In the early 1940’s, the scientist David MacAdam tested the threshold sensibility of the eye to chromaticity variations. He tried to provide a guideline of how accurate the average people’s colour perception is and their ability to distinguish between similar colours.

The observer had to match a target colour against another colour of the same range and identify which one of these hues corresponded to the reference colour. Each difference between the target colour and the sample colour detected as identical was plotted in the 1931 CIE chromaticity colour space as a point. Joining all these points, an ellipse comes out. Then the scientist asked each subject to repeat the experiment on 25 different target colours. As a result, he obtained 25 ellipses, the so called “MacAdam Ellipses”. Each MacAdam ellipse can be divided into 7 steps called “standard deviation of colour matching” (SDCM). For the average human eye, colour changes within the first 3 MacAdam steps are undetectable.