A spectrometer for measuring the intensity of light, comprises a grating having a major axis, a first entrance aperture aligned with the grating major axis and configured to direct light energy onto the grating, wherein the grating is adapted to produce a focused light beam, a first exit aperture aligned with the grating major axis and configured to accept the focused light beam, a second entrance aperture configured to direct the light energy onto the grating, wherein the second entrance aperture is offset from the grating major axis, and a second exit aperture configured to accept the focused light beam.