An imaging riometer for ionospheric studies (IRIS) is described. This phased-array system has been continuously operating in northern Finland since late 1994, and permits spatial observation of ionospheric absorption of radio waves over the surrounding area. The paper gives an introduction to the principles of riometry and ionospheric absorption; it describes the physical implementation of the system and presents some initial results. Consideration is also given to the effect of the absorption on radio links operating in the auroral regions, which can be severe.