  One of the key ingredients in establishing the relation between input signal
and output flux from a spectrometer is accurate determination of the
spectrophotometric calibration. In the case of spectrometers onboard
satellites, the accuracy of this part of the calibration pedigree is ultimately
linked to the accuracy of the set of reference SEDs that the spectrophotometric
calibration is built on. In this paper, we deal with the spectrophotometric
calibration of infrared (IR) spectrometers onboard satellites in the 2 to 200
micron range. We aim at comparing the different reference SEDs used for the IR
spectrophotometric calibration. The emphasis is on the reference SEDs of
stellar standards with spectral type later than A0, with special focus on the
theoretical model atmosphere spectra. Using the MARCS model atmosphere code,
spectral reference SEDs were constructed for a set of IR stellar standards (A
dwarfs, solar analogs, G9-M0 giants). A detailed error analysis was performed
to estimate proper uncertainties on the predicted flux values. It is shown that
the uncertainty on the predicted fluxes can be as high as 10%, but in case
high-resolution observational optical or near-IR data are available, and IR
excess can be excluded, the uncertainty on medium-resolution SEDs can be
reduced to 1-2% in the near-IR, to ~3% in the mid-IR, and to ~5% in the far-IR.
Moreover, it is argued that theoretical stellar atmosphere spectra are at the
moment the best representations for the IR fluxes of cool stellar standards.
When aiming at a determination of the spectrophotometric calibration of IR
spectrometers better than 3%, effort should be put into constructing an
appropriate set of stellar reference SEDs based on theoretical atmosphere
spectra for some 15 standard stars with spectral types between A0V and M0III.
