SMOKE DETECTOR

A smoke detector (1) has a spherical chamber (2) including a plurality of holes (15,17) for allowing smoke and other particles to enter the chamber. The majority of the internal surface (3) of the chamber (2) is covered with a high reflectivity Lambertian surface, that is a material that scatters incident light equally in all directions and at all wavelengths. The remaining portion of the internal surface (3) is coated with a light absorbing material (13) such as a matt black coating. A scatter sensor (9) is directed towards the absorbing coating (3), and an integrating detector (5) is configured to detect radiation directly from the entire Lambertian surface. A first LED (19) emits blue light into the chamber (2), and a second LED (21) emits infrared light into the chamber. Processing means (23, 25 and 27) are provided to analyse the signals from the detectors (5,9), including means for discriminating between signals from the sensors indicative of different frequencies of received radiation. The processing means can distinguish between different smoke types, such as white/grey smoke and black smoke, and can also distinguish between smoke and other particles.