The appts. has a radiation source and a receiver at the centre of the radiation beam passing through the medium. It is used for detection of radiation extinction by suspended particles. At least one further radiation receiver (4) is provided outside the radiation beam centre (2) in a zone directly adjacent to the first radiation detector (3), receiving radiation scattered from the beam. The two receivers (3, 4) are connected to an evaluation circuit which delivers similar signals when irradiation of the central receiver (3) decreases, and irradiation of the further receiver (4) increase.
