Colour has Quantity and quality in tooth shade selection
The common units used in photometry are the lumen for luminous flux, the lux (lumen per sq m) for luminance on a surface, and the candela per sq m for the luminance or objective brightness of a surface.
The perceptual attribute corresponding to luminance is luminosity. Brightness is subjective if the object is a source of light, or lightness if the object is not a source of light. But the surface color is judged in relationship to its surroundings.
The quality of color needs a pair of concepts or variables to describe it. Three dimensions are needed. One method of dealing with this is to sort out all samples of a given lightness (eliminating luminance as a variable), when it is found that they can always be arranged systematically in a two – dimensional array.
The arrays of samples considered above, one array for each distinct level of lightness, could be placed on stiff cards and assembled with spacers one above the other with progressive increase in lightness.
The complete set of samples would now exist, set out in three dimensions, to delineate what is called a color solid or color space. Such a color solid could also be formed from the same large collection of samples in a different fashion by selecting all non-gray samples of a given hue and arranging them systematically on a vertical card with variations of lightness increasing upwards and variations of saturation increasing outwards.
This second method of arranging all possible colors of samples to make a systematic color solid has been much more popular than the one described first, and forms the basis of most types of color atlas.