LED colour homogeneity

High-quality indoor lighting solutions require pure white colour. White light with a colour tint is undesirable. But at which point is the human eye able to differentiate between two colour points? David Lewis MacAdam already tried to tackle this question as early as the 1940s. Several light colours were compared with a defined colour point under CIE 1931. Classification was then made according to whether the same colouring or unequal colouring was apparent. It was found that all of the colour points of the same colour could be circumscribed with an ellipse. It is now recognised that within these elliptical surfaces and their centre, differences in colour between colour points cannot be detected. The unit, consequently known as the “1 Step MacAdam’s Ellipse”, is now taken as the measure of colour homogeneity. 

To define larger ranges in colour, the radius of the ellipse is incrementally increased - doubled, tripled, and so on - thus resulting in a 2 step or 3 step ellipse, etc. However, as MacAdam only drew conclusions on 25 colour points (see graphic), the unit can be converted to any colour point in the CIE 1931 standard using the SDCM (Standard Deviation Colour Matching) scale. Example: A 3 Step MacAdam ellipse can also be specified as “≤ 3 SDCM”.