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Color rendering

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The color rendering of a light source refers to its ability to reveal the colors of various objects faithfully 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 and art restoration. It is defined by the International Commission on Illumination (CIE) as follows:[1]

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.

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 (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.[2]
  • Spectral similarity index (SSI), AMPAS 2016 (revised 2020). Ditches the concept of color samples in CRI and TLCI to directly address the shape of the spectrum. Published as SMPTE 2122.[3]
  • IES TM-30, 2015 (revised 2020). A spiritual descendant of CRI with updated color transformation, more (99) color samples, and scoring for additional factors such as gamut size and hue shift. Still uses human participants. Endorsed by CIE in 2015 to replace CRI.[4]

Scales

Color rendering index

Main article: 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. 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 chromaticity is resolved with a chromatic adaptation transform 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.[5]

Television lighting consistency index

As early as 1971, an analogue of CRI for televisions have been devised by workers at the BBC.[6] 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.[2]

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 (CCT) is found, which provides the reference illuminant. Under the test and reference illuminant, an image of the ColorChecker 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.[7]

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.[3] 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 or still cameras.[8] (In theory, color gels also introduce variations that are hard to be captured by TLCI.)

The SSI is calculated by taking two normalized SPDs in the 10-nm intervals from 380 to 670 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.[8]

References

  1. ^ "CIE 17.4-1987 International Lighting Vocabulary". Archived from the original on 2010-02-27. Retrieved 2008-02-19.
  2. ^ a b "Television Lighting Consistency Index 2012". tech.ebu.ch. 31 May 2016.
  3. ^ a b "Spectral Similarity Index (SSI)". Oscars.org | Academy of Motion Picture Arts and Sciences. 21 April 2017.
  4. ^ Michael Royer (March 31, 2016). "Evaluating Color Rendering With TM-30" (PDF). ENERGY STAR Webinar (US DOE).
  5. ^ Rea, M.; Deng, L.; Wolsey, R. (2004). "Light Sources and Color". NLPIP Lighting Answers. Troy, NY: Rensselaer Polytechnic Institute. Archived from the original on 2010-06-11. Retrieved 2010-06-17.
  6. ^ Sproson, W. N. & Taylor, E. W. (1971). A colour television illumination consistency index. BBC Research Department Report 1971-45
  7. ^ European Broadcasting Union. "[Tech 3355] Method for the Assessment ofthe colorimetric properties of luminaires: The Television Lighting Consistency Index (TLCI-2012) &Television Luminaire Matching Factor (TLMF-2013)" (PDF). Retrieved 16 November 2021.
  8. ^ a b Academy of Motion Picture Arts and Sciences (2020-09-16). "Academy Spectral Similarity Index (SSI): Overview" (PDF).
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