Alliance for Lighting Information

Color Rendering Index Values for Exterior Sources

by David M. Keith

The rendering of colors by some MH and LED lamps may be considered one of the most significant benefits from such premium sources.

The most widely cited metric of a source's rendering capability is Color Rendering Index (CRI), defined in the CIE publication 13.3-1995 "Method of Measuring and Specifying Colour Rendering Properties of Light Sources" including specifically defined calculation procedures and surfaces. In section 5.3 "Tolerance for reference illuminant", the term "chromaticity difference" (DC) is defined. The section also states:

"If the chromaticity difference between the lamp to be tested and the reference illuminant is greater than the tolerance of DC = 5.4 10-3 the resulting Colour Rendering Indices may be expected to become less accurate."

A summary of the CRI and associated values calculated according to CIE 13.3-1995 for available SPDs is shown below, with the DC values, and sources with DC over the specified criterion of 0.0054 are identified:

Name			  X	  Y	 CCT	 CRI	  DC	

Skylight			95.41	100	11385	99	0.0008	
Daylight (D65)		95.05	100	6502	100	0	
LED cool white		92.53	100	6273	75	0.0046	
Fluorescent 6500K		93.79	100	6088	87	0.0025	
LED			97.91	100	5899	65	0.0046	
Sunlight			96.57	100	5517	99	0.0016	
Fluorescent		93.05	100	5193	76	0.0057	*
Metal halide		93.86	100	4917	65	0.0084	*
LED			94.44	100	4863	71	0.0075	*
LED neutral		100.41	100	4466	81	0.0020	
Fluorescent		95.3	100	4452	82	0.0079	*
Fluorescent		90.79	100	4415	5	0.0173	*
Metal halide		94.83	100	4319	68	0.0098	*
Fluorescent		93.47	100	4279	80	0.0129	*
Metal halide		102.08	100	4181	91	0.0035	
Metal halide		94.9	100	4160	64	0.0109	*
Metal halide		101.27	100	4086	90	0.0013	
Metal halide		96.64	100	4061	53	0.0082	*
Fluorescent		99.27	100	4009	79	0.0033	
Metal halide		96	100	4002	64	0.0101	*
Fluorescent 4100K		101.65	100	3813	87	0.0004	
Metal halide		98.81	100	3776	58	0.0067	*
Mercury Vapor coated	97.1	100	3758	53	0.0105	*
Ceramic Metal Halide	102.88	100	3618	65	0.0002	
Ceramic Metal Halide	103.16	100	3610	65	0.0003	
Fluorescent		100.86	100	3560	75	0.0052	
Fluorescent 3500K		105.1	100	3326	87	0.0002	
Metal Halide		109.09	100	3068	82	0.0043	
Fluorescent		106.6	100	3056	82	0.0018	
Ceramic Metal Halide	106.21	100	3049	79	0.0028	
Ceramic Metal Halide	106.41	100	3031	79	0.0027	
Fluorescent 3000K		108.44	100	2919	85	0.0013	
Compact Fluor.		106.96	100	2886	81	0.0058	*
Ceramic Metal Halide	109.39	100	2874	66	0.0005	
Incandescent CIE_A	109.85	100	2856	100	0	
Incan. Tung. Halogen	109.56	100	2830	99	0.0015	
Incan. Tung. Halogen	110.72	100	2788	100	0.0003	
Incan. Tung. Halogen	110.85	100	2775	100	0.0004	
Incandescent		111.77	100	2709	99	0.0006	
Incan. Neodymium		116	100	2604	73	0.0056	*
High Pressure Sodium	121.44	100	2174	21	0.0025	
High Pressure Sodium	123.74	100	2116	16	0.0007	
High Pressure Sodium	123.72	100	2108	32	0.0012	
High Pressure Sodium	125.32	100	2061	19	0.0007	
High Pressure Sodium	125.3	100	2060	16	0.0008	
High Pressure Sodium	125.49	100	2056	17	0.0006	
High Pressure Sodium	126.29	100	2005	7	0.0025	
High Pressure Sodium	127.69	100	2001	21	0.0007	
High Pressure Sodium	126.51	100	1998	7	0.0025	
Low Pressure Sodium	130.34	100	1832	-40	0.0077	*
Low Pressure Sodium	131.4	100	1807	-44	0.0074	*

No description of any relationship between DC values over 0.0054 and the accuracy of the corresponding CRI is given in CIE 13.3-1995.

This does indicate that the validity of the CRI values for many fluorescent, MH and LED sources are questionable, as they may fail to meet the DC criterion in CIE 13.3-1995.

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