We demonstrate a static multiplex spectrometer based on a Fabry-Perot interferometric filter for measuring the mean spectral content of diffuse sources. By replacing the slit of a low-dispersion grating spectrometer with a Fabry-Perot interferometric filter, we improve the resolving power of the instrument while simultaneously overcoming the free spectral range limitation of the Fabry Perot. The resulting instrument is smaller than conventional spectrometers having the same resolving power. We present experimental results from the spectrometer using neon lamp, He-Ne laser, and diode laser sources over a wavelength range from 620 nm to 660 nm. © 2009 SPIE-IS&T.