In addition, since Lab is a uniform color space close to human perception, the distance between two points in Lab space is often defined as color difference. Values of L*, a*, and b* can also be measured with a device such as a colorimeter (the device converts the tristimulus values by software). Specifically, the converted value of Y is L* (lightness), and the difference multiplied by a constant is a*, and the difference multiplied by a constant is b*. The values of L*, a*, and b* can be given by converting the values of XYZ with a simple conversion formula that expresses the characteristics of vision (see this graph).
Generally, human sensation tends to be proportional to the logarithm of the stimulus value, not to the stimulus value itself (the Weber-Fechner law). * Illuminant C is assumed for the Lab space above. On the other hand, in the Lab color space, the difference in the size of the ellipse is small, showing that the color space is much more uniform. In the xy color space, the size of the ellipse varies considerably from place to place. Usually, the "Lab" color system means the CIE L*a*b* color system.Īn ellipse represents the color difference discrimination limit (referred to as MacAdam's ellipse). In the Lab system colors are represented by three numerical values, L*, a*, and b* (* is read as "star"). Originally developed by Hunter, the CIE (International Commission on Illumination) made improvements in 1976 and called it the L*a*b* color system.
The Lab color system, as an absolute color space, was devised as a uniform color space closer to human perception (see the figure below). In the XYZ color system using tristimulus values themselves as the absolute color space, it is known that the amount of color difference that can be recognized by humans differs considerably depending on the color region.
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