Â© OSA 2015. Unexpected diffraction effects can become a real issue for the correct operation of far infrared wide field spatio-spectral interferometers now being proposed. Beam sizes may be too small in terms of the wavelength to remain collimated over the inter-component distances encountered in such systems. This results in some components being located in the Fresnel diffraction zone of an image of the pupil or the sky and geometrical optics becoming unreliable. In the presentation we demonstrate the application of various long-wavelength quasi-optical approaches to such systems. We also discuss how for wide field imaging interferometry it is possible to describe the operation of the interferometer with an efficient quasi-optical multi-mode approach.