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 and art restoration. 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 given by 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 a fluorescent 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 98 for the best multi-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 A newer version of the CRI, R96, has been developed, but it has not replaced the better-known Ra general color rendering index.
History
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 (1) a great variety of colours, (2) makes it easy to distinguish slight shades of colour, and (3) 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.
Principle
The colorful appearance of an illuminated surface depends on its physical characteristics, those of the light that illuminates it, and the main light from the point of view of the observer. The lighting designer and the decorator play with all these effects: the light of an incandescent lamp is gilded in the light of day that comes from a window; on a stage, a gray surface dyes with a color projector. It is therefore difficult to compare absolutely two sources of light.
To simplify the problem, we agree that the sources are the main lights. A surface colored by a pigment can be described by its absorption spectrum, which indicates, for each wavelength, the proportion of light that it returns. Thus, a surface that absorbs much more blue and green than red appears reddish, compared to a white surface, or neutral gray, which also reflects all wavelengths. This reddish sensation persists even though the light that illuminates it is enhanced in blue and green, as long as the reddish surface occupies only a small part of the field of view. As a result, color seems to attach to objects, while the light that comes to the eye is different.
The ability to distinguish two colors depends on the amount of light that illuminates it in the regions of the visible spectrum that characterize it. Thus, a pale blue made with a mixture of ultramarine and white appears gray in the light of a candle. The light of the candle contains a negligible amount of blue light. Overseas returns only blue. It behaves like black in the light of the candle. This effect is the main difference between two light sources. The more the color temperature approaches that of daylight, the more we can distinguish shades in the blue.
The problem is complicated by fluorescence-based light sources. Illuminating a white surface, which also reflects light of all visible wavelengths, they balance the blue, green and red areas of the spectrum, so that this surface appears white in comparison with that illuminated by the day. But the detail of their spectrum is different, so that two colors that would be similar under the same light now appear different. This is what specialists call a metamerism problem.
Comparing the performance of two lights to work with colors therefore involves comparing the rendering of several colored surfaces. The choice of absoption characteristics is decisive. Since two different spectra can produce the same color, we need to define their spectrum, not just their colorimetry. Some pigments give spectra with zones of absorption more marked, but narrower than others giving the same color. The choice of specimen spectra had to be the subject of many experiments, so that the index does not contradict the user experience too much.
The color temperature is the main aspect of the differences between illuminants, the index is calculated with respect to an ideal source of the same color temperature.
For each frequency band, the light emission coefficient is multiplied by the complement to one of the absorption coefficient of the color range, and the result is multiplied by the coefficient of the colorimetric function. The resulting colorimetry is the sum of all the results obtained for each colorimetric function. This operation is repeated with the reference light.
The index represents the arithmetic mean of the color deviations calculated for each sample between the result with the light to be rated and with the reference light, corrected by the von Kries transformation, which represents the chromatic visual adaptation to the color difference. between the ideal illuminant and illuminating it.
Measurement of the color rendering index
Both sources are used to illuminate several standard samples. The colors perceived with the reference and the source to be tested (measured according to the CIE 1931 standard) are compared using a conventional formula5 and averaged over all the samples to obtain the CRI of the source to be quantified. Since eight samples are often used, manufacturers generally use the “octo-” prefix for their high IRC lamps.
As the sun and incandescent lamps are approximately black bodies, their CRIs are worth 100.
The color rendering index was created to allow a comparison of the “approximately white” luminaires, ie, at the time of its definition, the fluorescent tubes, which also applies to their fluocompact variant . Since its introduction, color professionals have noted its insufficiency to qualify lighting and cases of metamerism showing two colored surfaces as identical or different under lighting yet of the same color temperature and high color rendering index . The development of LED lighting has led the CIE to define a color fidelity index, which includes a color space based on color differences where are best distributed 99 color samples and their absorption spectra instead of 15 generally reduced to 8 for 1995 CRI. However, the Commission notes that, even more precisely, the color fidelity index still can not be used as a quality index for lighting, and that groups of users may judge different luminaires whose results are identical for the index.
A reference source, such as blackbody radiation, is defined as having a CRI of 100. This is why incandescent lamps have that rating, as they are, in effect, almost blackbody radiators. The best possible faithfulness to a reference is specified by a CRI of one hundred, while the very poorest is specified by a CRI below zero. A high CRI by itself does not imply a good rendition of color, because the reference itself may have an imbalanced SPD if it has an extreme color temperature.
Criticism
Ohno (2006) and others have criticized CRI for not always correlating well with subjective color rendering quality in practice, particularly for light sources with spiky emission spectra such as fluorescent lamps or white LEDs. Another problem is that the CRI is discontinuous at 5000 K, because the chromaticity of the reference moves from the Planckian locus to the CIE daylight locus. Davis & Ohno (2006) identify several other issues, which they address in their Color Quality Scale (CQS):
The color space in which the color distance is calculated (CIEUVW) is obsolete and nonuniform. Use CIELAB or CIELUV instead.
The chromatic adaptation transform used (Von Kries transform) is inadequate. Use CMCCAT2000 or CIECAT02 instead.
Calculating the arithmetic mean of the errors diminishes the contribution of any single large deviation. Two light sources with similar CRI may perform significantly differently if one has a particularly low special CRI in a spectral band that is important for the application. Use the root mean square deviation instead.
The metric is not perceptual; all errors are equally weighted, whereas humans favor certain errors over others. A color can be more saturated or less saturated without a change in the numerical value of ∆Ei, while in general a saturated color is experienced as being more attractive.
The CRI cannot be calculated for light sources that do not 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 (i.e., 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 6500 K to the gamut area for the test source. 6500 K 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.
Rea and Freyssinier have developed another index, the Gamut Area Index (GAI), in an attempt to improve over the flaws found in the CRI. They have shown that the GAI is better than the CRI at predicting color discrimination on standardized Farnsworth-Munsell 100 Hue Tests and that GAI is predictive of color saturation. Proponents of using GAI claim that, when used in conjunction with CRI, this method of evaluating color rendering is preferred by test subjects over light sources that have high values of only one measure. Researchers recommend a lower and an upper limit to GAI. Use of LED technology has called for a new way to evaluate color rendering because of the unique spectrum of light created by these technologies. Preliminary tests have shown that the combination of GAI and CRI used together is a preferred method for evaluating color rendering.
Pousset, Obein & Razet (2010) developed a psychophysical experiment in order to evaluate light quality of LED lightings. It is based on colored samples used in the “Color Quality Scale”. Predictions of the CQS and results from visual measurements were compared.
CIE (2007) “reviews the applicability of the CIE color 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 color 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.”
For a comprehensive review of alternative color rendering indexes see Guo & Houser (2004).
Smet (2011) reviewed several alternative quality metrics and compared their performance based on visual data obtained in 9 psychophysical experiments. It was found that a geometric mean of the GAI index and the CIE Ra correlated best with naturalness (r=0.85), while a color quality metric based on memory colors (MCRI) correlated best for preference (r=0.88). The differences in performance of these metrics with the other tested metrics (CIE Ra; CRI-CAM02UCS; CQS; RCRI; GAI; geomean(GAI, CIE Ra); CSA; Judd Flattery; Thornton CPI; MCRI) were found to be statistically significant with p<0.0001. Dangol et al (2013) performed psychophysical experiments and concluded that people’s judgments of naturalness and overall preference could not be predicted with a single measure, but required the joint use of a fidelity-based measure (e.g., Qp) and a gamut-based measure (e.g., Qg or GAI.). They carried out further experiments in real offices evaluating various spectra generated for combination existing and proposed colour rendering metrics ( see Dangol et al. 2013,Islam et al. 2013,Baniya et al. 2013 for details). Film and video high-CRI LED lighting incompatibility Problems have been encountered attempting to use otherwise high CRI LED lighting on film and video sets. The color spectra of LED lighting primary colors does not match the expected color wavelength bandpasses of film emulsions and digital sensors. As a result, color rendition can be completely unpredictable in optical prints, transfers to digital media from film (DI's), and video camera recordings. This phenomenon with respect to motion picture film has been documented in an LED lighting evaluation series of tests produced by the Academy of Motion Picture Arts and Sciences scientific staff. To that end, various other metrics such as the TLCI (Television Lighting Consistency Index) have been developed to replace the human observer with a camera observer. Similar to the CRI, the metric measures quality of a light source as it would appear on camera on a scale from 0 to 100. Some manufacturers say their products have TLCI values of up to 99. Source From Wikipedia