Color rendering
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The color rendering 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 terah ...
refers to its ability 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 assoc ...
s 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 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: Effect of an illuminant on the color appearance of objects by conscious or subconscious comparison with their color appearance under a reference illuminant.


Quantitative Measures

A wide variety of quantitative measures have been devised to measure the color rendering of a light source, to the human eye or to the camera. Notable ones include: *
Color rendering index 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 a natural or standard light source. Light sources with a high CRI are desirable in ...
(CRI), CIE 1974. Currently acknowledged as flawed, but still widely used for consumer lighting. Updated 1999, but rarely followed. * Television lighting consistency index (TLCI), EBU 2012. Address the spectral response of cameras and screens, which can have significantly different results with high-CRI LED lighting. * Spectral similarity index (SSI),
AMPAS 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 motion ...
2016 (revised 2020). Ditches the concept of color samples in CRI and TLCI to directly address the shape of the spectrum. As there are no color samples, SSI doesn't measure color rendering, but is included here because it is often used to indicate the potential color rendering quality of a test light source as compared to a reference source. Published as
SMPTE The Society of Motion Picture and Television Engineers (SMPTE) (, rarely ), founded in 1916 as the Society of Motion Picture Engineers or SMPE, is a global professional association of engineers, technologists, and executives working in the m ...
2122. * IES TM-30, 2015 (revised 2020). A spiritual descendant of CRI with updated color transformation, more (99) color samples, and scoring for additional "pleasantness" factors such as gamut size and hue shift. Still uses human participants. Endorsed by CIE in 2015 to replace CRI.


Background

Researchers used 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 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 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 daylig ...
s 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 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.


Scales


Color rendering index

The color rendering index (CRI) of 1974 is the product of a CIE committee's study on the topic of color rendering. It uses the American colorimetric approach with a panel of human subjects instead of requiring
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 sp ...
. 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. The residual difference in chromaticity is resolved with a
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 ...
before comparing to the reference illuminant. Each color difference was translated to a sub-score, eight of which are averaged to produced the final score of Ra.


Television lighting consistency index

As early as 1971, an analogue of CRI for televisions have been devised by workers at the
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. At that time, the relatively broad-band nature of light sources meant that the CRI still approximated the color rendering for television cameras, an assumption quickly broken by the advent of LED lighting. As a result, the European Broadcasting Union re-introduced the concept of a television lighting consistency index (TLCI) in 2012, followed by a television luminaire matching dactor (TLMF) in 2013 for mixed lights. To calculate a TLCI, a full measure of the spectral power distribution (SPD) of the light source is first taken. From this SPD a
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) is found, which provides the reference illuminant. Under the test and reference illuminant, an image of the
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 ...
is simulated using known reflectivities and the color curves of an average HDTV camera and display. The differences are calculated in CIEDE2000. With the TLMF, the reference is not specified by a CCT, but by a user directly.


Spectral similarity index

The spectral similarity index (SSI) of 2016 is a scale that completely forgoes the comparison of color samples, instead directly comparing the SPDs of one light source to the reference. Its developers argue that difference among cameras mean that TLCI can only describe three-chip television cameras, not the more-varied spectral sensitivities of single-chip digital cinema, still cameras, or film. (In theory, color gels also introduce variations that are hard to be captured by TLCI.) The SSI is calculated by taking two integrated, normalized SPDs in the 5-nm intervals from 375 to 675 nm and finding a weighted relative difference between them. This weighted relative difference is convolved, and the magnitude of the result is translated into a 100-point value. A low SSI only warns of potential color-rendering issues, but neither confirms the presence of one nor indicates what errors are likely to occur.


TM-30

TM-30 is the current (as of 2021) CIE recommended measure for color rendering as perceived by humans. It generates a large set of outputs, including an overall fidelity index (Rf), an overall
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 ...
index (Rg) for changes in chroma, a gamut shape graph, and detailed values for chroma, hue, and color fidelity for each of the 16 hue ranges, plus color fidelity scores for each of the 99 sample colors. It uses the CAM02-UCS color space. The Rf has been adopted by the CIE as CIE 224:2017 "color fidelity index" (CFI). As with other newer scales, TM-30 is calculated from a SPD with reference to a SPD of the same CCT. The uniqueness of TM-30 is that it goes beyond ''fidelity'' (accuracy of color reproduction) to describe other aspects of color rendering. This extra information allows for, e.g. fidelity to be sacrificed for vividness of skin tones under a certain design criterion. Three reference design intents and priority levels are defined in TM-30 Annex E.


Other scales

Before the aforementioned scales are devised to replace CRI, a number of other measures have been proposed. None of them have seen wide use, however: ; R96a, 1999 : A revision of the CRI to account for the obsolescence/loss of original color samples and some improvements in colorimetry. Uses ColorChecker samples, CIELAB, and CIECAT94. ;
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 i ...
, 2005 : NIST-proposed replacement for CRI Ra. Uses more saturated samples, CIELAB, and CMCCAT2002. Modifications to scoring. ; Gamut area index, 2010 : A measure of
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. Use in conjunction with a fidelity measure (such as CRI) predicts preference better than using either alone.


Typical values


References


Works cited

* * * * {{refend Color Lighting