This publication discloses a remote identifier, a reader, and a method for activating a desired remote identifier. The remote identifier (2) includes a microcircuit (7), in which there is a memory and means for processing a radio-frequency signal, an antenna (6) connected to the microcircuit (7), by means of which both the signal and also electric power for the operating voltage of the microcircuit (7) can be received. According to the invention, a component (3), the electrical properties of which change due to the effect of infrared, or visible light, is electrically connected to the microcircuit (7) of the remote identifier and in the microcircuit (7) there are means, by which the combined effect of a change in the radio-frequency signal and the electrical component (3) can be expressed by a correlation method for the signals, in order to activate the remote identifier (2) for two-way data transmission.