Patent ID: 8088479

Claim:
An apparatus for indicating the passage of time, comprising: a substrate comprising one substance selected from the group consisting of the following: ceramics, metals, plastics, papers, fibers; and an indicating layer formed on said substrate, having a thickness smaller than or equal to 1000 μm, and comprising a polymer matrix and a predetermined amount of redox compound under the reduced form; wherein said redox compound is transformed from the oxidized form to the reduced form by light radiation treatment and said redox compound is dispersed in said polymer matrix and has the following properties: the reduced form of said redox compound reacts with oxygen to form the oxidized form of said redox compound; the hue or color density of the reduced form of said redox compound is different from that of the oxidized form of said redox compound; the hue or color density of said indicating layer varies as the time for the reduced form of said redox compound contacting with oxygen in environment increases to have the amount of the oxidized form of said redox compound increase so as to indicate the passage of time; and said polymer matrix is a photopolymerizable polymer matrix comprising one substance selected from the group consisting of the following: acrylics, epoxy resins, polycarbonates, polyesters, polyurethane, polyolefins, silicones and siloxanes.