| There is some conflation here between an art definition and the colorimetric definition. In colorimetry: Luminance is a spectrally weighted measure of physical light, it is not a perception, and luminance is linear to physical light. Lightness is the perception of luminance, along with darkness and brightness. In colorimetry, the term lightness is used to distinguish the perception of light (which is nonlinear to physical light) as opposed to luminance which is a linear measure of physical light. Luminance is denoted Y (relative) or L (absolute), Perceptual lightness is L* (Lstar) or J, brightness as Q (in most models). As you noted, the term lightness can also be used as a tonal metric (i.e. in art). However in colorimetry, the closeness to white, or achromatic, is described as less purity (less chroma or less saturation). And for the record, the site is not auto-generated. Nate Baldwin created that site. Nate is also the developer behind Adobe Leonardo, the color palette utility. As for the Helmholtz–Kohlrausch phenomenon, here is the official definition from the CIE - International Commission on Illumination Change in brightness of perceived colour, produced by increasing the purity of a colour stimulus while keeping its luminance constant within the range of photopic vision Note 1 to entry: For related colours, a change in lightness can also occur when the purity is increased while keeping the luminance factor of the colour stimulus constant. ----- The CIE points out that the change in perceived lightness occurs with increased saturation (purity). |