System has an optical sensor (1), an aperture with the optical element (2) with the changeable permeability and an activation light (3) emitting radiation, which is detected by the optical sensor (1). The activation light is connected with the switch (4) and it is placed within the reach of the optical sensor (1). Within a device which realizes the technological process with the luminous manifestation the instruction for the beginning of a given technological process is detected and on the basis of this instruction the activation light (3) lights up. This activates the optical sensor (1) by means of the activation light (3) before the optical sensor (1) detects the luminous manifestation of the technological process itself. The activation light (3) can be a simple infrared LED diode. The advantage lies in the fact that the protective device itself does not have to be modified in any way and its reaction time can be shortened to zero, or negative reaction time can be achieved since we use time passing during the initiation of the technological process before the appearance of the luminous manifestation for the darkening.
