Abstract: A security system is provided in which the monitoring of an article to detect an alarm condition involves sensing the effect of the article on ambient electromagnetic radiation, such as light, and initiating an alarm when the effect of the article on the radiation indicates an alarm condition. According to one aspect of the invention, the effect of the article on ambient radiation is sensed by comparing direct measurements of the radiation with measurements of the radiation along a path normally intercepted by the article. In one form, the security arrangement of the invention includes two light sensing devices, one of which is arranged behind the article to be protected, or is otherwise concealed by such article, and the other of which is arranged to sense ambient radiation. These lead directly into a group of comparators, the output of which is sensed for rate of change in order to control the sounding of a local alarm or a full-security alarm.