| 摘要 |
A light guide has a window through which radiation passes to a detector. The light guide is provided with a housing which defines a duct through which a flow of cooling fluid passes. The duct is arranged so that a proportion of the fluid flow passes over the external surface of the window to prevent deposition of contaminants thereon. A circumferential shield extends around the window to control the flow of radiation therethrough. The circumferential shield has a curved radial recess therein which imparts a rotational component of velocity to the flow of fluid passing over the surface of the window to promote cooling of the shield. The shield is further provided with an annular internal step which prevents radiation reflected from the inner wall of the shield passing through the shield.
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