After a space instrument is built, and when tests have shown that it can withstand the harsh launch and space environment, a thorough calibration campaign with the instrument is performed. In a dedicated set-up on ground that simulates the space environment, the instrument is illuminated extensively, with well-known light sources, under controlled conditions, to characterize its response. Analysis of the instrument data and the properties of the sources provides the conversion factors (calibration key data) that enable us to translate raw in-orbit measurements to physical quantities describing for example the light level, wavelength and degree of polarization. After launch, we also perform in-orbit calibrations to correct for changes that may occur once the instrument works in space, assuring proper scientific data quality throughout the mission lifetime (see e.g. Sentinel-5P/TROPOMI and SPEX).