Importance

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Abstract

To monitor that infra-red radiation element 22 within housing 10 has not been disarmed, for instance by rendering opaque an infra-red transparent element 16 masking a window 12, in the housing 10, through which radiation may enter and be focussed by optical system 18 onto the element 22, an infra-red radiation generator 32 comprising a low-temperature lamp 34 is mounted externally of the housing 10 so that element 22 receives radiation therefrom and provides a signal to a processor 30. The latter will provide an alarm in the event of absence of radiation from the generator 32 and/or change in the radiation reaching the element 22 commensurate with approach of a person.

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First Claim