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Colorimetry

 
Sci-Tech Dictionary: colorimetry
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(′kəl·ə′rim·ə·trē)

(optics) Any technique by which an unknown color is evaluated in terms of standard colors; the technique may be visual, photoelectric, or indirect by means of spectrophotometry; used in chemistry and physics.

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Britannica Concise Encyclopedia: colorimetry
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Measurement of the intensity of electromagnetic radiation in the visible spectrum transmitted through a solution or transparent solid. It is used to identify and determine the concentrations of substances that absorb light of a specific wavelength or colour according to Lambert's law, which relates the amount of light absorbed to the distance traveled through the absorbing medium, and Beer's law, relating it to the concentration of absorbing substance in the coloured solution. A photocell is often used to measure the amount of light transmitted through a glass tube containing the solution to be analyzed; the result is compared with results from a similar tube containing solvent alone. Most elements and many compounds, in appropriately treated samples, may be identified by colorimetry or spectrophotometry, a closely related technique.

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Sci-Tech Encyclopedia: Colorimetry
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Any technique by which an unknown color is evaluated in terms of known colors. Colorimetry may be visual, photoelectric, or indirect by means of spectrophotometry. These techniques are widely used in scientific studies involving the appearance of objects and lights, but are of greatest importance in the color specification of the raw materials and finished products of industry.

In visual colorimetry, the unknown color is presented beside a comparison field into which may be introduced any one of a range of known colors from which the operator chooses the one matching the unknown. To be generally applicable, the comparison field must not only cover a sufficient color range but must also be continuously adjustable in color.

In indirect colorimetry, the light leaving the unknown specimen is split into its component spectral parts by means of a prism or diffraction grating, and the amount of each component part is separately measured by a photometer. The quantity evaluated is spectral radiance of a light source, spectral transmittance of a filter (glass, plastic, gelatin, or liquid), or spectral reflectance of an opaque body.

In photoelectric colorimetry, the light leaving the specimen is measured separately by three photocells. The spectral sensitivity of these photocells is adjusted, usually by color filters, to conform as closely as possible to the three color-mixture functions for the average normal human eye (CIE standard observer). The responses of the photocells give directly the amounts of red, green, and blue primaries required to produce the color of the unknown specimen for the kind of vision represented by the three photocells.

If two objects have the same color because the light leaving one of them toward the eye is spectrally identical to that leaving the other, any type of colorimetry serves reliably to establish the fact of color match. If, however, the two lights are spectrally dissimilar, they may still color-match for any one observer; such pairs of lights are called metamers. Normal color vision differs sufficiently from person to person so that a metameric color match for one observer may be seriously mismatched for another. On this account, the question of color match of spectrally dissimilar lights can be reliably settled only by the indirect method which uses spectrophotometry combined with a precisely defined standard observer.

Computer Desktop Encyclopedia: colorimetry
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The science of measuring color. The International Commission on Illumination (Commission Internationale de l'Eclairage) governs this subject along with all aspects of lighting and illumination. See CIE Lab and colorimeter.

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Veterinary Dictionary: colorimetry
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The science of measuring color by defining it and estimating its intensity.

Wikipedia: Colorimetry
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This article is about measurement of color. For use in chemistry, see Colorimetry (chemical method).
Not to be confused with Calorimetry.

Colorimetry or Colourimetry is "the science and technology used to quantify and describe physically the human color perception."[1] It is similar to spectrophotometry, but is distinguished by its interest in reducing spectra to the physical correlates of color perception, most often the CIE XYZ tristimulus values and related quantities.[2]

Contents

Instruments

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

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 article: Tristimulus colorimeter

In 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 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.

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 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.

CRT phosphors

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. 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).[5] 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.[6] On the other hand, tristimulus colorimeters are purpose-built, cheaper, and easier to use.[7]

The CIE recommends using measurement intervals under 5 nm, even for smooth spectra.[4] 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 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.[8]

The normals are lines of equal correlated color temperature.

Internally the meter is typically a silicon photodiode tristimulus colorimeter.[8] The correlated color temperature can be calculated from the tristimulus values by first calculating the chromaticity co-ordinates in the CIE 1960 color space, then finding the closest point on the Planckian locus.

See also

References

  1. ^ Ohno, Yoshi (2000-10-16). "CIE Fundamentals for Color Measurements" (PDF). IS&T NIP16 Intl. Conf. on Digital Printing Technologies. Proc.. pp. 540-545. 
  2. ^ Gaurav Sharma (2002). Digital color imaging handbook. CRC Press. p. 15–17. ISBN 9780849309007. http://books.google.com/books?id=OxlBqY67rl0C&pg=PA17&dq=colorimetry+cie+xyz+tristimulus+values&lr=&as_brr=3&ei=QVM6Sq-3CoHgkwTfrsGPAw#PPA15,M1. 
  3. ^ a b ICC White Paper #5
  4. ^ a b c Lee, Hsien-Che (2005). "15.1: Spectral Measurements". Introduction to Color Imaging Science. Cambridge University Press. p. 369–374. ISBN 052184388X. "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" 
  5. ^ a b Schanda, János (2007). "Tristimulus Color Measurement of Self-Luminous Sources". Colorimetry: Understanding the CIE System. Wiley Interscience. doi:10.1002/9780470175637.ch6. ISBN 978-0-470-04904-4. 
  6. ^ Andreas Brant, GretagMacbeth Corporate Support (2005-01-07). "Colorimeter vs. Spectro". Colorsync-users Digest. http://lists.apple.com/archives/Colorsync-users/2005/Jan/msg00118.html. Retrieved 2008-05-06. 
  7. ^ Raymond Cheydleur, X-Rite (2005-01-08). "Colorimeter vs. Spectro". Colorsync-users Digest. http://lists.apple.com/archives/Colorsync-users/2005/Jan/msg00120.html. Retrieved 2008-05-06. 
  8. ^ a b Salvaggio, Carl (2007). Michael R. Peres. ed. The Focal Encyclopedia of Photography: Digital Imaging, Theory and Application (4E ed.). Focal Press. p. 741. ISBN 0240807405. 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. 

Further reading

External links


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