Colorimetry is "the science and technology used to quantify and describe physically the human

color perception Color vision, a feature of visual perception, is an ability to perceive differences between light composed of different wavelengths (i.e., different spectral power distributions) independently of light intensity. Color perception is a part of t ...

". It is similar to

spectrophotometry Spectrophotometry is a branch of electromagnetic spectroscopy concerned with the quantitative measurement of the reflection or transmission properties of a material as a function of wavelength. Spectrophotometry uses photometers, known as sp ...

, but is distinguished by its interest in reducing spectra to the physical correlates of color perception, most often the CIE 1931 XYZ color space tristimulus values and related quantities.

History

The Duboscq colorimeter was invented by

Jules Duboscq Louis Jules Duboscq (March 5, 1817 – September 24, 1886) was a French instrument maker, inventor, and pioneering photographer. He was known in his time, and is remembered today, for the high quality of his optical instruments. Life and wo ...

in 1870.

Instruments

Colorimetric equipment is similar to that used in spectrophotometry. Some related equipment is also mentioned for completeness. * A tristimulus colorimeter measures the tristimulus values of a color.ICC White Paper #5
/ref> * A spectroradiometer measures the absolute spectral

radiance In radiometry, radiance is the radiant flux emitted, reflected, transmitted or received by a given surface, per unit solid angle per unit projected area. Radiance is used to characterize diffuse emission and reflection of electromagnetic radiati ...

(intensity) or

irradiance In radiometry, irradiance is the radiant flux ''received'' by a ''surface'' per unit area. The SI unit of irradiance is the watt per square metre (W⋅m−2). The CGS unit erg per square centimetre per second (erg⋅cm−2⋅s−1) is often used ...

of a light source. * A spectrophotometer measures the spectral

reflectance The reflectance of the surface of a material is its effectiveness in reflecting radiant energy. It is the fraction of incident electromagnetic power that is reflected at the boundary. Reflectance is a component of the response of the electronic ...

transmittance Transmittance of the surface of a material is its effectiveness in transmitting radiant energy. It is the fraction of incident electromagnetic power that is transmitted through a sample, in contrast to the transmission coefficient, which is t ...

, or relative irradiance of a color sample. * A ''spectrocolorimeter'' is a spectrophotometer that can ''calculate'' tristimulus values. * A

densitometer A densitometer is a device that measures the degree of darkness (the optical density) of a photographic or semitransparent material or of a reflecting surface. The densitometer is basically a light source aimed at a photoelectric cell. It determ ...

measures the degree of light passing through or reflected by a subject. * A ''color temperature meter'' measures the

color temperature Color temperature is the color of light emitted by an idealized opaque, non-reflective body at a particular temperature measured in kelvins. The color temperature scale is used to categorize the color of light emitted by other light sources ...

of an incident illuminant.

Tristimulus colorimeter

digital imaging Digital imaging or digital image acquisition is the creation of a digital representation of the visual characteristics of an object, such as a physical scene or the interior structure of an object. The term is often assumed to imply or include t ...

, 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 In radiometry, photometry, and color science, a spectral power distribution (SPD) measurement describes the power per unit area per unit wavelength of an illumination ( radiant exitance). More generally, the term ''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 A monochromator is an optical device that transmits a mechanically selectable narrow band of wavelengths of light or other radiation chosen from a wider range of wavelengths available at the input. The name is from the Greek roots ''mono-'', ...

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 330px, Different lengths as in respect to the molecular scale. The nanometre (international spelling as used by the International Bureau of Weights and Measures; SI symbol: nm) or nanometer (American and British English spelling differences#-re, ...

increments is followed, the

visible light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 t ...

range of 400–700 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. The readings by themselves are typically not as useful as their tristimulus values, which can be converted into

chromaticity Chromaticity is an objective specification of the quality of a color regardless of its luminance. Chromaticity consists of two independent parameters, often specified as hue (h) and colorfulness (s), where the latter is alternatively called ...

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 In analysis, numerical integration comprises a broad family of algorithms for calculating the numerical value of a definite integral, and by extension, the term is also sometimes used to describe the numerical solution of differential equatio ...

(of the

color matching function The CIE 1931 color spaces are the first defined quantitative links between distributions of wavelengths in the electromagnetic visible spectrum, and physiologically perceived colors in human color vision. The mathematical relationships that defin ...

inner product In mathematics, an inner product space (or, rarely, a Hausdorff pre-Hilbert space) is a real vector space or a complex vector space with an operation called an inner product. The inner product of two vectors in the space is a scalar, often ...

with the illuminant's spectral power distribution). 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. On the other hand, tristimulus colorimeters are purpose-built, cheaper, and easier to use. The CIE (International Commission on Illumination) recommends using measurement intervals under 5 nm, even for smooth spectra. 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

cinematographer The cinematographer or director of photography (sometimes shortened to DP or DOP) is the person responsible for the photographing or recording of a film, television production, music video or other live action piece. The cinematographer is the ch ...

s use information provided by these meters to decide what 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 difference between the measurement and the reference, enabling the user to choose a corrective

color gel Color (American English) or colour (British English) is the visual perceptual property deriving from the spectrum of light interacting with the photoreceptor cells of the eyes. Color categories and physical specifications of color are associa ...

photographic filter In photography and cinematography, a filter is a camera accessory consisting of an optical filter that can be inserted into the optical path. The filter can be of a square or oblong shape and mounted in a holder accessory, or, more commonly, a ...

with the closest mired factor. Internally the meter is typically a silicon photodiode tristimulus colorimeter. The

correlated color temperature Color temperature is the color of light emitted by an idealized opaque, non-reflective body at a particular temperature measured in kelvins. The color temperature scale is used to categorize the color of light emitted by other light sources ...

can be calculated from the tristimulus values by first calculating the

co-ordinates in the

CIE 1960 color space The CIE 1960 color space ("CIE 1960 UCS", variously expanded ''Uniform Color Space'', ''Uniform Color Scale'', ''Uniform Chromaticity Scale'', ''Uniform Chromaticity Space'') is another name for the chromaticity space devised by David MacAdam. ...

, then finding the closest point on the Planckian locus.

References

External links

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.). {{Authority control Color Physical quantities Measurement Radiometry