Depending on the measurement taken, the participant could either manipulate the test light in isolation until a particular wavelength value was obtained (i.e., perception of unique green), or alternatively the test light could be used as a reference to which the mixing light was matched by varying the amount of each primary contained in the mixture. This half could also be occluded completely for performing unique green settings using just the bottom half of the field (resulting in a 0.67° × 1.33° viewing angle), which contained the “test” light. The top half of the field contained the “mixing light,” which could contain all, or a selection, of primaries blue, green, and red set at 460 nm, 530 nm, and 650nm, respectively. A square, bipartite viewing field (1.33° × 1.33°) was viewed monocularly through an eyepiece fitted with a doublet to counteract chromatic aberration of the human eye. This observation is highlighted by Kuehni ( 2004), who compiled data from various studies and found that unique yellow settings vary across a range of approximately 15 nm while the equivalent range for unique green is 54.6 nm.Ī Wright Colorimeter (Wright, 1928, 1939), originally built at Imperial College London in the early 1930s, was used for making Rayleigh matches, unique green settings and measurements of Macular Pigment Optical Density (MPOD). While all unique hues show some variability across subjects, the amount of variance is different for different hues and it has been found that unique green settings are the most variable while unique yellow settings are comparatively stable (Kuehni, 2004 Nerger, Volbrecht, & Ayde, 1995 Schefrin & Werner, 1990 Webster et al., 2002 Webster, Miyahara, Malkoc, & Raker, 2000). Later studies then began highlighting the distribution of the wavelength settings across the population for each of these unique hues (Dimmick & Hubbard, 1939 Purdy, 1931). During the late 19th century and early 20th century, a number of studies were carried out into the spectral locations of the unique hues (Hering, 1890 Maerz & Paul, 1930 Verbeek & Bazen, 1935 Westphal, 1910 von Bezold, 1876-reviewed by Dimmick & Hubbard, 1939). Subjects are able to set these points reliably using a monochromator and perceive them as being “unique” in that they do not contain a mixture of any other color, for instance, unique green (UG) appears neither yellowish nor bluish (Jordan & Mollon, 1995 Schefrin & Werner, 1990). The “unique hues” of red, green, yellow, and blue have been recognized as having special perceptual properties for well over a century. We model this effect using simulations of monochromatic unique green matching to broadband illuminations and show that matches in the region at approximately 500 nm exhibit particularly high variance both with respect to macular pigment density and also with respect to the precise shape of the broadband reference exemplar spectrum. Our results indicate that unique green wavelengths in our population are distributed unimodally and are correlated positively with macular pigment density individuals with a higher density of macular pigment select longer wavelengths of light as unique green than individuals with a lower density of macular pigment. We have also explored the possibility that individual differences in macular pigment density may be responsible for the variation in unique green wavelength. We have investigated this claim using a Wright colorimeter to measure the unique green wavelength of 58 participants and we have analyzed previous unique green literature by applying a rigorous statistical test to historical datasets. Some researchers have reported that this broad distribution of UG settings is bimodal and that the distribution results from the superposition of two or more subpopulations. Abstract Monochromatic unique green (UG) is more variable across the population than any other unique hue.
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