The optical density of the human lens changes during life. Literature concerning both the spectral density function and the rate of such changes is reviewed. Analysis indicates that two components govern the spectral lens density function, with one increasing gradually during life. The average lens density increases linearly at 400 nm by 0.12 density unit per decade between the ages of 20 and 60 and by 0.40 density unit per decade above age 60. A tabulation of the two components of the average 32-yr old lens is given, as are equations to derive the average spectral lens density functions for observers aged 20-80.

Several studies document rudimentary color vision under dim illumination. Here, hue perceptions of paper color samples were determined for a wide range of light levels, including very low light levels where rods alone mediate vision. The appearances of 24 paper color samples from the OSA Uniform Color Scales were gauged under successively dimmer illuminations from 10-0.0003 Lux. Triads of samples were chosen representing each of eight basic color categories; red, pink, orange, yellow, green, blue, purple, and gray. Samples within each triad varied in lightness. Observers sorted samples into groups that they could categorize with specific color names. Above 0.32 Lux, observers sorted the samples into the originally chosen color groups with few exceptions. For 0.1-0.01 Lux, the red and orange samples were usually correctly identified as either red or orange. The remaining samples tended to be grouped into two categories, associated with the scotopic sample reflectance. The lowest reflectance samples were below threshold and were named black. The higher reflectance group was named predominately as green or blue-green (three observers; the fourth observer used blue or achromatic). At the three dimmest levels (< or = 0.0032 Lux) there continued to be conspicuous color percepts. Color categories were reliably assigned based on relative sample scotopic lightness. Of the samples above threshold, those with lower reflectance were classified as red or orange (all observers) and the higher reflectance samples as green or blue-green (three observers) or achromatic or blue (the fourth observer). Rods and L-cones presumably mediated color percepts at the intermediate light levels used in the study. At the three lowest light levels there were distinct color appearances mediated exclusively by rods. We speculate that at these light levels the visual system estimates probable colors based on prior natural experience.

Heterochromatic modulation photometry is a method for obtaining equiluminance for a pair of heterochromatic lights presented in temporal alternation. A series of fixed standard luminance/test luminance ratios are presented, and at each ratio the modulation depth of the pair is reduced in tandem until the observer reports that flicker disappears. The data can be described by a luminance contrast template that appears V shaped when plotted on log-log coordinates. In the fitting of individual data, a free vertical scaling factor reflects the observer's sensitivity to luminance modulation and a free horizontal scaling factor reflects the observer's similarity in spectral sensitivity to the CIE standard observer. Data for red/green flicker photometric matches demonstrate the technique. Heterochromatic modulation photometry offers several advantages over flicker photometry: (1) a single fixed perceptual transition occurs on each trial series, namely, the transition from flicker to steady, and (2) luminance matches can be obtained at fixed frequencies at a number of luminance levels. The same procedure can be applied to the measurement of the minimally distinct border and to the identification of tritan pairs (stimulus pairs that differ only in their stimulation of short-wavelength-sensitive cones).

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People with normal trichromatic color vision experience variegated hue percepts under dim illuminations where only rod photoreceptors mediate vision. Here, hue perceptions were determined for persons with congenital color vision deficiencies over a wide range of light levels, including very low light levels where rods alone mediate vision. Deuteranomalous trichromats, deuteranopes and protanopes served as observers. The appearances of 24 paper color samples from the OSA Uniform Color Scales were gauged under successively dimmer illuminations from 10 to 0.0003 Lux~1.0 to Ϫ3.5 log Lux!. Triads of samples were chosen representing each of eight basic color categories; "red," "pink," "orange," "yellow," "green," "blue," "purple," and "gray." Samples within each triad varied in lightness. Observers sorted samples into groups that they could categorize with specific color names. Above Ϫ0.5 log Lux, the dichromatic and anomalous trichromatic observers sorted the samples into the original representative color groups, with some exceptions. At light levels where rods alone mediate vision, the color names assigned by the deuteranomalous trichromats were similar to the color names used by color normals; higher scotopic reflectance samples were classified as blue-green-grey and lower reflectance samples as red-orange. Color names reported by the dichromats at the dimmest light levels had extensive overlap in their sample scotopic lightness distributions. Dichromats did not assign scotopic color names based on the sample scotopic lightness, as did deuteranomalous trichromats and colour-normals. We reasoned that the reduction in color gamut that a dichromat experiences at photopic light levels leads to a limited association of rod color perception with objects differing in scotopic reflectance.

The PC pathway conveys both chromatic and achromatic information, with PC neurons being more responsive to chromatic (L-M) than to achromatic (L+M) stimuli. In considering the evolution of color vision, it has been suggested that the dynamic range of chromatic PC-pathway processing is tuned to the chromatic content of the natural environment. Anomalous trichromats, with reduced separation of their L- and M-cone spectral sensitivities, have diminished chromatic input to PC-pathway cells. Dichromats, with absent L or M cones, should have no chromatic input to PC-pathway cells. Therefore, the PC-pathway dynamic range of color defectives should be released from any constraint imposed by the chromatic environment. Here we ask whether this results in compensatory enhancement of achromatic PC-pathway processing in color defectives. This study employed a psychophysical method designed to isolate PC-pathway processing using achromatic stimuli. In a pulsed-pedestal condition, a four-square stimulus array appeared within a uniform surround. During a trial, one of the test squares differed from the other three, and the observer's task was to choose the square that was different. A four-alternative, forced-choice method was used to determine thresholds as a function of the contrast of the four-square array to the surround. Seven color defective and four normal observers participated. Results showed no systematic differences between normals and color defectives. There was no enhancement of achromatic processing as compensation for reduced chromatic processing in the PC-pathway system in color defectives. From physiological recordings, PC-pathway achromatic contrast gains of dichromatic and trichromatic New World primates and trichromatic Old World macaques have also been shown to be similar to each other. Our study and the animal studies imply that PC-pathway contrast gain parameters were regulated by factors other than the environmental chromaticity gamut, and may have arisen in a nontrichromatic common ancestor to both Old and New World primates.

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