| abstract |
Objects having a coating containing a phosphor are identified (e.g. to verify a document or to open a lock) by exposing the coating to UV radiation, filtering the resulting phosphorescent radiation, detecting the filtered phosphorescent radiation at one or more times during its decay, and producing a value representative of the detected phosphorescence, which value may be displayed. Measurement at two times overcomes the effects of ageing of the phosphor, and the use of more than one phosphor and filter gives improved security. |