Abstract: An optical arrangement for a light scattering type smoke detector has a radiation transmitter and receiver which are each preceded by a lens for obtaining a generally parallel field of view for each of the transmitter and receiver. The radiation is directed at a detection zone which is surrounded by diaphragms and the fields of view for the transmitter and receiver terminate in a light trap for preventing unwanted scattering and reflection. The transmitter and receiver are disposed at an acute angle with respect to each other and the receiver detects only back-scattered radiation from smoke particles in the detection area. This structure allows a relatively short structural length to be utilized along a common optical axis of the transmitter and receiver.