A colorimeter is generally any tool that characterizes colour samples to provide an objective measure of colour characteristics. In chemistry, the colorimeter is an apparatus that allows the absorbance of a solution at a particular frequency (color) of visual light to be determined. Colorimeters hence make it possible to ascertain the concentration of a known solute, since it is proportional to the absorbance.
Different chemical substances absorb varying frequencies of the visible spectrum. Colorimeters rely on the principle that the absorbance of a substance is proportional to its concentration i.e., a more concentrated solution gives a higher absorbance reading. A filter in the colorimeter is used to select the color of light which the solute absorbs the most, in order to maximise the accuracy of the experiment. Note that the colour of the absorbed light is the 'opposite' of the colour of the specimen, so a blue filter would be appropriate for an orange substance. Sensors measure the amount of light which has passed through the solution, compared to the amount entering, and a display reads the amount absorbed.
A quantitative reading for the concentration of a substance can be found by making up a series of solutions of known concentration of the chemical under study, and plotting a graph of absorbance against concentration. By reading off the absorbance of the specimen substance on the graph, a value for its concentration is found.
In other applications, colorimeters are used to characterize and correct colour response in video monitors, or by photographers to calibrate colours in a photographic print. Colorimeters are also available for disabled people who suffer from blindness or colour-blindness, where subjective colour names are announced based on objective measurements of colour parameters (e.g. hue, saturation and luminance)
See also Spectrometer for an overview of an instrument that similarly uses the property of colour in the assessment of chemical composition in astronomy and other applications.
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