The smoke detector has a fixing socket in which a detector insert with an optical module is fitted. The optical module has a light source (6) and a photodetector (7) contained within a measuring chamber having a base (11). The module further has a labyrinth system provided by plates (9) which project inwards from the periphery of the measuring chamber. All the critical components of the optical module have smooth surfaces, so that the non-absorbed light is reflected in a defined direction. The optical module components may be provided by plastics injection moulding.