Optical viewing device

A device for controlling the brightness of an optical signal overlaid on a specimen image includes: a main light source configured to illuminate a specimen with a main light source illumination; a main objective, configured to generate the specimen image in a viewer beam path; and a beam splitter configured to reflect the optical signal into the viewer beam path, where the optical signal is generated by a display, and an illumination of the display is selectable among the main light source illumination, a secondary light source illumination, and both, and where the secondary light source illumination is adjustable as a function of the main light source illumination. The device may be a surgical microscope. The illumination of the display by the main light source illumination may be indirect, where the display is illuminated substantially by light reflections of the main light source illumination from the specimen.