This artist's view shows the combined focal plane of the two instruments on board ESA's Planck spacecraft. The High Frequency Instrument (HFI) is visible as a circular forest of horns at the centre, surrounded by the Low Frequency Instrument (LFI) ring of horns.
The two instruments detect the collected radiation in different ways. LFI is designed to convert the lower energy microwaves into electrical voltages, rather like a transistor radio. HFI works by converting the higher energy microwaves to heat, which is then measured by a tiny electrical thermometer. The instruments share a common telescope.