Abstract: 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.