The photometry, the measurement of light, is an analytical method in chemistry to determine the concentration of a specific compound in solution by the measurement of the absorption of light. With the assistance of suitable reagents the substance to be analyzed is transformed into a colored compound, that absorbs light of a specific wave length. The intensity of the color shows a direct relationship to the concentration of the substance to be analyzed. 

By irradiating the colored sample with light of a suitable wavelength, the intensity of the incident light is attenuated by the absorption of the colored compound, the brightness decreases. The concentration of the substance to be analyzed can be calculated with the measured transmission and with the aid of calibration curves that are implemented in the photometer. 

The Finwell photometer is equipped with three LEDs as light sources, covering the wavelenghts of 520 nm, 570 nm, and 620 nm and three sensors tuned to the LEDs. With this setup of three different wavelengths, the best fitting wavelength for each color development can be picked, to provide a high accuracy of the measurement.