<title>Tunable Sources</title>

Abstract

Before the laser, if you wanted light at a particular optical frequency there were only two practical possibilities: you could either get lucky and find a spectral lamp which happened to have an emission line at the right frequency, or you would have to resort to using a broadband light source and filtering its output. The filter could take the form of an absorbing glass, a monochromator, or an interference filter -- depending on how narrow you wanted the spectrum and how little light you were prepared to put up with. Since all conventional incoherent light sources are approximately in thermal equilibrium at some well-defined temperature, their spectral radiance can never exceed that of a blackbody at the same temperature.