A colorimeter measures in either absorbance or percent transmittance, both measures of how much light of a particular color a solution transmits. This is a relative measurement, and only has meaning based on a pre-set standard of another solution. The colorimeter measures the concentration of a given solution based on the amount it affects the transmission of light, with greater effects indicating greater concentrations.
A colorimeter is a light measuring device that works by measuring the amount of light that passes through a solution, as described by the University of Michigan's Encyclopedia of Chemical Engineering Equipment. The color of the light varies depending on what is being measured. It works on a scientific principle known as Beer's law, which relates the concentration of a sample, its absorptivity, the curvature of the container, the light emitted and the light received.
Colorimeters can be used for various sorts of quality control and are able to measure important concentrations in food products and beverages. They are also used to measure the colored light coming from other sources, such as computer monitors. They are very quick to use, with readings available in under a second. They are also very portable – many handheld models are available and they have minimal power requirements. They are fairly expensive, and they less usable with reflective surfaces.