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Colorimetric matches between light from objects and a three-primary visual colorimeter are generally metameric, not spectral. The amount of difference between the spectral energy distributions from the object and the calorimeter determines the degree of metamerism. Three indexes of metamerism are considered that depend on this difference. One of these indexes is shown to correlate well with the chromaticity spreads found among observers’ settings for a series of metameric matches. This correlation is independent of luminance. The other indexes do not correlate with the spread of chromaticity of matches by observers.