MacAdam ellipse

In the study of color vision, a MacAdam ellipse is roughly a region on a chromaticity diagram which contains all colors which are indistinguishable, to the average human eye, from the color at the center of the ellipse. Specifically, it is the standard deviation of a number of experimental color matches to the central color. Assuming a bivariate normal distribution of these match points, a MacAdam ellipse thus contains about 39% of the color match points. A 2X MacAdam ellipse will contain about 86% of the match points, and a 3X MacAdam ellipse will contain about 99% of the match points. The just-noticeable differences of chromaticity is generally taken to be a 3X MacAdam ellipse. Standard Deviation Color Matching in LED lighting uses deviations relative to MacAdam ellipses to describe color precision of a light source.