Marking of articles with photochromic compounds

Abstract

A method of marking an article or substrate with a mark or device which is made visible or is enhanced on exposure to U.V. light which comprises printing onto the article or substrate a photochromic ink containing a photochromic compound which is reversibly converted to a colored form on exposure to U.V. light and reverts to a substantially colorless form in the absence of U.V. light and applying a superficial protective layer to said mark or device which protects the phototochromic compound against degradation by atmospheric moisture and oxygen. In addition the invention includes a security device in the form of a label, document or card which incorporates a mark or device which is partially or wholly invisible in light which does not contain a U.V. component, which comprises a substrate having printed thereon using a photochromic ink a mark or device and having a clear, superficial protective layer over said mark or device gives protection for the photochromic compound against degradation by atmospheric moisture and oxygen, said photochromic compound being converted to a colored form on exposure to U.V. light and reverting to a pale or substantially colorless form in the absence of U.V. light.

Dependencies are inferred from the claim text.