Passive infrared detectors

1. 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.

2. A passive infrared detector assembly according to claim 1 and wherein said at least one sensor is a pyroelectric sensor.

3. A passive infrared detector assembly according to claim 1 and wherein said at least one sensor is sensitive to radiation having wavelengths in the range of 5-14 μm.

4. A passive infrared detector assembly according to claim 1 and wherein the at least one infrared light emitter is operative to emit radiation having wavelengths in the range of 0.7-1 μm.

5. A passive infrared detector assembly according to claim 1 and wherein said infrared radiation receiver is operative to receive radiation reflected back from the near vicinity of said passive infrared detector.

6. A passive infrared detector assembly according to claim 1 and wherein said near infrared radiation receiver is located within said detector housing.

7. A passive infrared detector assembly according to claim 1 and wherein said at least one infrared light emitter is located within said detector housing.

8. A passive infrared detector assembly according to claim 1 and wherein at least one of pitch, height, curvature and finish of said plurality of teeth like portions is configured to cause said near infrared radiation receiver to differentiate between different liquids.

9. A method for detecting masking of a passive infrared detector, the method comprising:

providing a detector housing comprising a window transparent to far infrared radiation;

disposing within said detector housing at least one sensor sensitive to said far infrared radiation; and

providing 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.

10. A method according to claim 9 and wherein said disposing at least one sensor comprises disposing a pyroelectric sensor within said housing.

11. A method according to claim 9 and wherein said disposing at least one sensor comprises disposing at least one sensor which is sensitive to radiation having wavelengths in the range of 5-14 μm.

12. A method according to claim 9 and wherein the at least one infrared light emitter is operative to emit radiation having wavelengths in the range of 0.7-1 μm.

13. A method according to claim 9 and wherein said infrared radiation receiver is operative to receive radiation reflected back from the near vicinity of said passive infrared detector.

14. A method according to claim 9 and wherein said near infrared radiation receiver is located within said detector housing.

15. A method according to claim 9 and wherein said at least one infrared light emitter is located within said detector housing.

16. A method according to claim 9 and also comprising configuring at least one of pitch, height, curvature and finish of said plurality of teeth like portions to cause said near infrared radiation receiver to differentiate between different liquids.