The “unique hues” of red, green, yellow, and blue have been recognized as having special perceptual properties for well over a century. 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). 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). 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). 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). 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.
Despite general agreement that there is more variation in unique green wavelength settings than any other unique hue, it is unclear why this should be. Several studies have argued that the distribution of unique green is bimodal (Cobb,
1975; Richards,
1967; Rubin,
1961; Waaler,
1967), suggesting that the broad distribution observed is actually the superposition of two narrower distributions. However, further studies have not supported this claim (Hurvich, Jameson, & Cohen,
1968; Jordan & Mollon,
1995; Metz & Balliet,
1973; Schefrin & Werner,
1990).
Here, we explore the statistical distribution of unique green wavelength settings in our own measurements and in historical datasets using a modern statistical test for unimodality. We also investigate two possible causes for the variation of UG across individuals: variation in macular pigment optical density (MPOD), and slight variations in the spectra of unique green exemplars.