An optical element unit is provided comprising an optical element group; a housing receiving said optical element group and having an inner housing part and an exit end; and a purge unit connected to said housing and providing a purge medium to said inner housing part. The optical element group comprises an ultimate optical element. The ultimate optical element is located at the exit end of housing and separates the inner housing part from an environment external to the housing. The ultimate optical element and the housing define a sealing gap. The purge unit provides purge medium to the sealing gap to prevent intrusion of contaminants into the inner housing part.