In 1931 the CIE (Commission Internationale de l’Eclairage) published its famous Chromaticity graph. It meant that every colour that could be created by a mixture of red-green-blue could be given a unique number on the graph. That graph, and its successors, have been the mainstay of light-colour science ever since.
In 1942, David MacAdam, a physicist and colour scientist working with Kodak, posed the question: does every point on the CIE graph really represent a different colour ?
He ran a series of tests and demonstrated that, rather than a single colour being indicated by a point on a graph, it could be represented by an ellipse surrounding that point. Within the ellipse there would be no discernible difference in colour from the reference point at its centre.
And that was the birth of the MacAdam Ellipse.
The 1931 Chromaticity graph
Why is it important?
Until the introduction of the LED there was very little practical use for the MacAdam ellipse. Light sources were mass-produced and colour tolerances were sufficiently controlled that very few people complained about colour inconsistencies between two lamps.
However, with the introduction of the LED module, the method of light production changed. The manufacturing process is such that millions of LED chips are produced from an assembly line every day and it’s inevitable that there will be colour inconsistencies between individual chips and the modules built from them.
In the early days of LED production, quality of product was determined by the accuracy of the binning process of chips. This was the way that LEDs were separated into separate bins according to their colour output. It was an expensive and time consuming process. Improvements in LED module design have brought the Macadam Ellipse to the forefront of the colour checking process.
A perfect LED module assembly line will produce batches of modules operating within a once MacAdam Ellipse. There will be no discernible difference between any of the module outputs. LED modules produced at this level are used where colour performance and accuracy between fixtures is vitally important. Typically, good LED modules are produced within a two to three MacAdam ellipse range, here will be a visual difference if you look for it, but it is minor and generally considered to be acceptable in commercial usage.
Cheaper products will often use LED modules that have a range of MacAdam Ellipses beyond four, some going as high as eight. Fixtures using such modules need to be used with care. There may be general commercial of industrial areas where they are acceptable, but any requirement for colour sensitivity would rule them out.