous efficiency functions). However, in general, central foveal vision (1°–2° diameter) is not important or useful for rod or scotopic vision because the rods are largely missing from that region.

Univariance

The rods obey univariance, which means that the photoreceptor response varies only according to the number of photons that are absorbed. All that changes with photon wavelength is the probability that a photon will be absorbed, not the photoreceptor response after it has been absorbed. Thus, individual rod photoreceptors are color blind: Changes in wavelength are indistinguishable from changes in intensity. As a direct consequence of univariance, the shapes of rod spectral sensitivity and scotopic spectral luminous efficiency functions are identical. Moreover, both types of measure obey Abney’s law (see the section on Psychophysical Measures of Luminous Efficiency), are independent of the psychophysical measuring technique and are uninfluenced by the spectral distribution (chromatic properties) of the adapting light [31].