Colour rendition refers to how accurately an artificial light source reveals the colour of an object compared to how that colour appears when illuminated by a natural light source.
Colour Rendering Index (CRI)
CRI is measured on a scale from 0 to 100 and indicates the accuracy of a light when compared to a reference light source. A reference light source, such as incandescent or daylight, is used as a benchmark for the accuracy and suitability of the light. The closer the CRI is to 100, the more accurate the light source will render a color’s true appearance. For example, a CRI of 95 represents considerable value, while a low CRI rating indicates that some colours will not appear as vividly when illuminated.
How is CRI Calculated?
The value of CRI for a light source is calculated by testing colours. The “Commission Internationale de l’éclairage” (CIE) established a scale of 8 CIE Standard Colour Samples for the CRI method. There are an additional 7 CIE Special Colours that can be used, giving a total of 15 colour samples. The general Colour Rendering Index looks at the first 8 colour indices (Ri). Ra is the average of the 8 Ri values and is stated as CRI with a maximum value of 100. Extended CRI refers to the remaining 7 colours (9 to 15) including the saturated colours and skin tones. This is also often referred to as Re.
The test involves comparing either 8 or all 15 colour samples under the light source and then comparing it to a reference light source, usually sunlight. The average differences are then subtracted by 100 to get the CRI value. That is why light sources that show more “real” colours have higher CRI values, the average differences are smaller between the light source and the reference sunlight.
R9
R9 is one of the test colour samples used in the calculaton of extended CRI. It represents how accurately a light source will reproduce strong red colours. It is of special interest because skin or flesh tones are represented by this colour. For example, a high R9 rating can tell you if apples will appear fresh and vibrant.
California Title 24 notoriously requires 90+ CRI for residential light sources. It also calls for a minimum R9 of 50 in homes. The WELL Building Standard, feature 58: Color Quality also calls for a minimum R9 of 50.
TM-30-15
TM-30-15 is a new method created by the Illumination Engineering Society (IES) to assess the colour rendering performance of a light source. It has a comprehensive set of 99 reference colours. The new standard also quantifies colour saturation using the Gamut Index. Additionally, TM-30-15 also gauges how closely colour appearance of objects reproduce (render) compared to the colour appearance of objects under a reference light using the Colour Fidelity Index.
Learn more about the IES Method for Evaluating Light Source Colour Rendition.