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(New page: {{CogPsy}} Scale of saturation (0% at bottom). In colorimetry and color theory, '''colorfulness''', '''chroma''', and '''saturation''...)
 
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[[Image:saturationdemo.png|thumb|right|60px|Scale of saturation (0% at bottom).]]
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{{confuse|calorimetry}}
   
In [[colorimetry]] and [[color theory]], '''colorfulness''', '''chroma''', and '''saturation''' are related concepts referring to the intensity of a specific [[color]]. More technically, ''colorfulness'' is the perceived difference between the color of some [[stimulus (physiology)|stimulus]] and [[grey|gray]], ''chroma'' is the colorfulness of a stimulus relative to the [[brightness]] of a stimulus that appears white under similar viewing conditions, and ''saturation'' is the colorfulness of a stimulus relative to its own brightness.<ref>Mark D. Fairchild. “[http://www.cis.rit.edu/fairchild/PDFs/AppearanceLec.pdf Color Appearance Models: CIECAM02 and Beyond]”. Slides from a tutorial at the IS&T/SID 12th Color Imaging Conference. [[9 November]] [[2004]]. Retrieved [[19 September]] [[2007]].</ref> Though this general concept is intuitive, terms such as ''chroma'', ''saturation'', ''purity'', and ''intensity'' are often used without great precision, and even when well-defined depend greatly on the specific [[color model]] in use.
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'''Colorimetry''' is the science that describes [[color]]s in numbers, or provides a physical color match using a variety of measurement instruments. Colorimetry is used in [[chemistry]], and in industries such as color printing, textile manufacturing, paint manufacturing and in the food industry.
   
A highly colorful stimulus is vivid and intense, while a less colorful stimulus appears more muted, closer to gray. With no colorfulness at all, a color is a “neutral” gray. With three attributes—colorfulness (or chroma or saturation), [[lightness (color)|lightness]] (or brightness), and [[hue]]—any color can be described.
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Colorimetry is not an exact science due to the limitations inherent in the system ([[Metamerism (color)|metamerism]] being the most troublesome), the design of the measurement devices, the values used to estimate a given light source, etc. Colors that look the same seldom have the same spectral characteristics in any colorimetric system you employ, even assuming identical viewing conditions and identical observers with normal color vision.
   
== Saturation ==
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Colorimetry utilizes the standard color science recommendations provided by the [[International Commission on Illumination|CIE]].
   
Saturation is one of three coordinates in the [[HSL and HSV]] [[color space]]s.
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== Instruments ==
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* An absorption [[colorimeter]], in physical chemistry, determines the spectral [[absorbance]] characteristics of a [[solution]].
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* A [[colorimeter#Tristimulus colorimeter|tristimulus colorimeter]] measures the [[tristimulus value]]s of a color.<ref name=icc>[http://www.color.org/ICC_white_paper5glossary.pdf ICC White Paper #5]</ref>
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* A [[spectroradiometer]] measures the absolute spectral [[radiance]] (intensity) or [[irradiance]] of a light source.<ref name=lee>{{cite book|url=http://books.google.com/books?id=CzAbJrLin_AC&pg=PA369&dq=spectroradiometer+spectral&ei=xrewR_GKHZHCzASjo_DRAw&sig=ar9VSAEe55o94rK1FYs04fONBQA |title=Introduction to Color Imaging Science|first=Hsien-Che|last=Lee|page=369|chapter=15.1: Spectral Measurements|publisher=[[Cambridge University Press]]|year=2005|isbn=052184388X}}</ref>
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* A [[spectrophotometer]] measures the spectral [[reflectance]], [[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}}</ref>
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* A ''spectrocolorimeter'' is a spectrophotometer that can ''calculate'' tristimulus values.
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* A [[densitometer]] measures the degree of light passing through or reflected by a subject.<ref name=icc/>
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* A ''color temperature meter'' measures the [[color temperature]] of an incident illuminant.
   
The saturation of a color is determined by a combination of light intensity and how much it is distributed across the spectrum of different wavelengths. The purest color is achieved by using just one wavelength at a high intensity, such as in laser light. If the intensity drops, so does the saturation. To desaturate a color in a [[subtractive color|subtractive]] system (such as [[watercolor]]), you can add [[white]], [[black]], [[gray (color)|gray]], or the hue's [[complementary color|complement]].
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[[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.]]
   
Various correlates for saturation follow.
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=== Absorption colorimeter ===
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In [[physical chemistry]], a colorimeter is a device used to test the concentration of a solution by measuring its absorbance of a specific wavelength of light. To use this device, different [[solutions]] must be made, and a control (usually a mixture of [[distilled water]] and another solution) is first filled into a [[cuvette]] and placed inside a colorimeter to calibrate the machine. Only after the device has been calibrated can you use it to find the densities and/or concentrations of the other solutions. You do this by repeating the calibration, except with cuvettes filled with the other solutions.
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The filter on a colorimeter must be set to red if the liquid is blue.
   
; [[CIELUV]] : The chroma normalized by the luminance:
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Initially, the size of the filter chosen for the colorimeter is extremely important, as the wavelength of light that is transmitted by the colorimeter has to be same as that absorbed by the substance.
   
:<math>s_{uv}=13 \sqrt{(u'-u'_n)^2+(v'-v'_n)^2}=\frac{C^*_{uv}}{L^*}</math>
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=== Tristimulus colorimeter ===
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In [[digital imaging]], colorimeters are used for [[color calibration]]. Accurate [[ICC Profile|color profiles]] ensure consistency throughout the imaging workflow, from acquisition to output.
   
where <math>(u'_n,v'_n)</math> is the chromaticity of the white point, and the L*C*h color space is defined below.<ref>{{citation|last=Schanda|first=János|title=Colorimetry: Understanding the CIE System|publisher=[[Wiley Interscience]]|year=2007|isbn=978-0-470-04904-4 | url = http://books.google.com/books?id=g8VDAgAACAAJ&dq=intitle:Colorimetry+intitle:Understanding+intitle:the+intitle:CIE+intitle:System&lr=&as_brr=0&ei=p6aiR4KYOaOuiQGwwO1A }}, page 88.</ref>
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=== Spectroradiometer ===
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The absolute [[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 [[monochromator]] before reading it in narrow bands of wavelength.
   
; [[CIECAM02]] : The square root of the ''colorfulness'' divided by the ''brightness'':
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=== Spectrophotometer ===
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Reflected color can be measured using a ''spectrophotometer'', 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 [[visible light]] range of 400-700nm will yield 31 readings. These readings are typically used to draw the sample's [[reflectivity|spectral reflectance]] curve (how much it reflects, as a function of wavelength); the most accurate data that can be provided regarding its characteristics.
   
:<math>s=\sqrt{M/Q}</math>
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The readings by themselves are typically not as useful as their [[tristimulus]] values, which can be converted into [[chromaticity]] co-ordinates and manipulated through [[color translation|color space transformations]]. For this purpose, a ''spectrocolorimeter'' may be used, although tristimulus colorimeters offer a cheaper alternative. A spectrocolorimeter is simply a spectrophotometer that can estimate tristimulus values by [[numerical integration]] (of the [[color matching function]]s' [[inner product]] with the illuminant's spectral power distribution).<ref name=schanda/> The CIE recommends using measurement intervals under 5nm, even for smooth spectra<ref name=lee/>.
   
This definition is inspired by experimental work done with the intention of remedying [[CIECAM97s]]'s poor performance.<ref name=ciecam02>{{cite conference|title=The CIECAM02 Color Appearance Model|booktitle=IS&T/SID Tenth Color Imaging Conference|last=Moroney|first=Nathan|coauthors=Fairchild, Mark D.; Hunt, Robert W.G.; Li, Changjun; Luo, M. Ronnier; Newman, Todd|url=http://www.polybytes.com/misc/Meet_CIECAM02.pdf|location=[[Scottsdale, Arizona]]|id=ISBN 0-89208-241-0|year=2002|month=November 12|publisher=The [[Society for Imaging Science and Technology]]}}
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=== Color temperature meter ===
</ref><ref>{{cite conference|title=Magnitude estimation for scaling saturation|first=Lu-Yin G.|last=Juan|coauthor=Luo, Ming R.|url=http://spiedl.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PSISDG004421000001000575000001&idtype=cvips&gifs=yes| booktitle=[[Proceedings of SPIE]]|volume=4421|conference=9th Congress of the International Colour Association|editors=Robert Chung, Allan Rodrigues|month=June|year=2002|pages=575-578|doi=10.1117/12.464511}}</ref> It should be noted that M is proportional to the chroma C (<math>M=CF_L^{0.25}</math>), thus the CIECAM02 definition bears some similarity to the CIELUV definition. An important difference is that the CIECAM02 model accounts for the viewing conditions through the parameter <math>F_L</math>.<ref name=ciecam02/>
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[[Photographer]]s and [[cinematographer]]s use information provided by these meters to decide what [[color correction]] 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]] difference between the measurement and the reference, enabling the user to choose a corrective [[color gel]] or [[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=pg.741|edition=4E|isbn=0240807405|url=http://books.google.com/books?id=VYyldcYfq3MC&pg=RA1-PA741&lpg=RA1-PA741&dq=three+silicon+photodiodes+%22color+temperature%22&source=web&ots=sYm8zsOh8V&sig=uoNw3QoF85yaP99mmYOvlm7vJrE#PRA1-PA741,M1|}}</ref>
   
== Excitation purity ==
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Internally, the meter is typically a [[silicon photodiode]] tristimulus colorimeter.<ref name=fpe/>
   
[[Image:Excitation Purity.svg|300px|thumb|right|'''Excitation purity''' is the relative distance from the white point. Contours of constant purity can be found by shrinking the spectral locus about the white point. The points along the line segment have the same hue, with p<sub>e</sub> increasing from 0 to 1 between the white point and the spectral locus.]]
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== See also ==
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* [[Photometry (optics)|Photometry]]
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* [[Radiometry]]
   
The '''excitation purity''' (purity for short) of a stimulus is its difference from the illuminant's [[white point]] relative to the furthest point on the chromaticity diagram with the same hue ([[dominant wavelength]] for [[monochromatic]] sources); using the [[CIE 1931 color space]]:<ref>{{cite book|isbn=978-0-470-04904-4|url=http://books.google.com/books?id=CU7-2ZLGFpYC&pg=PA121&dq=%22excitation+purity%22&ei=YuKgR_jmJYjAiwHZyeWwCg&ie=ISO-8859-1&sig=7VmVRKi7b5dJQZKlfwEFQrTysls
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==References==
|title=The Focal Encyclopedia of Photography|publisher=[[Focal Press]]|first=Leslie D.|last=Stroebel|coauthor=Zakia, Richard D.]]|page=121|year=1993|isbn=0240514173|edition=3E}}</ref>
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<references/>
   
:<math>p_e = \sqrt{\frac{(x - x_n)^2 + (y - y_n)^2}{(x_I - x_n)^2 + (y_I - y_n)^2}}</math>
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==Further reading==
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* [http://nvl.nist.gov/pub/nistpubs/jres/112/3/V112.N03.A01.pdf Comparison of Calibration Methods for Tristimulus Colorimeters]
   
where <math>(x_I,y_I)</math> is the chromaticity of the white point and <math>(x_n,y_n)</math> is the point on the perimeter whose line segment to the white point contains the chromaticity of the stimulus. Different color spaces, such as CIELAB or CIELUV may be used, and will yield different results.
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== External links==
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* [http://www.optronik.de/Photometer.pdf Colorimetric equipment by X-Rite Optronik] An informative brochure with background information and specifications of their equipment.
== Chroma in CIE 1976 L*a*b* and L*u*v* color spaces ==
 
 
The naïve definition of saturation does not specify its response function. In the CIE XYZ and RGB color spaces, the saturation is defined in term of additive color mixing, and has the property of being proportional to any scaling centered at white or the white point illuminant. However, both color spaces are nonlinear in terms of psychovisually perceived color differences. It is also possible, and sometimes desirable to define a saturation-like quantity that is linearized in term of the psychovisual perception.
 
 
In the [[Lab color space|CIE 1976 L*a*b* and L*u*v* color spaces]], the unnormalized '''chroma''' is the radial component of the cylindrical coordinate CIE L*C*h (luminance, chroma, hue) representation of the L*a*b* and L*u*v* color spaces, also denoted as CIE L*C*h(a*b*) or CIE L*C*h for short, and CIE L*C*h(u*v*). The transformation of <math>(a^{*}, b^{*})</math> to <math>(C^{*}, h)</math> is given by:
 
 
:<math>C^{*} = \sqrt{a^{*2} + b^{*2}}</math><br><br>
 
:<math>h = \arctan \frac{b^{*}}{a^{*}}</math>
 
 
and analogously for CIE L*C*h(u*v*).
 
 
The chroma in the CIE L*C*h(a*b*) and CIE L*C*h(u*v*) coordinates has the advantage of being more psychovisually linear, yet they are non-linear in the in term of linear component color mixing. And therefore, chroma in CIE 1976 L*a*b* and L*u*v* color spaces is very much different from the traditional sense of "saturation".
 
 
===Chroma in color appearance models===
 
 
Another, psychovisually even more accurate, but also more complex method to obtain or specify the saturation is to use the color appearance model, like CIECAM. The '''chroma''' component of the JCh (lightness, chroma, hue) coordinate, and becomes a function of parameters like the chrominance and physical brightness of the illumination, or the characteristics of the emitting/reflecting surface, which is also psychovisually more sensible.
 
 
==Comparison==
 
 
<gallery>
 
Image:Surfing in Hawaii.jpg|Original image
 
Image:Surfing in Hawaii (retouched).jpg|Contrast and saturation increased
 
</gallery>
 
 
== References ==
 
 
{{reflist}}
 
 
==See also==
 
 
* [[Luminance (relative)]]
 
   
 
[[Category:Color]]
 
[[Category:Color]]
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[[Category:Physical quantity]]
   
 
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Colorimetry is the science that describes colors in numbers, or provides a physical color match using a variety of measurement instruments. Colorimetry is used in chemistry, and in industries such as color printing, textile manufacturing, paint manufacturing and in the food industry.

Colorimetry is not an exact science due to the limitations inherent in the system (metamerism being the most troublesome), the design of the measurement devices, the values used to estimate a given light source, etc. Colors that look the same seldom have the same spectral characteristics in any colorimetric system you employ, even assuming identical viewing conditions and identical observers with normal color vision.

Colorimetry utilizes the standard color science recommendations provided by the CIE.

Instruments Edit

File:ReflCurve.png

Absorption colorimeter Edit

In physical chemistry, a colorimeter is a device used to test the concentration of a solution by measuring its absorbance of a specific wavelength of light. To use this device, different solutions must be made, and a control (usually a mixture of distilled water and another solution) is first filled into a cuvette and placed inside a colorimeter to calibrate the machine. Only after the device has been calibrated can you use it to find the densities and/or concentrations of the other solutions. You do this by repeating the calibration, except with cuvettes filled with the other solutions. The filter on a colorimeter must be set to red if the liquid is blue.

Initially, the size of the filter chosen for the colorimeter is extremely important, as the wavelength of light that is transmitted by the colorimeter has to be same as that absorbed by the substance.

Tristimulus colorimeter Edit

In digital imaging, colorimeters are used for color calibration. Accurate color profiles ensure consistency throughout the imaging workflow, from acquisition to output.

Spectroradiometer Edit

The absolute 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 monochromator before reading it in narrow bands of wavelength.

Spectrophotometer Edit

Reflected color can be measured using a spectrophotometer, 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 visible light range of 400-700nm 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.

The readings by themselves are typically not as useful as their tristimulus values, which can be converted into chromaticity co-ordinates and manipulated through color space transformations. For this purpose, a spectrocolorimeter may be used, although tristimulus colorimeters offer a cheaper alternative. A spectrocolorimeter is simply a spectrophotometer that can estimate tristimulus values by numerical integration (of the color matching functions' inner product with the illuminant's spectral power distribution).[3] The CIE recommends using measurement intervals under 5nm, even for smooth spectra[2].

Color temperature meter Edit

Photographers and cinematographers use information provided by these meters to decide what color correction 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 difference between the measurement and the reference, enabling the user to choose a corrective color gel or photographic filter with the closest mired factor.[4]

Internally, the meter is typically a silicon photodiode tristimulus colorimeter.[4]

See also Edit

ReferencesEdit

  1. 1.0 1.1 ICC White Paper #5
  2. 2.0 2.1 2.2 Lee, Hsien-Che (2005). "15.1: Spectral Measurements" Introduction to Color Imaging Science, Cambridge University Press.
  3. 3.0 3.1 Schanda, János (2007). "Tristimulus Color Measurement of Self-Luminous Sources" Colorimetry: Understanding the CIE System, Wiley Interscience.
  4. 4.0 4.1 Salvaggio, Carl (2007). Michael R. Peres The Focal Encyclopedia of Photography: Digital Imaging, Theory and Application, 4E, Focal Press.

Further readingEdit

External linksEdit

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