a radiation detector , serving as an infrared movement monitor , which is reactive to thermal radiation includes a housing having a window admitting the radiation and optics . the window having a size being coordinated with the angle of coverage of the optics disposed behind it . the optics focussing the radiation onto an optical sensor , preferably through a fresnel lens . the window having a frame for accepting and guiding at least one masking element formed of a strip - shaped plastic film diminishing the infrared radiation and being slideably displaced in a guide channel along the window for covering the optics in a variable sub - region thereof .