A spectrophotometer is an instrument that measures the amount of light transmitted through a sample. It uses light energy in the ultraviolet (UV) or visible light spectrum to detect molecules in a solution. The spectrophotometer shines a beam of light on a sample, and measures how the sample interacts with the light through absorption, reflection, or transmission. By comparing the transmittance or absorbance values of an unknown sample to standards of known concentration, the spectrophotometer can determine the concentration of the unknown sample.