By manipulating the intensity of three suitably chosen monochromatic lights, an observer can match a mixture of these three lights in luminance, color, and saturation to any other light of arbitrary spectral composition. This observation is the foundation of trichromatic theory. Once established, these physically different lights that are matched for the visual system (a color match) continue to appear to be the same to the visual system over a wide range of stimulus conditions. The generally accepted interpretation of trichromacy is that it is established at the first stage of the visual system, in the photoreceptors, and the visual system maintains a trichromatic organization at all subsequent levels. Thus two different light distributions that are a color match represent two physically different stimuli that cause identical quantal absorptions in the three cone photoreceptors. As a result, despite the physical difference between them, the two stimuli are physiologically identical. It is this strong link to photoreceptor responses that has made color matching a powerful technique for studying the spectra,1–6
and regeneration kinetics11,12
of the cone photoreceptors. In recent years there has been an increasing interest in the use of color matching to study individual variations in the extinction spectra3–6
of the cone photopigments.
However, it is also known that there are stimulus conditions that cause changes in color matches. These conditions consist of changing the retinal position or field size of the color-matching field,7,10,15
changing the retinal illuminance of a test field,7,10,16–19
and changing the entry position of a test field through the pupil of the eye.9,20,21
There are a number of potential reasons for these stimulus-dependent changes in color matches, including differences in the optical density of the photopigments and preretinal filtering by the optic media and macular pigment. An additional factor that was proposed22
but that did not receive widespread attention is the potential for variations in the waveguide properties of the cones to affect color matches. There is considerable evidence that cone orientation plays a role in the spectral sensitivity of the cones. It is well known that the color of a light changes with pupil entry, an effect known as the Stiles–Crawford II (SCII) effect.23,24
This color change is quantified by a change of the color-matching functions with pupil entry.22,24
Because there is a large degree of intraobserver variation in the location of the Stiles–Crawford (SC) peak,25
contributions of spectral variation in waveguiding could also be a significant contributing factor to individual variability in color matching. In addition, it was proposed that the spectral waveguide properties of the cones change with retinal illuminance.26,27
The size of any possible waveguide effects on spectral sensitivity is limited. Brindley and Rushton28
showed that the change in color of monochromatic lights when the retina was transilluminated was of the same order as the SCII effect measured through the pupil. Miller8
both showed that optical density was a major factor in determining the wavelength dependence of the directional sensitivity of the cones in dichromatic observers. Wyszecki and Stiles19
showed that, by assuming that the only factor changing with bleaching was the optical density of the cones, they could derive reasonable photopigment spectra, although the actual spectra at which they arrived are somewhat different from other estimates of photo-pigment spectra, a result reinforced by the analysis of MacLeod and Webster.6
found that bleaching dichromats decreased the wavelength dependence of their Stiles–Crawford I (SCI) effect. Likewise, Alpern9
showed that bleaching the photopigments decreased the effect of pupil entry position as would be expected if the major portion of the SCII effect were due to changes in optical density with pupil entry. However, Alpern et al.22
later found that the effects of changing pupil entry position on full-spectrum color matches in the unbleached state could not be attributed to self-screening alone. There is also indirect evidence that waveguides may not play a strong role in determining the absorption spectra of the cones. In patients with retinal diseases that severely affect the orientation and morphology of the cones, color matches are equivalent to those of a bleached normal observer.12,33
This result can also be seen by comparing the data of Smith et al.34
with bleached color matches from the same laboratory.7
In this paper we describe experiments that test whether optical-density changes alone can account for the effect of changes in retinal illuminance and pupil entry. We take the experimental approach of directly manipulating the optical density of the cones by bleaching. All these experiments are performed at long wavelengths (>540 nm, called the Rayleigh region) for two reasons. First, this is the spectral region in which the simplifications and accuracy of a two-primary match are available. Matches must be precise to allow us to test for the presence of a factor other than optical density with reasonable power. Second, there are other factors adding to variability in the blue region of the spectrum, such as changes in absorption because of lens and macular pigmentation.
In experiment 1 we measure the variation in color matches in the Rayleigh region with variations of the standard wavelength and retinal illuminance for color-normal observers. We then ask whether a reasonable selection of photopigment spectra and optical densities can fit the effects of both changing the standard wavelength and bleaching the cones to a lower optical density. In experiment 2 we measure color matches as a function of retinal illuminance, keeping the standard constant but varying the mixture primaries. With this technique, the physiological stimulus (determined by the standard light) is held constant as the physical composition of the mixture field is varied by changing the wavelength of the red primary. This design tests whether the physical composition of a stimulus affects the change in color matches at high retinal illuminances. Experiment 3 directly compares the effects of changing pupil entry position under bleached and unbleached conditions.