The composition of articles can be identified using luminescent codes printed or coated onto the articles (or labels on the articles) by means of an ink which contains long persistence luminescent materials. Radiation is used to excite the luminescent materials and this is then extinguished before the article passes beneath a spectrometer. An infrared beam then strikes the article in order to stimulate emission from the long persistence luminescent materials, via a detrapping mechanism, and the spectrometer detects the emission in order to identify the article.