平行窓は光学システムにおいて「保護」と「透過」の両方の役割を果たします。2つの研磨面は、繊細な内部光学系を複雑な外部環境から隔離するために、極めて高い平行度（通常は秒角で表されます）を維持する必要があります。これにより、光は歪みなく透過し、ビーム経路やシステムの結像品質も変化しません。
The two polished surfaces must maintain extremely high parallelism (usually expressed in arcseconds) to ensure that the transmitted light does not experience angular shift or ghosting.
High surface accuracy is required to minimize wavefront distortion and scattering of the transmitted beam, thus ensuring it does not affect the imaging or detection accuracy of downstream systems.
Its most basic function is to act as a "protective shield," protecting internal sensors, detectors, lenses, etc., from damage caused by external factors such as dust, moisture, and flying debris.
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