Vivianne C. Smith

Brightness of equal-luminance lights

The brightness of lights defined by heterochromatic flicker photometry as being of equal luminance was determined by direct comparison with a white standard. Stimuli included mixtures of white and colored lights. Eight different colorimetric purities were tested for each of eleven primaries. Adaptation to a known white was maintained. Resulting brightnesses were expressed in terms of the ratio of the amount of light required for a brightness match to the amount required for a flicker photometric match (B:F). B:F ratios varied with the primary, being largest for red and blue primaries. The variation of B:F ratio with colorimetric purity differed depending on the primary, although for any given dominant wavelength the highest-purity stimulus always had the largest B:F ratio. The data qualitatively exclude a number of simple models of brightness generation.

Metameric Matches Relevant for Assessment of Color Vision

In a color matching procedure, an observer attempts to match the appearance of a test light with a mixture light by adjustment of three primary lights; the test and primary lights are arranged pairwise in the test and mixture fields. The actual appearance of both fields is irrelevant; the importance of the procedure lies in their identical appearance. Once a match is established, the test and mixture fields are said to be metameric.

Improved Clinical Technique For Wald-Marré Functions

We have described a computer-controlled technique to assess Wald-Marre functions. The background fields are similar in spectral composition to those used by previous authors, however flicker rather than discrete flashes was used for the test stimulus. For estimation of long- and middle-wavelength cone sensitivities, a flicker rate of 15 Hz was used; for the short, 2.4 Hz was used.

S-cone discrimination sensitivity and performance on arrangement tests

Previous psychophysical data from our laboratory have revealed that tritan discriminations are characterized by a set of luminance-dependent threshold-vs-radiance templates. The templates were expressed in units of S-cone trolands. In this communication, we used the templates to evaluate performance on four arrangement tests: the Farnsworth-Munsell 100-hue test, the Farnsworth Panel D-15 test, the Lanthony desaturated Panel D-15 test and the Lanthony New Color Test. We calculated S-cone excitation in S cone trolands for the test caps and derived a value of AS for given cap errors at four illuminance levels. Our calculations showed that tritan errors were expected at reduced illuminations for all tests. The level of illumination at which tritan errors were predicted was consistent with published data.

Assessment of S-cone sensitivity

Several recent analyses of the Short-wavelength-sensitive (S-) cone threshold vs. retinal illuminance (TVR) function suggest that it should be possible to differentiate a number of potential types of insult with psychophysical procedures. An assumption frequently made in modelling sites of disease action is that S-cone sensitivity is independent of the activity of L- and M-cones. We review increment threshold S-cone isolation techniques that have been used in clinical measurements. These experimental conditions produce S-cone sensitivity estimates which include sensitivity regulation of the S-cone system by the L- and M-cone types. TVR functions measured with a discrimination paradigm along a tritan line yield a different pattern of sensitivity regulation by L- and M-cones. For some purposes paradigms incorporating equiluminant stimuli along tritan lines may be of utility in estimating S-cone sensitivity.

Aberrant flicker sensitivity revealed by heterochromatic modulation photometry

Heterochromatic modulation photometry is a method in which 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 expectation is a distinct minimum modulation sensitivity at the standard/test ratio representing the luminance match. At other luminance ratios, flicker sensitivity should vary with the luminance difference between standard and test. We have devised theoretical templates to describe modulation sensitivity as a function of standard/test ratio. The results of other flicker studies indicate that flicker sensitivity would be expected to fall within the domain between a linear system (sensitivity proportional to the amplitude of modulation) and a Weberian system (sensitivity proportional to percent modulation). Using ‘red’ and ‘green’ lights, at low photopic luminances this expectation is well realized. At higher luminances and when the ‘red’ light is of higher luminance than the ‘green’, observers are much less sensitive to modulation than the models predict.

Sensitivity of screening tests for S-cone discrimination

We evaluated tritan screening tests in color normals by producing S-cone discrimination loss with reduced illumination. The Tokyo Medical College plates, the Rabkin plates, the Velhagen plates, and the Minimalist test were compared to the Standard Pseudoisochromatic Plates, Part 2 (SPP-2) for which we have previously reported data. We used six light levels ranging from 0.1 to 63 lux. Seventeen observers with normal color vision served in the experiment; seven older observers (median age 49) and ten younger observers (median age 28). All performed all tests correctly at 63 lux. There were statistically significant differences between the younger and older groups on all tests. For pseudoisochromatic plates, the Rabkin plates were the least sensitive to reduced illumination while the Tokyo Medical College plates showed high sensitivity. The four plates of the Velhagen test, showed differing sensitivity. The error scores on the Minimalist test increased with decreasing illumination level. The error scores of the tritan series were always greater than those of the protan and the deutan series. The Velhagen and the Minimalist test allow estimation of graded sensitivity loss comparable to the SPP-2.