Abstract: The invention can be used in biology and for environmental studies using fluorimeters. The inventive method consists in producing exiting light pulses having equal amplitude and modifiable duration, in measuring fluorescent chlorophyll characteristics at a constant background illumination simulating the irradiation intensity of an object during studies in the natural conditions and after the adaption thereof in the dark and in determining the state of a photosynthetic apparatus according to the entirety of fluorescent intensity values. The inventive device comprises the even number of measuring light sources, a current stabiliser of the light sources, the outputs of which are connected to electric inputs of the light sources, the input of which is connected to a control unit and a natural irradiation sensor is connected to the current stabiliser of the light sources through said control unit.