Projection system, lithographic apparatus and device manufacturing method

A lithographic apparatus comprising:

• a projection system configured to project a patterned radiation beam onto a target portion of a substrate, the projection system comprising an optical element having a first face and a second face, wherein the first face is configured to be exposed to an external gaseous environment connected to the outside of the lithographic apparatus and 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;
• and
• 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.

A lithographic apparatus comprising:

• a projection system configured to project a patterned radiation beam onto a target portion of a substrate, the projection system comprising an optical element having a first face and a second face, wherein the first face is configured to be exposed to an external gaseous environment connected to the outside of the lithographic apparatus and 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 and there being, in use, a pressure differential between the internal gaseous environment and the external gaseous environment;
• and
• a pressure compensation system configured to adjust the pressure in the internal gaseous environment in response to a measured change in the pressure differential.

A device manufacturing method, comprising:

• using a projection system to project a patterned radiation beam onto a substrate, wherein the projection system comprises an optical element having a first face and a second face, the first face exposed to an external gaseous environment connected to the outside of the projection system and the second face exposed to an internal gaseous environment, the internal gaseous environment being substantially isolated from the external gaseous environment;
• and
• adjusting the pressure in the internal gaseous environment in response to a change in pressure in the external gaseous environment.