SPECTRO-PHOTOMETRY (from Lat. spec trum, appearance, image, apparition, ± Gk. 063s, pluis, light + -per pia, -metric, measurement, from parpov, metron, measure), or SPECTRAL PHOTOMETRY (see also SPECTROSCOPY and PHOTOM ETRY). The study of the relative intensities of light of various colors from the same source or from different sources. Not only, as an ordinary photometry, may the relative intensities of two sources of white light or of monochromatic light be compared, but spectro-photometers are pro vided with dispersing prisms so arranged that the various colors of one beam of white light may be compared with the corresponding colors of an other. The general method is to bring some of the light from one source and some from the other source side by side in the same field of view, and by suitable means to alter the intensity of either beam (or of both) in a known degree until a `match' or photometric equality is secured; this condition of equality being determined by the vanishing of the line of separation between the two portions of the field of view, illuminated re spectively by the two sources to be compared. There is also a so-called 'method of contrast' de vised by Lumnier and Brodhun.
The most recent and efficient form of spectro photometer is that devised by Professor Brace of metrically situated with respect to the prism and provided with adjustable slits through which the light from the two sources enters the optical sys tem of the photometer. The amount of light entering the system through either collimator de pends upon the width of its slits. The width of one slit, say T, is, after the initial adjustments of the instrument, kept fixed throughout any one series of ohservations, while the width of the other collimator-slit (T') may be altered at will to secure a match in intensity of the two beams. Light of the same wave-length (i.e. of the same color) is thus brought by direct transmission from the collimator T, and after reflection at the silver strip from T', to the same focus in the observing telescope R. When the eyepiece is removed and the prism viewed through a slit in the focal plane of R, the eye sees three fields (as in Fig. 3), the central one,