Borescope
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A borescope is a rigid tube with an eyepiece on one end, an objective lens on the other linked together by a relay optical system in between. Borescopes are used for inspection work where the area to be inspected is inaccessible by other means. Rigid borescopes are similar to a fiberscope, but have a higher quality image and are not flexible. Rigid borescopes are therefore better suited to certain tasks such as inspecting automotive cylinders, fuel injectors, hydraulic manifold bodies and gunsmithing. Rigid or flexible borescopes may be fitted with a magnifying device and a way to illuminate the work being inspected, usually illumination fibers contained in the insertion tube of the borescope. The eyepiece may be fitted with a coupler lens to allow the borescope to be used with imaging devices such as a video or CCD camera.
Contrast endoscope
Borescopes are used in the visual inspection of aircraft, diesel and automotive engines. Aircraft engines require particular attention because of safety requirements.
Rigid borescopes provide a superior image to a flexible borescope, but have the limitation that access to what is to be viewed is a staight line. A flexible borescope can be used to access cavities which are around a bend, such as a combustion chamber or "Burner Cans" in order to view the condition of the compressed air inlets, turbine blades and seals without disassembling the engine.
Criteria for selecting a borescope is usually image clarity and access. For similar quality instruments, the largest rigid borescope that will fit the hole, will give the best image. Relay optics in rigid borescopes can be of 3 basic types, Hopkins rod lenses, achromatic doublets and gradient index rod lenses. For large diameter borescopes, the achromatic doublet relays work quite well, but as the diameter of the borecope tube gets smaller (less than about 4 millimeters) the Hopkins rod lens and gradient index rod lens designs provide superior images. For very small rigid borescopes, the gradient index lens relays are better.
Flexible borescopes suffer from pixelation and pixel cross talk due to the fiber image guide used in the relay. There is a wide variation in image quality between flexible borescopes caused by the variation in number of fibers and construction used in the fiber image guide. For flexible borescopes, articulation mechanism components, range of articulation, field of view and angles of view of the objective lens are also important. Fiber content in the flexible relay is also critical to provide the highest possible resolution to the viewer. Minimal quantity is 10,000 pixels and the best images are obtained with higher numbers of fibers in the 15,000 to 22,000 range for the larger diameter borescopes.