The invention relates to an apparatus for illuminating a viewing field, for example a specimen (10) under a microscope (7), by means of at least two light sources (1a, 1b). The illumination apparatus comprises an apparatus for measuring the brightness of the light sources (1a, 1b). The result of this measurement can be transmitted to the observer (9), either acoustically or visually, via a computer (4) that compares the signal with a definable threshold value. The beam paths (103, 104) of the two light sources (1a, 1b) are guided via a combining light-guide coupler (109), a common light guide (2), and a separating light-guide coupler (111) having two light-guide arms (112, 15), one of the two light-guide arms (112) being delivered to the microscope (7) as a main light source, and the other light-guide arm (15) to a handpiece (16) as a second light source of the microscope (7). At least one of the two light sources (1a, 1b) is configured as a changeable unit (illumination changer), and at least one of the input light-guide arms (107, 108) is configured as a changeable unit (light-guide changer).