A colorimeter is a device used for measuring colours, or colorimetry. It measures the absorbance of different wavelengths of light in a solution. It can be used to measure the concentration of a known solute.

Different chemical substances absorb different wavelengths of light. When the concentration of the solute is higher, it absorbs more light in a specific wavelength. This is known as the Beer-Lambert law.

The most important parts of a colorimeter are:

• a light source, which is usually an ordinary filament lamp
• an aperture, which can be adjusted
• a set of filters in different colors
• a detector that measures the light which has passed through the solution

Different filters are used to select the wavelength of light which the solution absorbs the most. This makes the colorimeter more accurate. Solutions are usually placed in glass or plastic cuvettes. The usual wavelengths used are between 400 and 700 nanometers. If it is necessary to use ultraviolet light (below 400 nanometers) then the lamp and filters must be changed.

The output of the colorimeter may be shown in graphs or tables, by an analogue or digital meter. The data may be printed on paper, or stored in a computer. The output 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. A Transmittance-Absorbance conversion table may be seen here.[1] Archived 2018-04-17 at the Wayback Machine