The invention provides an optical treatment system comprising a first optical window (20), a light source (14), a control unit (18) for controlling the light source (14) such that optical radiation at the first optical window (20) has at least one optical parameter with a predetermined first value, and comprising at least one transmitting adjustment member (30) having a second optical window (34) and being positionable such that optical radiation is emittable via the second optical window (34), said optical treatment system further comprising an optical parameter control means (36, 38; 42; 46, 48, 50; 52; 54) that is arranged to control said at least one optical parameter to have a predetermined second value at the second optical window (34). The optical parameter control means may be active, and influence the control unit (18) or light source (14), and/or passive, and influence the radiation emitted through the first optical window (20) by filtering, reflection, absorption, etc. Thus, the system is able to provide radiation having well known, predetermined and thus more reliable properties, even if the second window has a different shape or area or the radiation has been influenced otherwise.