Colors of monochromatic lights that vary in contrast-induced brightness

Using a color-naming procedure, two subjects described monochromatic lights, ranging from 450 to 630 nm, that were surrounded by perceptually unique-white fields of variable retinal illuminance. The test fields were 0.6 deg and 10, 100, or 1,000 Td. The surrounds, which were 4.5 deg in outer diameter, ranged from 0 to approximately 31,000 Td. From the resulting color-naming functions, equal-hue contours were derived, with surround intensity plotted against wavelength, for the spectral unique hues and the binary hues blue-green, green-yellow, and yellow-red. The wavelengths for unique blue, unique yellow, and blue-green were essentially invariant with changes in surround intensity. The spectral locus for unique green was also invariant at higher test-field intensities, but, at lower levels, it generally shifted toward shorter wavelengths as surround intensity increased. Nonmonotonic shifts were found for green-yellow and yellow-red. The contrast and the wavelength requirements for the color brown were nearly invariant with the changes of test-field intensity. Over the full range of surround intensities, subjects described the test fields as consisting of one or two hues plus white or black, depending on the surround level, but never (except for one subject at the lowest test-field intensity) white and black simultaneously and cospatially. This opponent aspect of black and white was compared with that associated with the chromatically opponent processes.