| CPC H01J 37/32935 (2013.01) [G02B 7/007 (2013.01); H01J 37/32798 (2013.01); H01J 37/32091 (2013.01)] | 19 Claims |

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1. An apparatus comprising:
a frame defining an aperture that extends between an interior side and an exterior side;
a first window sealed against the frame, sized larger than the aperture, and completely overlapping the aperture when viewed along a direction perpendicular to a largest surface of the first window;
a glass window; and
a polymeric window sized larger than the aperture, completely overlapping the aperture when viewed along a direction perpendicular to a largest surface of the polymeric window, and secured to the exterior side of the frame, wherein:
the first window is positioned closer to the interior side of the frame than the polymeric window,
the glass window is interposed between the first window and the polymeric window,
the first window is made of a fluorine-resistant, high-temperature stable material that has optical transmissivity of 80% in at least 80% of the visual light spectrum, and
the frame supports the first window, the glass window, and the polymeric window in a spaced-apart configuration.
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