| 摘要 |
<p>A device for keeping an outer surface of an optical element clean, said element being located in surroundings where deposits can be quickly formed on said element. The device has a protective device or screen with an opening placed in front of the surface to be protected. This opening has inner walls extending out from said surface of the optical element. The screen has a slot, which extends peripherally around the edge of the opening facing the surface to be protected, and gas is forced to stream through the slot. At least a part of the inner surface of the opening has an inwardly projecting part having a surface remote from the optical element which is parallel to or inclined outwardly from a symmetrical plane through the element.</p> |
