A fundamental parameter for assessing the quality of LEDs is the index MacAdam that is useful for identifying the color constancy.

In the early 1940s, David L. MacAdam performed an empirical experiment to define the sensitivity of the human eye to the color.

The result was the definition of ellipses to describe the color distances on the XY coordinates of the chromatic scale. The area enclosed by each ellipse includes all the colors identical to the standard reference color, the size of the ellipse is given in SDMC (Standard Deviation of Color Matching) and is evaluated on a 7-point scale.