An illuminating apparatus for a screen operable in at least two operating modes, viz B1 for a free viewing mode and B2 for a restricted viewing mode. The illuminating apparatus comprising: a backlight with a planar extension radiating light in a restricted angular range; a plate-shaped light guide that is, in a viewing direction, arranged in front of the backlight and with outcoupling elements one of the large surfaces and/or within its volume. The light guide is transparent to at least 70% of the light emitted by the backlight and light sources are arranged laterally at the edges of the light guide. Various parameters are specified for the number of outcoupling elements per unit area and for their extension, and in operating mode B2, the backlight is switched on and the light sources are switched off. In operating mode B1 at least the light sources are switched on.