Water quality photometers and colorimeters are ion-specific computer-interfaced probes designed to determine the concentration of a solution from its color intensity. The color of a solution may be inherent or derived by adding another reagent to it. Monochromatic light from an LED light source passes through a cuvette containing a solution sample. Some of the incoming light is absorbed by the solution. As a result, light of a lower intensity strikes a photodiode. The amount of light that penetrates a solution is known as transmittance. Transmittance can be expressed as the ratio of the intensity of the transmitted light, It, and the initial intensity of the light beam, Io, as expressed by the formula:
