A device for monitoring a measurement object, comprising: an active unit having a light source emitting light with a wavelength spectrum and an optical detector. An optical link passes the emitted light to a at least one passive unit. Each passive unit comprises a sensor and a selector for diverting the emitted light to the sensor. The sensor comprises a luminescent material being directly or indirectly affected by the emitted light diverted by the selector. The sensor is sensitive to an external influence by the measurement object for producing a modulated signal, which is passed to said detector via the optical link. The luminescent material may be a fluorescent material, which is directly irradiated by the emitted light from the light source.