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What is the relationship between the CIELAB a-b opponent axes and the perceptual unique hues?

April 26, 2017Axes CIELAB hues opponent perceptual relationship

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What is the relationship between the CIELAB a*-b* opponent axes and the perceptual unique hues?

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The unique hues do not directly line up with the a*-b* axes. One reason for this that we perceive different numbers of discriminable hues between each of the four unique hues (e.g. we see more purple hues between unique red and unique blue than greenish-yellow hues between unique yellow and unique green). In terms of CIELAB hue angle, the unique hues fall at approximately R:24 deg., Y:90 deg., G:162 deg., and B: 246 deg. Only unique yellow happens to line up with a CIELAB axis (positive b*).