Projection system, lithographic apparatus and device manufacturing method

Various configurations of a projection system, of a lithographic apparatus, and of a device manufacturing method are disclosed. According to a disclosed configuration, the projection system is configured to project a patterned radiation beam onto a target portion of a substrate. The projection system includes an optical element having a first face and a second face. The first face is configured to be exposed to an external gaseous environment connected to the outside of the lithographic apparatus. The second face is configured to be exposed to an internal gaseous environment, the internal gaseous environment being substantially isolated from the external gaseous environment. The projection system further includes a pressure compensation system configured to adjust the pressure in the internal gaseous environment in response to a change in pressure in the external gaseous environment or a pressure differential between the internal gaseous environment or the external gaseous environment.