Colourimetry is "the science and technology used to quantify and describe physically the human colour perception." It is similar to spectrophotometry, but is distinguished by its interest in reducing spectra to the physical correlates of colour perception, most often the CIE 1931 XYZ colour space tristimulus values and related quantities.
Colourimetric equipment is similar to that used in spectrophotometry. Some related equipment is also mentioned for completeness.
- A tristimulus colourimeter measures the tristimulus values of a colour.
- A spectroradiometer measures the absolute spectral radiance (intensity) or irradiance of a light source.
- A spectrophotometer measures the spectral reflectance, transmittance, or relative irradiance of a colour sample.
- A spectrocolourimeter is a spectrophotometer that can calculate tristimulus values.
- A densitometer measures the degree of light passing through or reflected by a subject.
- A colour temperature meter measures the colour temperature of an incident illuminant.
Spectroradiometer, spectrophotometer, spectrocolourimeter
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 colour 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 colour sample. If the custom of taking readings at 10 nanometer increments is followed, the visible light 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 co-ordinates and manipulated through colour space transformations. For this purpose, a spectrocolourimeter may be used. A spectrocolourimeter is simply a spectrophotometer that can estimate tristimulus values by numerical integration (of the colour matching functions' inner product with the illuminant's spectral power distribution). One benefit of spectrocolourimeters over tristimulus colourimeters 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 colourimeters are purpose-built, cheaper, and easier to use.
The CIE 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.
Colour temperature meter
Photographers and cinematographers use information provided by these meters to decide what colour balancing should be done to make different light sources appear to have the same colour temperature. If the user enters the reference colour temperature, the meter can calculate the mired difference between the measurement and the reference, enabling the user to choose a corrective colour gel or photographic filter with the closest mired factor.
Internally the meter is typically a silicon photodiode tristimulus colourimeter. The correlated colour temperature can be calculated from the tristimulus values by first calculating the chromaticity co-ordinates in the CIE 1960 colour space, then finding the closest point on the Planckian locus.
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- Gaurav Sharma (2002). Digital Colour Imaging Handbook. CRC Press. pp. 15–17. ISBN 978-0-8493-0900-7.
- ICC White Paper #5
- Lee, Hsien-Che (2005). "15.1: Spectral Measurements". Introduction to Colouuur Imaging Science. Cambridge University Press. pp. 369–374. ISBN 0-521-84388-X. "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"
- Schanda, János (2007). "Tristimulus Colour Measurement of Self-Luminous Sources". Colourimetry: Understanding the CIE System. Wiley Interscience. doi:10.1002/9780470175637.ch6. ISBN 978-0-470-04904-4.
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- Bala, Raja (2003). "Device Characterization". In Gaurav Sharma. Digital Colour Imaging Handbook. CRC Press. ISBN 978-0-8493-0900-7.
- Gardner, James L. (May–June 2007). "Comparison of Calibration Methods for Tristimulus Colourimeters" (PDF). Journal of Research of the National Institute of Standards and Technology 112 (3): 129–138. doi:10.6028/jres.112.010.
- MacEvoy, Bruce (2008-05-08). "Overview of the development and applications of colourimetry". Handprint.com. Retrieved 2008-07-17.
- Optronik - Photometers An informative brochure with background information and specifications of their equipment.
- Konica Minolta Sensing - Precise Colour Communication – from perception to instrumentation
- Hunter LAB - A guide to colour and appearance A comprehensive guide to measuring colour 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. TS
- NIST Publications related to colourimetry.
- X-Rite - A Guide to Understanding Colour Communication