Colour Rendering Index
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A color rendering index (CRI) is a quantitative measure of the ability of a
light source Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 terahe ...
to reveal the
color Color (American English) or colour (British English) is the visual perceptual property deriving from the spectrum of light interacting with the photoreceptor cells of the eyes. Color categories and physical specifications of color are associ ...
s of various objects faithfully in comparison with a natural or standard light source. Light sources with a high CRI are desirable in color-critical applications such as
neonatal care Neonatal nursing is a sub-specialty of nursing care for newborn infants up to 28 days after birth. The term neonatal comes from neo, "new", and natal, "pertaining to birth or origin". Neonatal nursing requires a high degree of skill, dedication an ...
and
art restoration The conservation and restoration of cultural property focuses on protection and care of cultural property (tangible cultural heritage), including artworks, architecture, archaeology, and museum collections. Conservation activities include prev ...
. It is defined by the
International Commission on Illumination The International Commission on Illumination (usually abbreviated CIE for its French name, Commission internationale de l'éclairage) is the international authority on light, illumination, colour, and colour spaces. It was established in 1913 a ...
(CIE) as follows:
Color rendering The color rendering of a light source refers to its ability to reveal the colors of various objects faithfully (i.e. to produce illuminant metamerism) in comparison with an ideal or natural light source. Light sources with good color rendering are ...
: Effect of an
illuminant A standard illuminant is a theoretical source of visible light with a spectral power distribution that is published. Standard illuminants provide a basis for comparing images or colors recorded under different lighting. CIE illuminants The Inter ...
on the color appearance of objects by conscious or subconscious comparison with their color appearance under a reference or
standard illuminant A standard illuminant is a theoretical source of visible light with a spectral power distribution that is published. Standard illuminants provide a basis for comparing images or colors recorded under different lighting. CIE illuminants The Inter ...
.
The CRI of a light source does not indicate the apparent color of the light source; that information is given by the correlated color temperature (CCT). The CRI is determined by the light source's
spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors i ...
. An
incandescent lamp An incandescent light bulb, incandescent lamp or incandescent light globe is an electric light with a wire filament heated until it glows. The filament is enclosed in a glass bulb with a vacuum or inert gas to protect the filament from oxid ...
has a
continuous spectrum In physics, a continuous spectrum usually means a set of attainable values for some physical quantity (such as energy or wavelength) that is best described as an interval of real numbers, as opposed to a discrete spectrum, a set of attainable ...
, a
fluorescent lamp A fluorescent lamp, or fluorescent tube, is a low-pressure mercury-vapor gas-discharge lamp that uses fluorescence to produce visible light. An electric current in the gas excites mercury vapor, which produces short-wave ultraviolet lig ...
has a discrete line spectrum; implying that the incandescent 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 whose
spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors i ...
is identical to the spectrum of daylight, very close to that of a
black body A black body or blackbody is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. The name "black body" is given because it absorbs all colors of light. A black body ...
(incandescent lamps are effectively black bodies), dropping to negative values for some light sources. Low-pressure sodium lighting has a negative CRI;
fluorescent light A fluorescent lamp, or fluorescent tube, is a low-pressure mercury-vapor gas-discharge lamp that uses fluorescence to produce visible light. An electric current in the gas excites mercury vapor, which produces short-wave ultraviolet lig ...
s range from about 50 for the basic types, up to about 98 for the best multi-phosphor type. Typical white-color
LEDs A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light (cor ...
have a CRI of 80 or more, while some manufacturers claim that their LEDs achieve a CRI of up to 98. CIE Ra's ability to predict color appearance has been criticized in favor of measures based on
color appearance model A color appearance model (CAM) is a mathematical model that seeks to describe the perceptual aspects of human color vision, i.e. viewing conditions under which the appearance of a color does not tally with the corresponding physical measurement of ...
s, such as
CIECAM02 In colorimetry, CIECAM02 is the color appearance model published in 2002 by the International Commission on Illumination (CIE) Technical Committee 8-01 (''Color Appearance Modelling for Color Management Systems'') and the successor of CIECAM97s. ...
and for
daylight Daylight is the combination of all direct and indirect sunlight during the daytime. This includes direct sunlight, diffuse sky radiation, and (often) both of these reflected by Earth and terrestrial objects, like landforms and buildings. Sunligh ...
simulators, the CIE
metamerism index In colorimetry, metamerism is a perceived matching of colors with different (nonmatching) spectral power distributions. Colors that match this way are called metamers. A spectral power distribution describes the proportion of total light given o ...
. CRI is not a good indicator for use in visual assessment of light sources, especially for sources below 5000 
kelvin The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based engineer and phys ...
(K). (A verbatim re-publication of the 1974, second edition. Accompanying dis
D008: Computer Program to Calculate CRIs
)
New standards, such as the
IES TM-30 The color rendering of a light source refers to its ability to reveal the colors of various objects faithfully (i.e. to produce illuminant metamerism) in comparison with an ideal or natural light source. Light sources with good color rendering are ...
, resolve these issues and have begun replacing the usage of CRI among professional lighting designers. However, CRI is still common among household lighting products.


History

Researchers use daylight as the benchmark to which to compare color rendering of electric lights. In 1948, daylight was described as the ideal source of illumination for good color rendering because "it (daylight) displays (1) a great variety of colors, (2) makes it easy to distinguish slight shades of color, and (3) the colors 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 light Lighting or illumination is the deliberate use of light to achieve practical or aesthetic effects. Lighting includes the use of both artificial light sources like lamps and light fixtures, as well as natural illumination by capturing dayligh ...
s to accurately reproduce colors. European researchers attempted to describe illuminants by measuring the
spectral power distribution In radiometry, photometry, and color science, a spectral power distribution (SPD) measurement describes the power per unit area per unit wavelength of an illumination (radiant exitance). More generally, the term ''spectral power distribution'' ...
(SPD) in "representative" spectral bands, whereas their North American counterparts studied the
colorimetric Colorimetry is "the science and technology used to quantify and describe physically the human color perception". It is similar to spectrophotometry, but is distinguished by its interest in reducing spectra to the physical correlates of color ...
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 Spectrophotometry is a branch of electromagnetic spectroscopy concerned with the quantitative measurement of the reflection or transmission properties of a material as a function of wavelength. Spectrophotometry uses photometers, known as spec ...
, with a set of
Munsell Munsell may refer to: * Albert Henry Munsell (1858–1918), American painter, teacher of art, and the inventor of the Munsell color system * Harvey M. Munsell, American soldier in the Civil War. *Munsell Color Company *Munsell color system In c ...
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 Colorimetry is "the science and technology used to quantify and describe physically the human color perception". It is similar to spectrophotometry, but is distinguished by its interest in reducing spectra to the physical correlates of color ...
, which is based on the trichromatic nature of the
human visual system The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina containing photoreceptor cells, the optic nerve, the optic tract and the visual cortex) which gives organisms the sense of sight (the a ...
. 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 A black body or blackbody is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. The name "black body" is given because it absorbs all colors of light. A black body ...
with the same color temperature for lamps with a CCT of under 5000 K, or a phase of CIE
standard illuminant A standard illuminant is a theoretical source of visible light with a spectral power distribution that is published. Standard illuminants provide a basis for comparing images or colors recorded under different lighting. CIE illuminants The Inter ...
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 Chromatic adaptation is the human visual system’s ability to adjust to changes in illumination in order to preserve the appearance of object colors. It is responsible for the stable appearance of object colors despite the wide variation of light w ...
. There are two extent versions of CRI: the more commonly used Ra of (actually from 1974) and R96a of .


Test method

The CRI is calculated by comparing the color rendering of the test source to that of a "perfect" source, which is a black body radiator for sources with correlated color temperatures under 5000 K, and a phase of daylight otherwise (e.g., D65).
Chromatic adaptation Chromatic adaptation is the human visual system’s ability to adjust to changes in illumination in order to preserve the appearance of object colors. It is responsible for the stable appearance of object colors despite the wide variation of light w ...
should be performed so that like quantities are compared. The ''Test Method'' (also called ''Test Sample Method'' or ''Test Color Method'') needs only
colorimetric Colorimetry is "the science and technology used to quantify and describe physically the human color perception". It is similar to spectrophotometry, but is distinguished by its interest in reducing spectra to the physical correlates of color ...
, rather than
spectrophotometric Spectrophotometry is a branch of electromagnetic spectroscopy concerned with the quantitative measurement of the reflection or transmission properties of a material as a function of wavelength. Spectrophotometry uses photometers, known as spec ...
, information. # Using the 2° standard observer, find the
chromaticity Chromaticity is an objective specification of the quality of a color regardless of its luminance. Chromaticity consists of two independent parameters, often specified as hue (h) and colorfulness (s), where the latter is alternatively called ...
co-ordinates of the test source in the
CIE 1960 color space The CIE 1960 color space ("CIE 1960 UCS", variously expanded ''Uniform Color Space'', ''Uniform Color Scale'', ''Uniform Chromaticity Scale'', ''Uniform Chromaticity Space'') is another name for the chromaticity space devised by David MacAdam. ...
. # Determine the
correlated color temperature Color temperature is the color of light emitted by an idealized opaque, non-reflective body at a particular temperature measured in kelvins. The color temperature scale is used to categorize the color of light emitted by other light sources ...
(CCT) of the test source by finding the closest point to the
Planckian locus In physics and color science, the Planckian locus or black body locus is the path or ''locus'' that the color of an incandescent black body would take in a particular chromaticity space as the blackbody temperature changes. It goes from deep r ...
on the (''u'', ''v'') chromaticity diagram. # If the test source has a CCT < 5000 K, use a black body for reference, otherwise use CIE
standard illuminant A standard illuminant is a theoretical source of visible light with a spectral power distribution that is published. Standard illuminants provide a basis for comparing images or colors recorded under different lighting. CIE illuminants The Inter ...
D. Both sources should have the same CCT. # Ensure that the chromaticity distance (DC) of the test source to the Planckian locus is under 5.4×10−3 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. \text = \Delta_ = \sqrt. # Illuminate the first eight standard samples, from the fifteen listed below, alternately using both sources. # Using the 2° standard observer, find the co-ordinates of the light reflected by each sample in the
CIE 1964 color space The CIE 1964 (''U''*, ''V''*, ''W''*) color space, also known as CIEUVW, is based on the CIE 1960 UCS: :U^*=13W^*(u-u_0), \quad V^*=13W^*(v-v_0), \quad W^*=25Y^\frac13-17 where is the white point and ''Y'' is the luminous tristimulus value of t ...
. # Chromatically adapt each sample by a
von Kries transform Chromatic adaptation is the human visual system’s ability to adjust to changes in illumination in order to preserve the appearance of object colors. It is responsible for the stable appearance of object colors despite the wide variation of light w ...
. # For each sample, calculate the
Euclidean distance In mathematics, the Euclidean distance between two points in Euclidean space is the length of a line segment between the two points. It can be calculated from the Cartesian coordinates of the points using the Pythagorean theorem, therefor ...
\Delta E_i between the pair of co-ordinates. # Calculate the special (i.e., particular) CRI using the formula R_i = 100 - 4.6 \Delta E_i # Find the general CRI (Ra) by calculating the
arithmetic mean In mathematics and statistics, the arithmetic mean ( ) or arithmetic average, or just the ''mean'' or the ''average'' (when the context is clear), is the sum of a collection of numbers divided by the count of numbers in the collection. The colle ...
of the special CRIs. Note that the last three steps are equivalent to finding the mean
color difference In color science, color difference or color distance is the separation between two colors. This metric allows quantified examination of a notion that formerly could only be described with adjectives. Quantification of these properties is of great ...
, \Delta \bar_ and using that to calculate R_a: R_a = 100 - 4.6 \Delta \bar_.


Chromatic adaptation

uses this von Kries chromatic transform equation to find the corresponding color (''u''''c'',''i'', ''v''''c'',''i'') for each sample. The mixed subscripts (''t'', ''i'') refer to the
inner product In mathematics, an inner product space (or, rarely, a Hausdorff space, Hausdorff pre-Hilbert space) is a real vector space or a complex vector space with an operation (mathematics), operation called an inner product. The inner product of two ve ...
of the test illuminant spectrum and the spectral reflexivity of sample ''i'': u_ = \frac, v_ = \frac, c = (4.0 - u - 10.0 v) / v, d = (1.708 v - 1.481 u + 0.404) / v, where subscripts ''r'' and ''t'' refer to reference and test light sources respectively.


Test color samples

As specified in , the original test color samples (TCS) are taken from an early edition of the
Munsell Munsell may refer to: * Albert Henry Munsell (1858–1918), American painter, teacher of art, and the inventor of the Munsell color system * Harvey M. Munsell, American soldier in the Civil War. *Munsell Color Company *Munsell color system In c ...
Atlas. The first eight samples, a subset of the eighteen proposed in , 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 R_a. 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 , and their approximate Munsell notations are listed aside.


R96a method

In the CIE's 1991 Quadrennial Meeting, Technical Committee 1-33 (Color Rendering) was assembled to work on updating the color rendering method, as a result of which the R96a method was developed. The committee was dissolved in 1999, releasing , but no firm recommendations, partly due to disagreements between researchers and manufacturers. The R96a method has a few distinguishing features: * A new set of test color samples * Six reference illuminants: D65, D50, black bodies of 4200 K, 3450 K, 2950 K, and 2700 K. * A new chromatic adaptation transform: CIECAT94. * Color difference evaluation in CIELAB. * Adaptation of all colors to D65 (since CIELAB is well-tested under D65). It is conventional to use the original method; R96a should be explicitly mentioned if used.


New test color samples

As discussed in , recommends the use of a
ColorChecker File:CIE1931xy_ColorChecker_SMIL.svg, 300px, Nominal chromaticities of ColorChecker patches in the CIE 1931 xy chromaticity diagram (in thSVG version hover over a color swatch to highlight it; click it to select and deselect it) defaul The Col ...
chart owing to the obsolescence of the original samples, of which only
metameric In biology, metamerism is the phenomenon of having a linear series of body segments fundamentally similar in structure, though not all such structures are entirely alike in any single life form because some of them perform special functions. In ...
matches remain. In addition to the eight ColorChart samples, two skin tone samples are defined (TCS09* and TCS10*). Accordingly, the updated general CRI is averaged over ten samples, not eight as before. Nevertheless, has determined that the patches in give better correlations for any color difference than the ColorChecker chart, whose samples are not equally distributed in a uniform color space.


Example

The CRI can also be theoretically derived from the spectral power distribution (SPD) of the illuminant and samples, since physical copies of the original color samples are difficult to find. In this method, care should be taken to use a sampling resolution fine enough to capture spikes in the SPD. The SPDs of the standard test colors are tabulated in 5 nm increments , so it is suggested to use interpolation up to the resolution of the illuminant's spectrophotometry. Starting with the SPD, let us verify that the CRI of reference illuminant F4 is 51. The first step is to determine the
tristimulus value The CIE 1931 color spaces are the first defined quantitative links between distributions of wavelengths in the electromagnetic visible spectrum, and physiologically perceived colors in human color vision. The mathematical relationships that defin ...
s using the 1931 standard observer. Calculation of the
inner product In mathematics, an inner product space (or, rarely, a Hausdorff space, Hausdorff pre-Hilbert space) is a real vector space or a complex vector space with an operation (mathematics), operation called an inner product. The inner product of two ve ...
of the SPD with the standard observer's color matching functions (CMFs) yields (''X'', ''Y'', ''Z'') = (109.2, 100.0, 38.9) (after normalizing for ''Y'' = 100). From this follow the ''xy'' chromaticity values: x = \frac = 0.4402, y = \frac = 0.4031. The next step is to convert these chromaticities to the CIE 1960 UCS in order to be able to determine the CCT: u = \frac = 0.2531, v = \frac = 0.3477. Examining the CIE 1960 UCS reveals this point to be closest to 2938 K on the Planckian locus, which has a coordinate of (0.2528, 0.3484). The distance of the test point to the locus is under the limit (5.4×10−3), so we can continue the procedure, assured of a meaningful result: \begin \text &= \sqrt \\ &= 8.12 \times 10^ < 5.4 \times 10^. \end We can verify the CCT by using McCamy's approximation algorithm to estimate the CCT from the ''xy'' chromaticities: \text_\text = -449 n^3 + 3525 n^2 - 6823.3 n + 5520.33, where n = \frac. Substituting (x, y) = (0.4402, 0.4031) yields ''n'' = 0.4979 and CCTest. = 2941 K, which is close enough. ( Robertson's method can be used for greater precision, but we will be content with 2940 K in order to replicate published results.) Since 2940 < 5000, we select a Planckian radiator of 2940 K as the reference illuminant. The next step is to determine the values of the test color samples under each illuminant in the CIEUVW color space. This is done by integrating the product of the CMF with the SPDs of the illuminant and the sample, then converting from CIEXYZ to CIEUVW (with the ''u'', ''v'' coordinates of the reference illuminant as white point): From this we can calculate the color difference between the chromatically adapted samples (labeled "CAT") and those illuminated by the reference. (The Euclidean metric is used to calculate the color difference in CIEUVW.) The special CRI is simply R_i = 100 - 4.6 \Delta E_. Finally, the general color rendering index is the mean of the special CRIs: 51.


Typical values

A reference source, such as blackbody radiation, is defined as having a CRI of 100. This is why
incandescent lamp An incandescent light bulb, incandescent lamp or incandescent light globe is an electric light with a wire filament heated until it glows. The filament is enclosed in a glass bulb with a vacuum or inert gas to protect the filament from oxid ...
s have that rating, as they are, in effect, almost blackbody radiators. The best possible faithfulness to a reference is specified by CRI = 100, 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.


Special value: R9

Ra is the average value of R1–R8; other values from R9 to R15 are not used in the calculation of Ra, including R9 "saturated red", R13 "skin color (light)", and R15 "skin color (medium)", which are all difficult colors to faithfully reproduce. R9 is a vital index in high-CRI lighting, as many applications require red lights, such as film and video lighting, medical lighting, art lighting, etc. However, in the general CRI (Ra) calculation R9 is not included. R9 is one of the numbers of Ri refers to test color samples (TCS), which is one score in extended CRI. It is the number rates the light source's color revealing ability towards TCS 09. And it describes the specific ability of light to accurately reproduce the red color of objects. Many lights manufacturers or retailers do not point out the score of R9, while it is a vital value to evaluate the color rendition performance for film and video lighting, as well as any applications that need high CRI value. So, generally, it is regarded as a supplement of color rendering index when evaluating a high-CRI light source. R9 value, TCS 09, or in other words, the red color is the key color for many lighting applications, such as film and video lighting, textile printing, image printing, skin tone, medical lighting, and so on. Besides, many other objects which are not in red color, but actually consists of different colors including red color. For instance, the skin tone is impacted by the blood under the skin, which means that the skin tone also includes red color, although it looks much like close to white or light yellow. So, if the R9 value is not good enough, the skin tone under this light will be more paleness or even greenish in your eyes or cameras.


Criticism

Ohno 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
LED A light-emitting diode (LED) is a semiconductor Electronics, device that Light#Light sources, emits light when Electric current, current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy i ...
s. Another problem is that the CRI is discontinuous at 5000 K, because the chromaticity of the reference moves from the
Planckian locus In physics and color science, the Planckian locus or black body locus is the path or ''locus'' that the color of an incandescent black body would take in a particular chromaticity space as the blackbody temperature changes. It goes from deep r ...
to the CIE daylight locus. identify several other issues, which they address in their
color quality scale Color quality scale (CQS) is a color rendering score – a quantitative measure of the ability of a light source to reproduce colors of illuminated objects. Developed by researchers at NIST the metric aims to overcome some of the issues inherent in ...
(CQS): * The color space in which the color distance is calculated (CIEUVW) is obsolete and nonuniform. Use
CIELAB The CIELAB color space, also referred to as ''L*a*b*'' , is a color space defined by the International Commission on Illumination (abbreviated CIE) in 1976. (Referring to CIELAB as "Lab" without asterisks should be avoided to prevent confusion ...
or
CIELUV In colorimetry, the CIE 1976 ''L''*, ''u''*, ''v''* color space, commonly known by its abbreviation CIELUV, is a color space adopted by the International Commission on Illumination (CIE) in 1976, as a simple-to-compute transformation of the 1931 CI ...
instead. * The chromatic adaptation transform used (
Von Kries transform Chromatic adaptation is the human visual system’s ability to adjust to changes in illumination in order to preserve the appearance of object colors. It is responsible for the stable appearance of object colors despite the wide variation of light w ...
) 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 The root-mean-square deviation (RMSD) or root-mean-square error (RMSE) is a frequently used measure of the differences between values (sample or population values) predicted by a model or an estimator and the values observed. The RMSD represents ...
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 ∆''E''''i'', while in general a saturated color is experienced as being more attractive. * A negative CRI is difficult to interpret. Normalize the scale from 0 to 100 using the formula R_\text = 10 \ln \left exp(R_\text/10) + 1\right/math>. * 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 In color reproduction, including computer graphics and photography, the gamut, or color gamut , is a certain ''complete subset'' of colors. The most common usage refers to the subset of colors which can be accurately represented in a given circ ...
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.


Alternatives

"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 . reviewed several alternative quality metrics and compared their performance based on visual data obtained in nine 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., 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 color rendering metrics. Due to the criticisms of CRI many researchers have developed alternative metrics, though relatively few of them have had wide adoption.


Gamut area index (GAI)

Developed in 2010 by Rea and Freyssinier, the gamut area index (GAI) is 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.


Color quality scale (CQS)

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.


Film and video high-CRI LED lighting

Problems have been encountered attempting to use 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 (DIs), 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 The Academy of Motion Picture Arts and Sciences (AMPAS, often pronounced ; also known as simply the Academy or the Motion Picture Academy) is a professional honorary organization with the stated goal of advancing the arts and sciences of motio ...
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 that their products have TLCI values of up to 99.


References


Sources

* * * . Carried out by TC 1-69: color Rendering of White Light Sources. * * * * * * * * * * * * * * * * *


External links


MATLAB script for calculating measures of light source color
Rensselaer Polytechnic Institute Rensselaer Polytechnic Institute () (RPI) is a private research university in Troy, New York, with an additional campus in Hartford, Connecticut. A third campus in Groton, Connecticut closed in 2018. RPI was established in 1824 by Stephen Van ...
, 2004.
Excel spreadsheet with a cornucopia of data
Lighting Laboratory of the
Helsinki University of Technology Helsinki University of Technology (TKK; fi, Teknillinen korkeakoulu; sv, Tekniska högskolan) was a technical university in Finland. It was located in Otaniemi, Espoo in the metropolitan area of Greater Helsinki. The university was founded in ...
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, Metrologia">Uncertainty evaluation for measurement of LED colour , Metrologia

United States Department of Energy, Office of Energy Efficiency and Renewable Energy (EERE)
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Alliance for Solid State Illumination Systems and Technologies, Color Rendering

What is the difference between CRI and CQS?
Edaphic Scientific Knowledge Base

Lumens {{DEFAULTSORT:Color Rendering Index Color Lighting