tristimulus colorimeter
{{short description|Device to measure color values}}
{{Other uses|Colorimeter (disambiguation){{!}}Colorimeter}}
A tristimulus colorimeter, colloquially shortened to colorimeter or colourimeter, is used in digital imaging to profile and calibrate output devices.
It takes a limited number of wideband spectral energy readings along the visible spectrum by using filtered photodetectors; e.g. silicon photodiodes.{{cite book|last=Schanda|first=János|author2=George Eppeldauer |author3=Georg Sauter |title=Colorimetry: Understanding the CIE System|publisher=Wiley Interscience|date=2007|isbn=978-0-470-04904-4|chapter=Tristimulus Color Measurement of Self-Luminous Sources|doi=10.1002/9780470175637.ch6|editor=János Schanda}}
A colorimeter with the known value of absolute error allows measuring (x,y)-chromaticity coordinates in red, green, blue and white colors. Measured values are used for calculation of LCD profile coefficients.{{cite journal|url=http://ntv.ifmo.ru/en/article/13462/metod_programmnoy_kompensaciitehnologicheskogo_razbrosa_koordinat_cvetnostizhidkokristallicheskih_paneley.htm|title=Method of software-based compensation of technological variation in chromaticity coordinates of LCD panels|author=Zharinov I.O., Zharinov O.O.|journal=Scientific and Technical Journal of Information Technologies, Mechanics and Optics|volume=15|issue=3|pages=387–397|year=2015|doi=10.17586/2226-1494-2015-15-3-387-397|doi-access=free}}
Originally, three glass filters whose transmittance spectra mimicked the CIE color matching functions (shown below) were employed.{{cite journal|title=Photoelectric tristimulus colorimetry with three filters|journal=JOSA|first=Richard Sewall|last=Hunter|date=September 1942|volume=32|issue=9|pages=509–538| url=http://www.opticsinfobase.org/abstract.cfm?URI=josa-32-9-509|doi=10.1364/JOSA.32.000509|hdl=2027/mdp.39015077308396|hdl-access=free}} A filter bank may be used to decompose the individual color matching functions if more accuracy is desired.{{cite journal|url=http://nvl.nist.gov/pub/nistpubs/jres/103/6/j36epp.pdf|title=Spectral Response Based Calibration Method of Tristimulus Colorimeters|first=George|last=Eppeldauer|journal=Journal of Research of the National Institute of Standards and Technology|volume=103|issue=6|date=Nov–Dec 1998|pages=615–619|doi=10.6028/jres.103.040|pmid=28009362|pmc=4890952}}
A camera or colorimeter is said to be colorimetric if it satisfies the {{Interlanguage link multi|Luther condition|de|3=Luther-Bedingung}} by {{Interlanguage link multi|Robert Thomas Dietrich Luther{{!}}Robert Luther|de|3=Robert Thomas Dietrich Luther}} (1868–1945) (also called the "Maxwell–Ives criterion"),{{cite journal|title=Aus dem Gebiet der Farbreizmetrik|last=Luther|first=Robert T.D.|authorlink=:de:Robert Thomas Dietrich Luther|date=1927|journal=Zeitschrift für technische Physik|volume=8|pages=540–558}} reducing observer metamerism color errors, if the product of the spectral responsivity of the photoreceptor and the spectral transmittance of the filters is a linear combination of the CMFs.{{cite book|title=Colorimetry|doi=10.1002/0470094745.ch5|page=154|first=Noboru|last=Ohta|author2=Robertson, Alan R. |isbn=978-0-470-09472-3|chapter=Measurement and Calculation of Colorimetric Values|date=2005|publisher=John Wiley & Sons, Ltd.}}
A colorimeter or a digital camera with a color filter array can, under certain conditions, be used as an alternative to a spectrophotometer.{{cite journal | author = Imai, Francisco H. |author2=Berns, Roy S. | date = 1999 | title = Spectral estimation using trichromatic digital cameras | journal = Proceedings of the International Symposium on Multispectral Imaging and Color Reproduction for Digital Archives | volume = 42 | url = http://www.cis.rit.edu/research/mcsl2/online/Spectral/TechnicalPapers/SpecEstimRGBCamera_Chiba99.pdf | accessdate = 2008-05-06 }}{{cite thesis | last = Solli | first = Martin |date=16 December 2004 | title = Filter characterization in digital cameras | url = http://www.diva-portal.org/diva/getDocument?urn_nbn_se_liu_diva-2667-1__fulltext.pdf | accessdate = 2008-05-06 }}
The illuminant and observer conditions should be specified when citing a measurement (e.g. D65/10°).{{cite book|url=https://books.google.com/books?id=oEsZiCt5VOAC&q=%22tristimulus+colorimeter%22+OR+spectrocolorimeter&pg=PA42|title=The Colour Image Processing Handbook|first=Stephen J.|last=Sangwine|author2=Horne |author3=Robin E. N. |page=41|publisher=Springer|date=1998|isbn=0-412-80620-7}}
The quality of a colorimeter may be assessed using the means in CIE publication 179:2007.{{cite book|title=Methods for characterising tristimulus colorimeters for measuring the colour of light|author=CIE TC 2-16 "Characterization of the Performance of Tristimulus Colorimeters"|isbn=978-3-901906-60-2|series=Publication 179:2007|date=2007|url=https://cie.co.at/publications/methods-characterising-tristimulus-colorimeters-measuring-colour-light|publisher=CIE Central Bureau|location=Vienna}}
There are various calibration methods for tristimulus colorimeters.{{cite journal | last = Gardner | first = James L. | title = Comparison of Calibration Methods for Tristimulus Colorimeters | journal = Journal of Research of the National Institute of Standards and Technology | volume = 112 | issue = 3 | pages = 129–138 | date = May–June 2007 | url = http://nvl.nist.gov/pub/nistpubs/jres/112/3/V112.N03.A01.pdf | doi = 10.6028/jres.112.010 | pmid = 27110460 | pmc = 4656001 | url-status = dead | archiveurl = https://web.archive.org/web/20080528051448/http://nvl.nist.gov/pub/nistpubs/jres/112/3/V112.N03.A01.pdf | archivedate = 2008-05-28 }}
File:Spyder4Elite color sensor.JPG|Color sensor of a colorimeter
File:CIE 1931 XYZ Color Matching Functions.svg|The CIE 1931 XYZ color matching functions