Editing Metamerism (color) (section)

== Measuring metamerism ==
{{main|Color rendering}}
The best-known measure of metamerism is the [[color rendering index]] (CRI), which is a linear function of the mean [[Euclidean distance]] between the test and reference [[spectral reflectance]] vectors in the [[CIE 1964 color space]].  CRI has been replaced with an updated metric, IES:TM30, that provides a more accurate assessment of fidelity and adds features for assessing how a test light will change the saturation and hue of colorants as compared to the reference light.<ref>{{Cite journal |last=Royer |first=Michael P. |date=2022-04-03 |title=Tutorial: Background and Guidance for Using the ANSI/IES TM-30 Method for Evaluating Light Source Color Rendition |url=https://www.tandfonline.com/doi/full/10.1080/15502724.2020.1860771 |journal=LEUKOS |language=en |volume=18 |issue=2 |pages=191–231 |doi=10.1080/15502724.2020.1860771 |issn=1550-2724}}</ref>  Another metric, for daylight simulators, is the '''MI''', the CIE metamerism index,<ref>{{Cite web |url=http://www.cie.co.at/publ/abst/15-2004.html |title=CIE Publication 15 |access-date=2008-01-19 |archive-url=https://web.archive.org/web/20080213035758/http://www.cie.co.at/publ/abst/15-2004.html |archive-date=2008-02-13 |url-status=dead }}</ref> which is derived by calculating the mean [[color difference]] of eight metamers (five in the [[visible spectrum]] and three in the [[ultraviolet]] range) in [[CIELAB]] or [[CIELUV]]. The salient difference between CRI and MI is the color space used to calculate the color difference, the one used in CRI being obsolete  and not [[perceptually uniform]].

MI can be decomposed into MI<sub>vis</sub> and MI<sub>UV</sub> if only part of the spectrum is being considered. The numerical result can be interpreted by rounding into one of five letter categories:<ref>[http://www.knt.vein.hu/staff/schandaj/SJCV-Publ-2005/397.doc CIE Standards for assessing quality of light sources] {{webarchive|url=https://web.archive.org/web/20110112155836/http://www.knt.vein.hu/staff/schandaj/SJCV-Publ-2005/397.doc |date=2011-01-12 }}, J Schanda, [[Veszprém]] University, Department for Image Processing and Neurocomputing, Hungary</ref>

{| class="wikitable"
|-
! Category
! MI (CIELAB)
! MI (CIELUV)
|-
| A
| <&nbsp;0.25
| <&nbsp;0.32
|-
| B
| 0.25–0.5
| 0.32–0.65
|-
| C
| 0.5–1.0
| 0.65–1.3
|-
| D
| 1.0–2.0
| 1.3–2.6
|-
| E
| >&nbsp;2.0
| >&nbsp;2.6
|}