The PRobe far-Infrared Mission for Astrophysics (PRIMA) is an infrared observatory for the next decade, currently in Phase A, with a 1.8 m telescope actively cooled to 4.5 K. On board, an infrared camera, PRIMAger, equipped with ultra-sensitive kinetic inductance detector arrays, will provide observers with a coverage of mid-infrared to far-infrared wavelengths from 24 to 264 μm. PRIMAger will offer two imaging modes: the hyperspectral mode will cover the 24 to 84 μm wavelength range with a spectral resolution R ≥ 8, whereas the polarimetric mode will provide polarimetric imaging in four broadbands from 80 to 264 μm. These observational capabilities have been tailored to answer fundamental astrophysical questions such as black hole and star-formation co-evolution in galaxies, the evolution of small dust grains over a wide range of redshifts, and the effects of interstellar magnetic fields in various environments, as well as to open a vast discovery space with versatile photometric and polarimetric capabilities. PRIMAger is being developed by an international collaboration bringing together French institutes (Laboratoire d’Astrophysique de Marseille and CEA) through the Center National d’Etudes Spatiales (CNES, Paris, France), the Netherlands Institute for Space Research (SRON, Leiden, Netherlands), and the Cardiff University (Cardiff, UK) in Europe, as well as the Jet Propulsion Laboratory and Goddard Space Flight Center in the United States.

We present the preliminary design of the calibration unit of the future E-ELT instrument METIS. This independent subunit is mounted externally to the main cryostat of METIS and will function both as calibration reference for science observations, as well as verification and alignment tool during the AIT phase. In this paper, we focus on describing its preliminary layout and foreseen functionalities, based on the performance requirements defined at system level and the constraints imposed by warm IR background. We discuss the advantage of employing an integrating sphere as common radiation emitter, leading to a novel and versatile design, where the source's spatio-spectral properties can be varied with high fidelity and repeatability. By combining only few tuneable sources and mechanisms we show how a large instrument such as METIS can be calibrated and tested, without the need of a complex cold calibration unit.