A spectrophotometer is a device for measuring the relative amounts of radiant energy or radiant flux as a function of wavelength. Spectrophotometers consist of two instruments, a spectrometer for producing light of any selected color (wavelength), and a photometer for measuring the intensity of light. There are two major classes of spectrophotometers; single beam and double beam. A double beam spectrophotometer measures the ratio of the light intensity on two different light paths, and a single beam spectrophotometer measures the absolute light intensity.