A beam detector (10) including a light source (32), a receiver (34), and a target (36), acting in co-operation to detect particles in a monitored area (38). The target (36), reflects incident light (40), resulting in reflected light (32) being returned to receiver (34). The receiver (34) is a receiver is capable of recording and reporting light intensity at a plurality of points across its field of view. In the preferred form the detector (10) emits a first light beam (3614) in a first wavelength band; a second light beam (3618) in a second wavelength band; and a third light beam (3616) in a third wavelength band, wherein the first and second wavelengths bands are substantially equal and are different to the third wavelength band.