What is the basis for 18% reflectance being the center of the lightness scale in many different systems? For example: Munsell, Ansel Adams zone system, and CIELAB. Coincidence? I doubt it.
You are correct that it is not a coincidence. The number you are referring to ranges between about 18% and 20% reflectance in various systems. There are two bases for this number. The first is that the average reflectance of natural scenes tends to be about 18-20%. This was measured by various researchers, perhaps the most famous of which was by Jones and Condit at Kodak in 1941. This value has been used to set proper exposures in photography. The second basis is human perception. The appearance of middle gray (perceptually half way between white and black) has a reflectance of about 20% (and hence the definition of Value 5 in Munsell or L*=50 in CIELAB). Is it a coincidence that the average reflectance of natural scenes and the human perception of middle gray have the same reflectance factor? I don’t think so. The human visual systems functions by sensing contrast from the background average and evolved in a world with this average reflectance. No coincidence there.
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