Colour rendering
MacAdam and CRI (Ra) are two key concepts in lighting that help us to define which colours are perceived as equivalent to the human eye and measure how well a light source renders colours compared to natural light. These concepts are important to understand to create the desired atmosphere and colour quality.
LED colour temperature
Although lighting manufacturers specify the same colour temperature for their LEDs, there may be differences between them. The colour temperature of an LED can also change over time. Even in the production of the LEDs, there is great variation in both colour temperature and luminous flux. Therefore, the LEDs are sorted to make it possible to select LEDs from a limited range – a process called LED binning.
The light colour quality or chromaticity of an LED module is a measurement of dispersion in the light colour. Colour deviation of LED modules is indicated by MacAdam ellipses, also called SDCM (Standard Deviation Colour Matching), according to the CIE 1964 standard.
MacAdam specifies dispersion in the light colour
A MacAdam ellipse is an area of the colour triangle where the colour differences within the ellipse are indistinguishable from the colour at the centere of the ellipse to the average human eye. The MacAdam ellipses are different sizes in the colour spaces because individuals have different abilities to see differences depending on the colour.
The MacAdam ellipses represent the noticeable differences in chromaticity. Standard Deviation Colour Matching in LED lighting uses deviations from MacAdam ellipses to describe the colour accuracy of a light source. A large variation in colour quality results in a larger ellipse and a higher MacAdam value.
The MacAdam system rates chromaticity (the strength, purity and saturation of a colour) on a scale of 1-10, with 1 being the best.
Colour quality in different applications
Between 1 and 3 it is difficult to see a difference in colour, but further up in the scale you can experience clear colour differences in the light, especially on white walls that are illuminated with wallwashers or spotlights. A good rule of thumb is to avoid MacAdam 4 SDCM or higher indoors, although requirements usually state MacAdam 2-3 SDCM. In most outdoor environments, the MacAdam 5 SDCM can be considered fully adequate.
The following MacAdam values can be used as a guideline:
- Macadam 2 SDCM: museums, galleries
- Macadam 3 SDCM: offices, shops, schools, hospitals, outdoor exterior lighting
- Macadam 5 SDCM: other outdoor applications
Colour rendering
The concept of colour rendering uses the light bulb as a reference. The Colour Rendering Index (CRI) or Rendering Average (Ra) scale ranges from 0-100 and is a measure of a light source’s ability to render colours. A light bulb has a CRI/Ra of 100, and the colour rendering of an LED is given on the same scale. Colour rendering, depending on the LED, can typically vary from 70 to 95.
Colour rendering is assessed according to the CIE method and is normally measured using eight colours according to CRI Ra where the index runs from 1-8 (see illustration below). Colour rendering is given as an average value. A light source can thus reproduce seven of these well but the eighth worse. As a supplementary scale, Ra 1-14 contains six additional colours.
As the illustration shows, the current LED is not able to optimally reproduce the red colour (number nine). As a result, the average value for Ra 1-14 is lower than for Ra 1-8. Regardless of the average value, it can still be seen that rendering of red is lower, which is common for LED lighting.
General Colour Fidelity Index
In January 2025, the International Commission on Illumination (CIE) declared that the CRI system will eventually be replaced by a system called the General Colour Fidelity Index. Here the result is given as an Rf value, based on the rendering of 99 colours. The system has been evaluated since 2017 (publication CIE 224:2017). As the CRI system has been around for a long time and is well established, both CRI (Ra) and Rf will be published on product sheets for the foreseeable future.
