A method of initiating a requested action in a computer system via a printed substrate. The substrate has human-readable information and machine-readable coded data printed thereon. The coded data is indicative of a substrate identity and of a plurality of locations on the substrate. The method comprises the steps of: (a) interacting with the substrate using an optically imaging sensing device; (b) imaging the coded data; (c) generating indicating data in the sensing device using the imaged coded data, the indicating data identifying the substrate identity and a position of the sensing device relative to the substrate; and communicating the indicating data to the computer system so that the computer system can initiate the requested action. The coded data is printed with an ink comprising an IR-absorbing naphthalocyanine dye.