According to the general visual inspection rules, a mirrored borescope or a fiber optic viewing system can be used where access for direct visual inspection is restricted. This is usually by agreement between the contracting parties.
So, What is a Borescope?
A borescope (occasionally called a boroscope, though this spelling is unusual) is an optical device consisting of;
- A rigid or flexible tube with an eyepiece or displays on one end,
- An objective lens or camera on the other connected together by an optical or electrical system in between.
In some cases, the optical system is enclosed 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 an imaging or video device. For medical use, similar instruments are called endoscopes.
What is a Borescope Used For?
Borescope is commonly used in the visual inspection of engines, turbines, automotive and industrial manufactured parts that require particular attention due to their quality, safety and maintenance requirements. But the main reason to use a borescope is simply because the area to be inspected is inaccessible by other means.
Borescopes are regularly used in nondestructive testing techniques for recognizing defects or imperfections. Similar devices use by doctors to inspect the inside of the human body are referred to as endoscopes.
Difference Between Borescope and Endoscope
Endoscope is a type of borescope and both have similar function but with some slight different in diameter, flexibility, magnification, level of light, direction of views and length. These differences are mostly due to their application whether for the inspection of machine parts or the human body.
For example, endoscopes tend to have dimmer lights than borescopes because they are more preferable to accommodate the internal structure of the human body.
Types of Borescope
There are 3 main types of borescope. They are;
- Flexible borescopes
- Video borescopes
- Rigid borescopes
The traditional 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.
Traditional 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.
Some high-end borescopes offer a “visual grid” on image captures to assist in evaluating the size of an area with a problem. For flexible borescopes, articulation mechanism components, the range of articulation, the field of view and angles of view of the objective lens are also important.
The 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.
The ability to control the light at the end of the insertion tube allows the borescope user to make adjustments that can greatly improve the clarity of video or still images.
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. The end of the insertion tube includes a light which makes it possible to capture video or still images deep within equipment, engines and other dark spaces.
As a tool for remote visual inspection, the ability to capture video or still images for later inspection is a huge benefit. A display at the other end shows the camera view, and in some models, the viewing position can be changed via a joystick or similar control.
Because a complex fiber optic waveguide in a traditional borescope 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). Costs for high-end video borescopes can range from about $8,000 to $50,000 depending upon manufacturer, options, and specifications.
Easy-to-use, battery-powered video borescopes, with 3″ LCD displays of 320×240 pixels or better, became available circa 2012 from several manufacturers for prices between $100 and $400 and are adequate for some applications. On many of these models, the video camera and flexible tube are submersible. Later models offered improved features, such as lower cost, better resolution, adjustable illumination or replacing the built-in display with a computer connection, such as a USB cable. Hobbyist models of this type are now available starting at around $10.
Rigid borescopes are similar to fiberscopes but generally provide a superior image at a 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 (over 12mm), the achromatic doublet relays work quite well, but as the diameter of the borescope tube gets smaller the Hopkins rod lens and gradient index rod lens designs provide superior images. For very small rigid borescopes (under 3mm), the gradient index lens relays are better.