Colorimeter

Colorimeter

[kuhl-uh-rim-i-ter]
A colorimeter, an instrument used in colorimetry, can refer to one of several related devices.

  • In scientific fields the word generally refers to the device that measures the absorbance of particular wavelengths of light by a specific solution. This device, invented by Jan Szczepanik, is most commonly used to determine the concentration of a known solute in a given solution by the application of the Beer-Lambert law, which states that the concentration of a solute is proportional to the absorbance.
  • In digital imaging, tristimulus colorimeters, also shortened to colorimeters, are used to profile and calibrate output devices.

Construction

The essential parts of a colorimeter are:

  • a light source (often an ordinary low-voltage filament lamp)
  • an adjustable aperture
  • a set of colored filters
  • a cuvette to hold the working solution
  • a detector (usually a photoresistor) to measure the transmitted light
  • a meter to display the output from the detector

In addition, there may be:

  • a voltage regulator, to protect the instrument from fluctuations in mains voltage.
  • a second light path, cuvette and detector. This enables comparison between the working solution and a "blank", consisting of pure solvent, to improve accuracy.

Filters

Changeable optics filters are used in the colorimeter to select the wavelength of light which the solute absorbs the most, in order to maximize accuracy. The usual wavelength range is from 400 to 700 nanometres (nm). If it is necessary to operate in the ultraviolet range (below 400 nm) then some modifications to the colorimeter are needed. In modern colorimeters the filament lamp and filters may be replaced by several light-emitting diodes of different colors.

Cuvettes

In a manual colorimeter the cuvettes are inserted and removed by hand. An automated colorimeter (as used in an AutoAnalyzer) is fitted with a flowcell through which solution flows continuously.

Controls

Output

The output from a colorimeter may be displayed by an analogue or digital meter and may be shown as transmittance (a linear scale from 0-100%) or as absorbance (a logarithmic scale from zero to infinity). The useful range of the absorbance scale is from 0-2 but it is desirable to keep within the range 0-1 because, above 1, the results become unreliable due to scattering of light.

In addition, the output may be sent to a chart recorder, data logger, or computer.

Tristimulus colorimeter

A tristimulus colorimeter—colloquially shortened to colorimeter—takes a limited number of wideband spectral energy readings (~3-7) along the visible spectrum by using filtered photodetectors; e.g. silicon photodiodes.

Originally, three glass filters whose transmittance spectra mimicked the CIE color matching functions (shown on the right) were employed. A filter bank may be used to decompose the individual color matching functions if more accuracy is desired.

A camera or colorimeter is said to be colorimetric if it satisfies the Luther condition (also called the "Maxwell-Ives criterion"), reducing observer metamerism color errors, if the product of the spectral responsivity of the photoreceptor and the spectral transmittance of the filters is a linear combination of the CMFs.

A colorimeter or a digital camera with a color filter array can, under certain conditions, be used as an alternative to a spectrophotometer.

The illuminant and observer conditions should be specified when citing a measurement (e.g. D65/10°).

The quality of a colorimeter may be assessed using the means in CIE publication 179:2007.

References

External links

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