Abstract: A photoelectric smoke detector including a housing defining a test zone and an opening for admitting smoke thereinto, a light source arranged to direct light to the zone and a light responsive element arranged to receive light scattered by smoke particles in the zone. An optical barrier prevents the direct transmission of light through the zone and between the light source and light responsive element, the barrier defining a light passage for transmitting light directly therebetween. Projecting into the light passage is a mechanical gate that can be adjusted to alter the size of the light passage and thereby vary the level of light transmitted directly therethrough between the light source and the light responsive element. In response to selective adjustment of the mechanical gate, the light responsive element can provide a background signal that obviates normal electronic noise.