| 摘要 |
An electronic surveillance system in which a passive label attached to an article under surveillance is interrogated by means of a transmitted signal in a first form of energy, the label using the energy of that signal to return a signal to a receiver which gives an indication of the presence of the label if a reply signal has predetermined characteristics. In order to enable the receiver to distinguish the reply signal from the original transmitted signal the label is constructed to produce the reply signal in a form of energy different from that of the original transmitted signal. It is preferred that the first form of energy is accoustic energy and the second form of energy is electromagnetic energy. |
