Abstract: Illumination unit (2) for an optical system, including an illumination system (1) which comprises, in this order, a radiation source (3) and a first optical integrator (11). The illumination unit (2) further comprises a detection system (45) including a radiation-sensitive detector (47). The illumination system (1) comprises a second optical integrator (13). The two integrators (11, 13) enclose a prism system (15) comprising at least one prism (17). The prism system (15) has a coupling-out surface via which light can be coupled out of the illumination system (1), and an exit surface via which light can be coupled out of the prism system (15), without the intensity in the main light path being essentially influenced. The detection system (45) is arranged proximate to the exit surface of the prism system (15) and comprises light-integrating means.

Abstract: An imaging apparatus and an illumination unit for use in such an apparatus are described, which unit comprises an illumination housing (LH) accommodating a radiation source (LA) and at least a reflector (RL). To prevent a decrease of the radiation power supplied by the illumination housing, this housing is provided with means (LA, SH, L.sub.29) which prevent a diffuse deposit of silicon-containing particles present in the ambient air from being formed on optical components within the illumination housing.