Alliance for Lighting Information

Color Rendering Index Values for MH

by David M. Keith

The rendering of colors by MH lamps is considered one of the most significant benefits.

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:

Source			Abbr.  	  x	  y	CCT	 CRI 	   DC

Equal Energy		EqE	0.33	0.33	5455	95	0.0077 *
CIE D65			C_D	0.31	0.33	6502	100	0.0000
CIE Illuminant A	C_A	0.45	0.41	2856	100	0.0000
HPS 100W		H10	0.52	0.42	2060	16	0.0008
HPS 150W		H15	0.51	0.42	2174	21	0.0025
HPS 250W		H25	0.52	0.41	2001	21	0.0007
HPS 400W		H40	0.52	0.42	2116	16	0.0007
MH 250W Horiz		M2h	0.39	0.40	4002	64	0.0101 *
MH 250W Univ		M2u	0.38	0.40	4061	53	0.0082 *
MH 400W Horiz		M4h	0.37	0.39	4319	68	0.0098 *
MH 400W Univ		M4u	0.40	0.40	3776	58	0.0067 *
Fluorescent 3000K	F30	0.44	0.41	2919	85	0.0013
Fluorescent 3500K	F35	0.42	0.40	3326	87	0.0002
Fluorescent 4100K	F41	0.39	0.38	3813	87	0.0004
Fluorescent 6500K	F65	0.32	0.34	6088	87	0.0025

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 the MH sources is questionable, as they are the only ones (the Equal Energy source is for reference only) that fail to meet the DC criterion in CIE 13.3-1995.

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