A method and device for monitoring a measurement object. A passive unit has a light source, which radiates radiated light modulated by at least one external measurement influence from the measurement object. An active unit has an optical detector, which receives the radiated and modulated light via an optical link. In addition, there is a transmitter unit emitting energy, such as optical energy, sound energy, electromagnetic energy etc. The emitted energy is coded by an energy signature and sent to the passive unit via an energy link. The passive unit receives the emitted energy by a receiver unit, which decodes the energy signature and moderates the radiated and modulated light in dependence of the energy signature. The received energy is also used to drive the light source. A processor unit may decode the signal for discriminating the signal from error sources.