A new look at colour rendering

A new look at colour rendering

A widely used system for assessing color perception in a specific light, developed by the CIE’s International Commission on Illumination, called the CRI Color Rendering Index, is increasingly criticized and viewed as inadequate. 

By using the new TM30-15 method and the determination of the Rg index, a useful aspect of colour saturation change vector graphics has been created, in which the influence of the light source on specific colours can be quickly and easily recognised.

The arrows indicate whether the light measured against the reference source reduces colour saturation or increases. In addition, you can also see the extent to which the colour shade differs from the reference light. Each arrow identifies 1 of the 16 colour cells. The TM30-15 method therefore introduces a lot of changes and presents more information and eliminates many errors occurring in CRI(Ra). It should be expected that the new method may soon replace the already used colour rendering index.

The CRI system, developed by CIE, is increasingly becoming criticised for being based on far too small a range of test colours to be taken into account in determining Ra values. In addition, there is a lack of saturated test colours in the samples, which contributes to a situation where two different light sources have the same Ra-value but a significantly different colour intensity. Colour mapping alone is therefore not an overall measure defining light sources where the colour parameters characterising intensity/saturaturation are not included.

The imperfections of CRI have led to a completely new system developed by the IES Illuminating Engineering Society of North America called TM 30-15.

The colour reproduction of a light source is described by two indicators:

• The Rf fidelity index describing the similarity of the light source tested to the reference source, on 99 colour samples. It basically describes the same as the traditional Ra factor, but the values themselves cannot be compared. The index Rf shall in most cases be less than the value of Ra. The new Rf index is expressed by values between 0 and 100

• The Gamut Rg index, which indicates the change in colour saturation. In determining the index, the coordinates of the sample illuminated by both the test light and the reference light are determined. 16 cells are determined from their chromaticity coordinates, which were determined from the test light. The average of each cell determines the points which form planes for both the test and reference light. In the case of Rg, the values may be greater than 100, which indicates greater saturation than the reference source.

However, the CIE has issued document 224: 2017 defining the Rf fidelity index, which has been modified from document TM30-15. It is based on the calculation of the Rf fidelity index in the IES TM-30-15, with only a few minor changes. These modifications result in small differences in Rf values and have little impact on index values for typical light sources. 

At the LED line® Light Research and Measurement Centre, thanks to advanced software, new indexes are determined in accordance with method TM30-15 as well as CIE224:2017

However, to ensure that the Rf index is understood and calculated equally, IESNA has issued a revised document TM-30-18 leading to the standardisation of the Rf index. This is the first step towards the globalisation of the new colour fidelity index. It is therefore only necessary to wait for the new method to officially replace the existing Ra marking system.

Nevertheless, due to consumer habits and general knowledge of the CRI, it should remain, and be a recognisable addition to the new method

TM30-15 and its modifications TM30-18

Norbert Chrzanowski

Head of Research and Development LED line®

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