Patent ID: 7875852

Claim:
A passive infrared detector assembly comprising: a detector housing comprising a window transparent to far infrared radiation; at least one sensor sensitive to said far infrared radiation and disposed within said detector housing; and an anti-masking subassembly, including: at least one light guide having at least one serrated light aperture, said light aperture defining a plurality of teeth like portions and a plurality of grooves and being disposed close to the outside of said window; at least one infrared light emitter operative to emit near infrared radiation, via said light aperture of said at least one light guide, outside of said at least one light guide; and a near infrared radiation receiver operative to receive said near infrared radiation emitted by said at least one near infrared light emitter via said light aperture and to measure the level of received near infrared radiation and to provide a masking alarm signal upon detection of a predetermined change in the level of said received near infrared radiation, wherein masking agents accumulating in said plurality of grooves change the level of said received near infrared radiation by a degree which exceeds a predetermined alarm threshold thereby causing the provision of a masking alarm signal, and wherein water accumulating in said plurality of grooves change the level of said received near infrared radiation by a degree which does not exceed said predetermined alarm threshold thereby not causing the provision of a masking alarm signal.