From the word 'spectrometer' and the Greek 'phot' meaning light. A spectrophotometer is an optical spectrometer that is designed for computation of the ratio of the radiant energy of two different light beams as a function of wavelength. One of the beams is typically called the ‘baseline’, 'reference', or ‘white reference’ and is often the diffuse equivalent of the ‘incident’ radiation. The output is typically ‘transmittance’, ‘reflectance’, or transformations of either.