Black-body measurements made in the laboratory are used to determine the small scale structure of the RSRF's for all bands. If the use of these RSRF's on standard observations result in similar `features' being seen in many sources, and it is proven these `features' come from the RSRFs files, they are removed by comparing the RSRF files with AOT 1 or AOT 6 observations of standard astronomical objects for which accurate SEDs are known.
For the shorter wavelengths (2.36-16 microns) 
Lyrae (HR7001) is being
used as the primary in-orbit calibration source. For the longer wavelengths,
the asteroids Ceres and Pallas were initially chosen as the primary calibration
sources. Unfortunately, the solar system tracking performance of the ISO
satellite disturbs these measurements. Therefore, NML-CYG and Gamma Draconis
(HR6705) have been observed in order to uncover possible discrepancies between
lab and in-orbit measurements. Other objects used to derive the currents RSRFs
are Boo (HR5340), 
Leo (HR4534), Delta Draconis (HR7310) and
Pegasus (HR8775).
Models used for comparison include ones by Cohen, Kurucz and Marcs.