borescope

A borescope is an optical device consisting of a rigid or flexible tube with an eyepiece on one end, an objective lens on the other linked together by a relay optical system in between. The optical system is usually surrounded by optical fibers used for illumination of the remote object and a rigid or flexible protective outer sheath. The remote object is illuminated and an internal image formed by the objective lens is relayed to the eyepiece which magnifies the internal image and presents it to the viewer's eye. Borescopes are used for inspection work where the area to be inspected is inaccessible by other means.
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