  Photometric instruments operating at far infrared to millimetre wavelengths
often have broad spectral passbands (central wavelength/bandwidth ~ 3 or less),
especially those operating in space. A broad passband can result in significant
variation of the beam profile and aperture efficiency across the passband,
effects which thus far have not generally been taken into account in the flux
calibration of such instruments. With absolute calibration uncertainties
associated with the brightness of primary calibration standards now in the
region of 5% or less, variation of the beam properties across the passband can
be a significant contributor to the overall calibration accuracy for extended
emission. We present a calibration framework which takes such variations into
account for both antenna-coupled and absorber-coupled focal plane
architectures. The scheme covers point source and extended source cases, and
also the intermediate case of a semi-extended source profile. We apply the new
method to the Herschel-SPIRE space-borne photometer.
