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Something about CRI (Color Rrendering Index)

Edit: East Bright Technology Limited    Date: Jul 26, 2016

A color rendering index (CRI) is a quantitative measure of the ability of a light source to reveal the colors of various objects faithfully in comparison with an ideal or natural light source. Light sources with a high CRI are desirable in color-critical applications such as neonatal care, photography and cinematography. It is defined by the International Commission on Illumination (CIE) as follows:

Color rendering: Effect of an illuminant on the color appearance of objects by conscious or subconscious comparison with their color appearance under a reference illuminant

The CRI of a light source does not indicate the apparent color of the light source; that 

information is under the rubric of the correlated color temperature (CCT). The CRI is determined by the light source's spectrum. The pictures on the right show the continuous 

spectrum of an incandescent lamp and the discrete line spectrum of afluorescent lamp; the 

former lamp has the higher CRI. The value often quoted as 'CRI' on commercially available 

lighting products is properly called the CIE Ra value, 'CRI' being a general term and CIE Ra 

being the international standard color rendering index.

Numerically, the highest possible CIE Ra value is 100, and would only be given to a source identical to standardized daylight or a black body (incandescent lamps are effectively black bodies), dropping to negative values for some light sources. Low-pressure sodium lighting has negative CRI; fluorescent lights range from about 50 for the basic types, up to about 90 for the best tri-phosphor type. Typical LEDs have about 80+ CRI, while some manufacturers claim that their LEDs have achieved up to 98 CRI. 

CIE Ra's ability to predict color appearance has been criticized in favor of measures based on color appearance models, such as CIECAM02 and, for daylight simulators, the CIE Metamerism Index. CRI is not a good indicator for use in visual assessment, especially for sources below 5000 kelvin (K). A newer version of the CRI, R96a, has been developed, but it has not replaced the better-known Ra general color rendering index.

Researchers use daylight as the benchmark to which to compare color rendering of electric lights. In 1948, Bouma described daylight as the ideal source of illumination for good color rendering because "it (daylight) displays a great variety of colours, makes it easy to distinguish slight shades of colour, and the colours of objects around us obviously look natural. 

Around the middle of the 20th century, color scientists took an interest in assessing the ability of artificial lights to accurately reproduce colors. European researchers attempted to describe illuminants by measuring the spectral power distribution (SPD) in "representative" spectral bands, whereas their North American counterparts studied the colorimetric effect of the illuminants on reference objects. 

The CIE assembled a committee to study the matter and accepted the proposal to use the latter approach, which has the virtue of not needing spectrophotometry, with a set of Munsell samples. 

Eight samples of varying hue would be alternately lit with two illuminants, and the color appearance compared. Since no color appearance model existed at the time, it was decided to base the evaluation on color differences in a suitable color space, CIEUVW. In 1931, the CIE adopted the first formal system of colorimetry, which is based on the trichromatic nature of the human visual system. CRI is based upon this system of colorimetry. 

To deal with the problem of having to compare light sources of different correlated color temperatures (CCT), the CIE settled on using a reference black body with the same color temperature for lamps with a CCT of under 5000 K, or a phase of CIE standard illuminant D (daylight) otherwise. This presented a continuous range of color temperatures to choose a reference from. Any chromaticity difference between the source and reference illuminants were to be abridged with a von Kries-type chromatic adaptation transform.

As specified in CIE (1995), the original test color samples (TCS) are taken from an early edition

 of the MunsellAtlas. The first eight samples, a subset of the eighteen proposed in Nickerson 

(1960), are relatively low saturated colors and are evenly distributed over the complete range 

of hues. These eight samples are employed to calculate the general color rendering index Ra. The 

last six samples provide supplementary information about the color rendering properties of the 

light source; the first four for high saturation, and the last two as representatives of

 well-known objects. The reflectance spectra of these samples may be found in CIE (2004), 

and their approximate Munsell notations are listed below.

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