Device for determination of the light intensity of a light beam (7) has a beam splitter (1) and a detector (11). The beam splitter divides off a measurement beam (23) from the incident beam and the ratio of the intensity of the light in the incident beam to that of the measurement beam is measured using the detector to ensure that the ratio remains constant. Beam splitter and detector form a single unit. Independent claims are also made for: (1) A microscope; (2) A microscopy method in which the intensity of the incident illumination beam is monitored.
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