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. An internal image of the illuminated object is formed by the objective lens and magnified by the eyepiece which presents it to the viewer’s eye. Rigid or flexible borescopes may be fitted with a video or CCD camera. The terms boroscope and borescope are often used to describe the same device.
Borescopes are used for inspection work where the area to be inspected is inaccessible by other means. Devices for use inside the human body are referred to as endoscopes.
Borescopes are commonly used in the visual inspection of aircraft engines, aeroderivative industrial gas turbines, steam turbines, diesel engines, and automotive and 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. Other common uses include forensic applications in law enforcement and building inspection, and in gunsmithing for inspecting the interior bore of a firearm.
Fiberscopes or Flexible Borescopes
A flexible borescope includes a bundle of optical fibers which divide the image into pixels. It is also known as a fiberscope and can be used to access cavities which are around a bend, such as a combustion chamber or “burner can”, in order to view the condition of the compressed air inlets, turbine blades and seals without disassembling the engine.
Flexible borescopes suffer from pixelation and pixel crosstalk due to the fiber image guide. 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.
Videoscopes or Video Borescopes
A video borescope or “inspection camera” is similar to the flexible borescope but uses a miniature video camera at the end of the flexible tube. A display in the handle shows the camera view. Because the complex optical waveguide is replaced with an inexpensive electrical cable, video borescopes can be much less costly and potentially better resolution (depending on the specifications of the camera). Digital models often have an integrated recorder and can save images in common digital formats.
Rigid borescopes are similar to fiberscopes but generally provide a superior image at lower cost compared to a flexible borescope. Rigid borescopes have the limitation that access to what is to be viewed must be in a straight line. Rigid borescopes are therefore better suited to certain tasks such as inspecting automotive cylinders, fuel injectors and hydraulic manifold bodies, and gunsmithing.
Criteria for selecting a borescope are usually image clarity and access. For similar-quality instruments, the largest rigid borescope that will fit the hole gives the best image. Optical systems in rigid borescopes can be of 3 basic types: Harold 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 3 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.