Abstract: Smoke detection apparatus including a pair of photoelectric cells which receive illumination in a predetermined ratio from a very low level light source. An improved simulating device is included for the purpose of testing the correct functioning of the alarm system connected to a sensing circuit formed by the photocells. The effects normally produced by smoke present in a chamber are simulated by selectively directing light from an auxiliary light source onto the sensing or "reflected" cell in the sensing circuit. In addition, a specially designed and constructed sensor head is provided for avoiding discrepancies in the placement of the critical elements; moreover, improved flow of any smoke present is promoted by the specific geometry of the sensor head.