Schematics of the MpI concept. Two options are shown for the reference source: (a) a de-focused image of the instantaneous field-of-view; and (b) a cold reference. The polariser P1 provides the needed polarised input to the MpI. P2 is the beam splitter defining the two arms, while P3, in the cryostat, dispatches the two projections of the polarised signal to the focal-plane arrays. The incoming beam is represented by a spectral distribution S(ν).