SWS photometric calibration will mainly rely on stellar sources. A preparatory programme to be used by all ISO experiments is being executed at ESO and other observatories. A preliminary account of this programme can be found in van der Bliek et al. (1992, The Messenger 70, 28). The goal is to obtain accurate near-infrared photometry, on a single homogeneous system, for a large number of stars selected for non-peculiar properties. Atmospheric models will be used to extrapolate the observations to longer wavelengths. At longer wavelengths within the SWS range, stars will be supplemented by asteroids from the set to be used for long-wavelength calibration of PHT and LWS. If found necessary, additional secondary calibrators for regular use may be obtained by tying more stars, compact planetary nebulae or ultracompact HII regions to the primary standards.
Grating wavelength calibration will be mainly based on H recombination lines and fine structure lines in planetary nebulae and ultracompact HII regions. For the Fabry-Pérot, H recombination lines in the very brightest planetary nebulae will provide the final reference, supplemented by water emission lines from the envelopes of oxygen-rich late-type stars.