A photometer is an analytical instrument designed to measure the intensity of light passing through a substance, providing quantitative data regarding light absorption or transmission. These devices operate based on principles of spectrophotometry, utilizing a light source, a monochromator to select a specific wavelength, and a detector to measure the transmitted light intensity. By measuring the rate of change in light absorbance across various wavelengths, photometers allow for the determination of concentrations of various solutes, particularly in chemical and biological samples. The function is critical in numerous scientific disciplines, including clinical diagnostics, environmental monitoring, and research chemistry, where the quantitative analysis of colorimetric reactions is necessary.