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.
Rigid borescopes are similar to a fiberscope, but are not flexible and generally much cheaper and provide a superior image. 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 are 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.
When in use inside the human body, this device is referred to as an endoscope.
Borescopes are commonly used in the visual inspection of aircraft engines, aeroderivative industrial gas turbines, steam turbines, diesel engines and automotive/truck engines.  Gas and steam turbines require particular attention because of safety and maintenance requirements.  Borescope inspection of engines can be used to prevent unnecessary maintenance, which can become extremely costly for large turbines.  They are also used in manufacturing of machined or cast parts to inspect critical interior surfaces for burrs, surface finish or complete through holes.  Forensic applications in law enforcement and building inspection are also common uses for borescopes.
Rigid borescopes generally provide a superior image at lower cost compared to a flexible borescope, but have the limitation that access to what is to be viewed is a straight 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 borescope 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.  Image quality varies widely among different models of flexible borescopes depending on the 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 while the best images are obtained with higher numbers of fibers in the 15,000 to 22,000 range for the larger diameter borescopes.