Abstract: A multi-pass spectrometer. Light from a source is expanded and collimated with a collimating optic into a collimated beam. The collimated beam illuminates a dispersing optic such as a grating from which light is reflected back toward the collimating optic. A transmitting-reflecting optic is positioned across the collimated beam to cause at least two reflections of at least a portion of the collimated beam from the dispersing optic. A photometer measures the double dispersed beam at a range of spacial locations in order to determine spectral characteristics of the light from the light source. In a preferred embodiment, a partially reflecting mirror is positioned between the grating and a collimated optic. This mirror is positioned at an angle slightly different from 90 degrees so that a portion of the light reflected from the dispersing optic is reflected back toward the dispersing optic at a slightly different range of angles.