This artist's impression shows the focal plane of the two instruments on board ESA's Planck spacecraft. The instruments detect the collected radiation differently.
The Low Frequency Instrument (LFI) is designed to convert the lower energy microwaves into electrical voltages, rather like a transistor radio. The High Frequency Instrument (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.