MacAdam ellipses are used to determine colour differences. They constitute the areas in the colour diagram in which the comparative colours surrounding a reference colour are perceived to be an equal colour distance apart (MacAdam; American physicist). Colour tolerances are practically imperceptible up to a value of 3 MacAdams. Colour differences become visible at higher values and these differences may increase over the service life. The CIE chromaticity clearly shows that the MacAdam ellipses in the green and yellow zones are considerably larger than in the blue or violet zones.
This effect shows that the human eye is less capable of recognising colour differences affecting green LEDs than differences between blue LEDs.