Colour rendering evaluation IES TM 30
The globally recognised colour rendering rating with the colour rendering index (CRI) was deemed too superficial by some American LED manufacturers. The eight test colours defined in the CIE 13.3 (1995) test procedure are not colours often seen in daily life, and their low number cannot reflect the range of all possible colours either. A second concern was that only colour fidelity and not colour saturation is considered. The Vienna-based International Commission on Illumination (CIE Commission International de l'Eclairage) did not want to endorse a new evaluation system.
Thus, American standardisers from the IES (Illuminating Engineering Society), based in New York, set about creating a more comprehensive colour rendering evaluation matrix. Colour fidelity was to be evaluated more extensively, and colour saturation was also to be considered. The IES TM 30-15 colour rendering rating is intended to complement the CIE 13.3 (1995) colour rendering rating. The CIE did not recognise the TM 30-15 rating. However, the Rf TM 30 colour fidelity assessment can still be used in Europe for scientific purposes. Regardless, the TM 30-15 method of evaluating light quality is useful and has become well-established in the American and Pacific regions. TM 30-15 has not yet become established in Europe.
The revised TM 30-18 version has been approved by ANSI (American National Standard Institute) as ANSI / IES TM 30-18.
Colour fidelity evaluation Rf (Rendition fidelity)
Instead of eight test colours, 99 test colours that appear in daily life were selected. The 99 test colours are evenly distributed in the colour space. The colour fidelity evaluation is highest when the test light source's coordinate (red square) is identical to the reference's (black dot) coordinate.
Colour saturation evaluation Rg (Rendition gamut)
To determine the colour saturation, the Rg value, the colour space was divided into 16 segments. All colour points in a segment are averaged to form a common colour point.
Display of the colour saturation rendition Rg
For the apparent representation of the saturation deviation, the colour space was stretched so that the 16 averaged colour points are on a circle (black line). The representation shown on the right is called the colour vector graphic. The red line shows the results of the test light source. The deviation between the test light source and the reference is clearly visible in this graph and is marked by black arrows.
Colour saturation evaluation Rg (Rendition gamut)
The colour distortion icon shows the differences to the reference even more clearly. The image on the left shows the colour distortion icon. The reference is a white line on a black background. The oversaturation or undersaturation is very clearly visible.
Rendition fidelity & Rendition gamut
The average of all 99 test colours is output as Rf (f for fidelity). To evaluate the colour saturation, the Rg value (g for gamut) has been defined. These two values are determined together.
For Rf, the highest value is 100; for Rg, a maximum value of 140 is theoretically possible. The closer the Rf value is to the maximum value of 100, the closer the Rg value is to the maximum of 100.
Joint graphical representation Rf and Rg
Which values are good, which values are excellent?
- good: Rf > 80; Rg between 85 and 115, red dashed line
- very good: Rf > 90; Rg between 90 and 110, yellow dashed line
- excellent: Rf 100; Rg 100, green dot
Where are the well-known CRI ≥ 80 and CRI ≥ 90 colour ratings in this matrix?
- typical CRI ≥ 80 lights sources are slightly below the value of Rf 80 and have a low saturation of Rg 80 pink range
- typical CRI ≥ 90 lights sources are slightly below the value of Rf 90 and have a saturation in the range of Rg 90 light yellow range
