Abstract: Radiation detection apparatus is disclosed including a first detector (6) for detecting radiation at a wavelength which is preferably in the infra-red region, and a device for sensing the presence of an obturating element (11), which element acts to prevent operation of the first detector. The sensing means includes a transmitter (14) which transmits a signal at a second wavelength, and a second detector (15) responsive to and arranged to receive the signal in the presence of the obturating element, preferably by reflection of the signal from the element. An alarm (10) may be activated in response to detection of radiation or of the transmitted signal.

Abstract: This invention is a radio frequency identity system in which each user has a token containing a low power transmitter. If he wants to enter a security area, a receiver operates to let him enter if it recognizes that the transmission is correct. A correct transmission is one having a set number of R.F. carrier cycles between successive modulation pulses.In an alarm system a user can identify himself in a similar manner.