colorimetry

(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.

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.

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 and colorimeter.

For more information on colorimetry, visit Britannica.com.

The science of measuring color by defining it and estimating its intensity.

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Not to be confused with calorimetry.

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.

Using a colorimeter

A colorimeter is the device used in colorimetry. 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.

Instruments

A colorimeter takes 3 wideband readings along the visible spectrum to obtain a rough estimate of a color sample. For critical color matching a spectrophotometer that takes readings 31 times along the visible spectrum would be employed. A densitometer is sufficient to measure lightness and darkness. A spectroradiometer measures the colors of light sources.

Range of challenges in colorimetry

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.

The measurement devices - in depth

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. Color can be measured using a spectrophotometer, which takes measurements in the visible region (and a little more on both ends,) of a given color sample. The spectral reflectance curve is the most accurate data that can be provided regarding a color's characteristics. However, a spectral reflectance curve is a graph of 31 readings taken at 10 nanometer increments along the electromagnetic spectrum from 400 to 700 nanometers. The plot is often referred to as the DNA of the color. However, the 31 values have little practical application. Thus the values are mathematically reduced to 3 values via a calculation that integrates the "standard observer" and your chosen light source, ending up with 3 tristimulus values, which need to be converted yet again into coordinates in the desired color space.

Colorimetry utilizes the standard color science calculations provided by the International Lighting Standards Commission (CIE) in 1931. 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.

See also

• Metamerism (color)
• Spectrophotometry
• spectral power distributions

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