View source for Physics:Colorimetry

{{Short description|Technique to quantify and describe physically the human color perception}}
'''Colorimetry''' is "the science and technology used to quantify and describe physically the human color perception".<ref>{{cite conference
 | first = Yoshi
 | last = Ohno
 | date = 16 October 2000
| title = CIE Fundamentals for Color Measurements
 | conference = IS&T NIP16 Intl. Conf. on Digital Printing Technologies
 | pages = 540–45
 | url = http://physics.nist.gov/Divisions/Div844/facilities/photo/Publications/OhnoNIP16-2000.pdf
 | access-date = 18 June 2009
 | archive-url = https://web.archive.org/web/20090515032644/http://www.physics.nist.gov/Divisions/Div844/facilities/photo/Publications/OhnoNIP16-2000.pdf
 | archive-date = 15 May 2009
 | url-status = dead
 }}</ref>
It is similar to [[Physics:Spectrophotometry|spectrophotometry]], but is distinguished by its interest in reducing spectra to the physical correlates of color perception, most often the [[CIE 1931 color space#Tristimulus values|CIE 1931 XYZ color space tristimulus values]] and related quantities.<ref>{{cite book
 | title = Digital Color Imaging Handbook
 | author = Gaurav Sharma
 | publisher = CRC Press
 | year = 2002
 | isbn = 978-0-8493-0900-7
 | pages = 15–17
 | url = https://books.google.com/books?id=OxlBqY67rl0C&q=colorimetry+cie+xyz+tristimulus+values&pg=PA17
 }}</ref>

==History==
The Duboscq colorimeter was invented by  Jules Duboscq in 1870.
<ref>[https://www2.humboldt.edu/scimus/HSC.36-53/Descriptions/Color_B%26L.htm#:~:text=The%20Duboscq%20colorimeter%20was%20invented,of%20the%20various%20colorimeter%20designs.&text=It%20is%20also%20an%20instrument,similar%20instruments%20still%20being%20sold Cal Poly Humboldt]humboldt.edu {{Webarchive|url=https://web.archive.org/web/20130508143804/http://www.humboldt.edu/scimus/HSC.36-53/Descriptions/Color_B%26L.htm#:~:text=The%20Duboscq%20colorimeter%20was%20invented,of%20the%20various%20colorimeter%20designs.&text=It%20is%20also%20an%20instrument,similar%20instruments%20still%20being%20sold |date=8 May 2013 }}</ref>

== Instruments ==
Colorimetric equipment  is similar to that used in spectrophotometry. Some related equipment is also mentioned for completeness.

* A [[Physics:Tristimulus colorimeter|tristimulus colorimeter]] measures the tristimulus values of a color.<ref name=icc>{{Cite web|url=https://www.color.org/ICC_white_paper5glossary.pdf|title=ICC White Paper #5|accessdate=28 May 2023}}</ref>
* A [[Physics:Spectroradiometer|spectroradiometer]] measures the absolute spectral [[Physics:Radiance|radiance]] (intensity) or [[Physics:Irradiance|irradiance]] of a light source.<ref name=lee>{{cite book|chapter-url=https://books.google.com/books?id=CzAbJrLin_AC&q=spectroradiometer+spectral&pg=PA369|title=Introduction to Color Imaging Science|first=Hsien-Che|last=Lee|pages=369–374|chapter=15.1: Spectral Measurements|publisher={{wipe|Cambridge University Press}}|year=2005|isbn=0-521-84388-X|quote=The process recommended by the CIE for computing the tristimulus values is to use 1 nm interval or 5 nm interval if the spectral function is smooth}}</ref>
* A [[Physics:Spectrophotometer|spectrophotometer]] measures the spectral [[Physics:Reflectance|reflectance]], [[Physics:Transmittance|transmittance]], or relative irradiance of a color sample.<ref name=lee/><ref name=schanda>{{cite book|last=Schanda|first=János|title=Colorimetry: Understanding the CIE System|publisher=Wiley Interscience|year=2007|isbn=978-0-470-04904-4|chapter=Tristimulus Color Measurement of Self-Luminous Sources|pages=135–157|doi=10.1002/9780470175637.ch6}}</ref>
* A ''spectrocolorimeter'' is a spectrophotometer that can ''calculate'' tristimulus values.
* A [[Engineering:Densitometer|densitometer]] measures the degree of light passing through or reflected by a subject.<ref name=icc/>
* A ''color temperature meter'' measures the [[Earth:Color temperature|color temperature]] of an incident illuminant.

[[Image:ReflCurve.png|right|thumb|Two spectral reflectance curves. The object in question reflects light with shorter wavelengths while absorbing those in others, lending it a blue appearance.]]

=== Tristimulus colorimeter ===
{{Main|Physics:Tristimulus colorimeter}}
In [[Digital imaging|digital imaging]], colorimeters are tristimulus devices used for color calibration. Accurate color profiles ensure consistency throughout the imaging workflow, from acquisition to output.

=== Spectroradiometer, spectrophotometer, spectrocolorimeter ===
The absolute [[Physics:Spectral power distribution|spectral power distribution]] of a light source can be measured with a ''spectroradiometer'', which works by optically collecting the light, then passing it through a [[Physics:Monochromator|monochromator]] before reading it in narrow bands of wavelength.

Reflected color can be measured using a ''spectrophotometer'' (also called ''spectroreflectometer'' or ''reflectometer''), which takes measurements in the visible region (and a little beyond) of a given color sample. If the custom of taking readings at 10 nanometer increments is followed, the [[Physics:Visible light|visible light]] range of 400–700&nbsp;nm will yield 31 readings. These readings are typically used to draw the sample's spectral reflectance curve (how much it reflects, as a function of wavelength)—the most accurate data that can be provided regarding its characteristics.

[[Image:CRT_phosphors.png|thumb|CRT phosphors]]
The readings by themselves are typically not as useful as their tristimulus values, which can be converted into [[Physics:Chromaticity|chromaticity]] co-ordinates and manipulated through color space transformations.  For this purpose, a ''spectrocolorimeter'' may be used. A spectrocolorimeter is simply a spectrophotometer that can estimate tristimulus values by [[Numerical integration|numerical integration]] (of the color matching functions' inner product with the illuminant's spectral power distribution).<ref name=schanda/> One benefit of spectrocolorimeters over tristimulus colorimeters is that they do not have optical filters, which are subject to manufacturing variance, and have a fixed spectral transmittance curve—until they age.<ref>{{cite web|title=Colorimeter vs. Spectro|volume=2|issue=3|work=Colorsync-users Digest|access-date=6 May 2008|author=Andreas Brant, GretagMacbeth Corporate Support|date=7 January 2005|url=http://lists.apple.com/archives/Colorsync-users/2005/Jan/msg00118.html|archive-date=11 July 2018|archive-url=https://web.archive.org/web/20180711021847/https://lists.apple.com/archives/Colorsync-users/2005/Jan/msg00118.html|url-status=dead}}</ref> On the other hand, tristimulus colorimeters are purpose-built, cheaper, and easier to use.<ref>{{cite web|title=Colorimeter vs. Spectro|volume=2|issue=3|work=Colorsync-users Digest|access-date=6 May 2008|author=Raymond Cheydleur, X-Rite|date=8 January 2005|url=http://lists.apple.com/archives/Colorsync-users/2005/Jan/msg00120.html|archive-date=10 July 2018|archive-url=https://web.archive.org/web/20180710195804/https://lists.apple.com/archives/Colorsync-users/2005/Jan/msg00120.html|url-status=dead}}</ref>

The CIE (International Commission on Illumination) recommends using measurement intervals under 5&nbsp;nm, even for smooth spectra.<ref name=lee/> Sparser measurements fail to accurately characterize spiky emission spectra, such as that of the red phosphor of a CRT display, depicted aside.

=== Color temperature meter ===
Photographers and cinematographers use information provided by these meters to decide what [[Color balance|color balancing]] should be done to make different light sources appear to have the same color temperature. If the user enters the reference color temperature, the meter can calculate the [[Mired|mired]] difference between the measurement and the reference, enabling the user to choose a corrective [[Physics:Color gel|color gel]] or [[Physics:Photographic filter|photographic filter]] with the closest mired factor.<ref name=fpe>{{cite book|first=Carl|last=Salvaggio|title=The Focal Encyclopedia of Photography: Digital Imaging, Theory and Application|publisher=Focal Press|editor=Michael R. Peres|year=2007|page=741|edition=4E|isbn=978-0-240-80740-9|url=https://books.google.com/books?id=VYyldcYfq3MC&q=three+silicon+photodiodes+%22color+temperature%22&pg=RA1-PA741}}</ref>

[[Image:Planckian-locus.png|thumb|The normals are lines of equal correlated color temperature.]]
Internally the meter is typically a silicon photodiode tristimulus colorimeter.<ref name=fpe/> The [[Earth:Correlated color temperature|correlated color temperature]] can be calculated from the tristimulus values by first calculating the [[Physics:Chromaticity|chromaticity]] co-ordinates in the [[CIE 1960 color space]], then finding the closest point on the [[Planckian locus]].

== See also ==
* [[Color science]]
* [[Physics:Photometry (optics)|Photometry]]
* [[Astronomy:Radiometry|Radiometry]]

==References==
{{Reflist|30em}}

==Further reading==
* {{cite book
 | last = Schanda
 | first = János D.
 | year = 1997
 | title = Handbook of Applied Photometry
 | pages = 327–412
 | publisher = OSA/AIP
 | isbn = 978-1-56396-416-9
 | editor = Casimer DeCusatis
 | chapter = Colorimetry
 | chapter-url = http://www.knt.vein.hu/Tantargyak/Szinmeres/Colour.pdf
 | access-date = 17 July 2008
 | archive-date = 17 September 2005
 | archive-url = https://web.archive.org/web/20050917045049/http://www.knt.vein.hu/Tantargyak/Szinmeres/Colour.pdf
 | url-status = dead
 }}
* {{cite book|editor=Gaurav Sharma|title=Digital Color Imaging Handbook|chapter=Device Characterization|first=Raja|last=Bala|publisher=CRC Press|year=2003|chapter-url=http://chester.xerox.com/~raja/papers/CRC_Chapter5.pdf|isbn=978-0-8493-0900-7|access-date=5 May 2008|archive-date=28 May 2008|archive-url=https://web.archive.org/web/20080528051448/http://chester.xerox.com/~raja/papers/CRC_Chapter5.pdf|url-status=dead}}
* {{cite journal|url=http://nvl.nist.gov/pub/nistpubs/jres/112/3/V112.N03.A01.pdf|title=Comparison of Calibration Methods for Tristimulus Colorimeters|volume=112|issue=3|date=May–June 2007|first=James L.|last=Gardner|journal=Journal of Research of the National Institute of Standards and Technology|pages=129–138|doi=10.6028/jres.112.010|pmid=27110460|pmc=4656001|s2cid=1949232|access-date=2 February 2008|archive-url=https://web.archive.org/web/20080528051448/http://nvl.nist.gov/pub/nistpubs/jres/112/3/V112.N03.A01.pdf|archive-date=28 May 2008|url-status=dead}}
* {{cite web|url=http://www.handprint.com/HP/WCL/color6.html#colorimetry
|access-date=17 July 2008
|title=Overview of the development and applications of colorimetry
|date=8 May 2008
|first=Bruce 
|last=MacEvoy
|work=Handprint.com
}}
* Optronik – [https://web.archive.org/web/20080528051449/http://www.optronik.de/Photometer.pdf Photometers] An informative brochure with background information and specifications of their equipment.
* Konica Minolta Sensing – [http://www2.konicaminolta.eu/eu/Measuring/pcc/en/part1/index.html Precise Color Communication – from perception to instrumentation]
* HunterLab – [https://www.hunterlab.com/en/color-measurement-learning/application-notes FAQ | How to Measure Color of a Sample & Use An Index] A guide to measuring color and appearance of objects. The section provides information on numerical scales and indices that are used throughout the world to remove subjective measurements and assumptions.
* NIST Publications [https://www.nist.gov/fusion-search?utf8=true&affiliate=nist-search&s=colorimetry&commit=Search related to colorimetry].

==External links==
* [http://isp.uv.es/code/visioncolor/colorlab.html Colorlab] MATLAB toolbox for color science computation and accurate color reproduction (by Jesus Malo and Maria Jose Luque, Universitat de Valencia). It includes CIE standard tristimulus colorimetry and transformations to a number of non-linear color appearance models (CIE Lab, CIE CAM, etc.).

{{Color topics}}

[[Category:Physical quantities]]
[[Category:Measurement]]
[[Category:Radiometry]]

{{Sourceattribution|Colorimetry}}