embodiment the film of selected detector elements is poled to a polarity opposite that of the other elements to cause internal cancellation of signals due to events which affect an even number of detectors simultaneously application_number US 94265978 A description referring now to the drawings a passive infrared intruder detection system according to a preferred embodiment of the invention is shown to comprise a generally cylindrical housing 10 a top view being shown in cross section in fig1 within which a spherical mirror 12 having a center of curvature c and a focal length f is positioned at one end 14 the opposite end 16 of housing 10 is sealed by an infrared radiation transmitting window 18 preferably fabricated from a thin sheet of polyethylene this material is relatively transparent to infrared radiation at the wavelengths of interest i e between 6 and 15 microns and acts to eliminate air currents and dust which could adversely affect the system performance preferably the concave reflecting surface 20 of mirror 12 is coated with a material which scatters or absorbs visible and near infrared radiation and is relatively transparent to wavelengths longer than approximately 4 microns such a material as is known in the art is magnesium oxide powder which scatters radiation up to 4 microns and becomes transparent at longer wavelengths thereby permitting the underlying mirror to focus target generated infrared images at the focus of mirror 12 is positioned an array 22 of pyroelectric detectors 24 as is well known a spherical mirror acts to focus incoming parallel rays at a point which is spaced from the mirror surface a distance f equal to one half the center of curvature thus.