Everything about Color Rendering Index totally explained
The
CIE Color Rendering Index (
CRI) (sometimes incorrectly called
Color Rendition Index), is a quantitative measure of the ability of a light source to reproduce 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 photography and cinematography.
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Note that the CRI by itself doesn't indicate what the
color temperature of the reference light source is; therefore, it's customary to also cite the
correlated color temperature (CCT).
According to, CRI is being deprecated in favor of measures based on
color appearance models, such as
CIECAM02 and, for
daylight simulators, the CIE
Metamerism Index. notes that CRI isn't a good indicator for use in visual assessment.
History
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 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
tristimulus values compared. 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 5000K, or a phase of CIE
standard illuminant D (daylight) otherwise.
Test Color Method
The CRI is calculated by comparing the color rendering of the test source to that of a "perfect" source which is generally a black body radiator, except for sources with color temperatures above 5000K, in which case a phase of daylight (for example
D65) is used.
Chromatic adaptation should be performed so that like quantities are compared. Specified in and republished in, the
Test Color Method needs only
colorimetric, rather than
spectrophotometric, information.
- Using the 2° standard observer, find the chromaticity co-ordinates of the test source in the CIE 1960 color space.
- Determine the correlated color temperature (CCT) of the test source by finding the closest point to the Planckian locus on the (u,v) chromaticity diagram.
- If the test source has a CCT<5000K, use a black body for reference, otherwise use CIE standard illuminant D. Both sources should have the same CCT.
- Ensure that the chromaticity distance (DC) of the test source to the Planckian locus is under 5.4E-3 in the CIE 1960 UCS. This ensures the meaningfulness of the result, as the CRI is only defined for light sources that are approximately white.
- The CRI can not be calculated for light sources that don't have a CCT (non-white light).
- Eight samples are not enough since manufacturers can optimize the emission spectra of their lamps to reproduce them faithfully, but otherwise perform poorly. Use more samples (they suggest fifteen for CQS).
- The samples are not saturated enough to pose difficulty for reproduction.
- CRI merely measures the faithfulness of any illuminant to an ideal source with the same CCT, but the ideal source itself may not render colors well if it has an extreme color temperature, due to a lack of energy at either short or long wavelengths (for example, it may be excessively blue or red). Weight the result by the ratio of the gamut area of the polygon formed by the fifteen samples in CIELAB for 6500K to the gamut area for the test source. 6500K is chosen for reference since it has a relatively even distribution of energy over the visible spectrum and hence high gamut area. This normalizes the multiplication factor.
"reviews the applicability of the CIE colour rendering index to
white LED light sources based on the results of visual experiments." Chaired by Davis, CIE TC 1-69(C) is currently investigating "new methods for assessing the colour rendition properties of white-light sources used for illumination, including solid-state light sources, with the goal of recommending new assessment procedures ... by March, 2010."
Further Information
Get more info on 'Color Rendering Index'.
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