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A colorimeter is a device used in colorimetry. 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 is 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.
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.
Changeable optics filters are used in the colorimeter to select the wavelength which the solute absorbs the most, in order to maximize accuracy. The usual wavelength range is from 400 to 700 nanometers (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.
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.
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.
- Nuffield Advanced Chemistry (2003)
-  Anzalone GC, Glover AG, Pearce JM. Open-Source Colorimeter. Sensors (2013); 13(4):5338-5346
- The Measurement of Colour
- The Nuffield Foundation 2003. March 30, 2003. 
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- "Colorimetry" Encyclopædia Britannica. Encyclopædia Britannica Online. Encyclopædia Britannica Inc. (2011) 17 November 2011. 
- Orion Colorimetry Theory. The Technical Edge. 
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