A Radial Sampling of the OSA Uniform Color Scales

The OSA Uniform Color Scales were derived using a unique geometry for the physical samples. Regular rhombohedral packing allows each sample to be compared to twelve other equally distant samples. While this sampling scheme provides an efficient geometry for sample comparison and allows multiple cleavage planes, it obscures the underlying perceptual attributes. However, it is relatively straightforward to compute a radial sampling of data points in OSA. This radial sampling results in a distance from the achromatic axis and an angular quantity and can be used to compare other color spaces. This paper presents a method and considerations for computing the radial sampling. The utility of this data is demonstrated by comparing the perceptual uniformity of five different color spaces. Introduction Color spaces and color order systems provide a means to organize and describe color. A uniform color space is one in which equal differences in any part of the space correspond to equal perceptual differences. There is an abundance of prior and ongoing research in the area of uniform color spaces. The OSA Uniform Color Scales or UCS is one such color space and was the result of extensive psychophyisical investigation and other considerations. The UCS exists both as physical samples and in equation form. The samples are based on a regular rhombohedral sampling scheme. This sampling results in twelve equally spaced neighbors for any given color and yields the closest uniform three dimensional sampling. This sampling scheme has a number of advantages, including visualization of color differences along multiple cleavage planes and efficient sampling for psychophysical evaluation. However this sampling scheme can make it more difficult to visualize color space uniformity. For example the OSA UCS space has been compared to other color spaces and color order systems, such as CIELAB or the Munsell Book of Color, or as a qualitative demonstration of perceptual uniformity for a physiologically based system of photometry and colorimetry. However the radial structure of the Munsell Book of Color and the regular rhombohedral structure of the OSA UCS can make visual comparisons difficult. However, aside from computational considerations there is no specific limitation or restriction on calculating alternative sampling schemes for the OSA UCS. This paper reviews the OSA UCS, the forward transformations and then proposes a radial sampling of the OSA UCS. Finally as an example of how this data can be used, the CIELAB, CIELUV, CIECAM97s, IPT and CIECAM02 color spaces are compared to each other using the radial OSA UCS data.