A source of polychromatic light with a continuous UV spectrum is separated by a prism or diffraction grating into component UV wavelengths between 200 and 400 nm (nanometers) and visible wavelengths between 400 and 800 nm. Each wavelength is split into two equal-intensity beams by a half-mirrored device. The sample beam passes through a container of the compound being studied. A reference beam passes through an identical container of sample solvent.
The spectrometer automatically scans all the component wavelengths passing through the containers. The intensity of each wavelength is measured by electronic detectors and compared. The reference beam intensities are subtracted to determine the absorption of energy by the sample at each wavelength.
The spectrometer records all the wavelengths at which absorption occurs together with the amount of absorption at each wavelength. The instrument then creates a graph, called a spectrum, of absorbance versus wavelength.