What is a MacAdam Ellipse?

This is an elliptical region on the CIE chromaticity diagram that contains all the colours that are indistinguishable to the average human eye, from the colour at the centre of the ellipse. Adjacent ellipses are “just distinguishable” in terms of colour. This are used to refine the binning process of LED colours. Slight colour differences in the appearance are measured in MacAdam ellipses or steps.