A passive infrared intrusion detection device uses a dual element pyroelectric detector and a housing for surrounding and containing the dual element pyroelectric detector. The housing has a plurality of Fresnel lens elements for gathering infrared radiation from a "look down" volume of space and...http://www.google.ca/patents/US5818337?utm_source=gb-gplus-sharePatent US5818337 - Masked passive infrared intrusion detection device and method of operation therefore 
