Apparatus and method for detecting tampering with an infra-red motion sensor

A passive infrared (PIR) motion detector comprising:

• a. an infra-red motion detector Fresnel lenslet body comprising:
• i. an array of Fresnel lenslets provided on said body and corresponding to motion detection zones in a monitored space for collecting and focusing light onto a passive infrared sensor target;
• and
• ii. at least one anti-masking optical structure provided on said body,
• b. a housing having said lenslet body mounted thereon;
• c. at least one passive infra-red radiation sensor within said housing substantially opposite said lenslet body to receive IR light collected from said zones in said monitored space, said infra-red sensor being sensitive to infra-red radiation wavelengths including infra-red radiation wavelengths emanated by a human body;
• d. at least one active light source for projecting light onto said anti-masking optical structure;
• and
• e. at least one tamper sensor for detecting said light source light redirected from said anti-masking optical structure,
• wherein said active light source is configured to project proximity detection light in front of said lenslet body, and said tamper sensor is configured to detect proximity detection light reflected from an object placed near said lenslet body.