A system for determining a calibrated spectral measurement includes a tunable Fabry-Perot etalon, a detector, and a processor. The tunable Fabry-Perot etalon has a settable gap. The detector measures light intensity. The processor is configured to determine the calibrated spectral measurement. The calibrated spectral measurement is based at least in part on a measurement set of detected light intensities for a plurality of settable gaps and a reconstruction matrix. The reconstruction matrix is based at least in part on calibration measurements. For a calibration measurement, a settable gap is selected and a set of input monochromatic source wavelengths is used to measure responses at a detector after transmission through the Fabry-Perot etalon. Each input monochromatic source wavelength is also measured using a radiometer to scale detector measurements.