Abstract: A perforation detector including a housing having a first portion and an oppositely disposed second portion for receiving the perforated area of a traveling web therebetween. One portion supports an optical signal emitter and the other portion supports a receiver for detecting the optical signal. The first portion is connected to a source of pressurized fluid such as filtered air. Such portion has a passage for directing the fluid flow, in the form of a fluid curtain, about the optical element in that portion and toward the optical element in the second portion. The fluid flow thus prevents accumulation of signal interferring particulate matter on and between the optical elements.