Alliance for Lighting InformationThe 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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