Borescopes (or boroscopes) are optical inspection tools which consist of a hollow tube with an eyepiece and a lens at opposite ends.
While borescopes are produced in many forms, all devices include a hollow tube, a light source, an eyepiece (which typically contains a means for magnification), and some form of relaying an image from the end of the scope to the eyepiece (often optical lenses or a video camera). They are primarily used for visual inspection of areas which are not accessible to other tools to check for cracks, coating defects, voids, burrs, corrosion, and other critical problem indicators. As noted below, borescopes can be flexible or rigid and use multiple means of transmitting images.
A typical rigid borescope is shown below. This particular device includes a flashlight adapter for providing image illumination, as shown in the image.
Image credit: Cornell University
Borescopes are useful for inspecting bores and cavities of many different devices and types of equipment, including those listed below.
- Cast parts
- Building interiors (forensics and general inspection)
- Automotive components - fuel injectors, engine cylinders, motors, and manifolds
- Tanks and boilers
- Pumps and valves
- Specialized military use - gun bore inspection, surveillance
Using a flexible borescope (fiberscope) to inspect a cavity.
Image credit: Gradient Lens Corp.
Borescopes may be classified in two different ways: by tube rigidity or image relay type.
Rigid vs. Flexible
Borescope tubing may be either rigid or flexible. Rigid borescopes include some of the most inexpensive designs, as they typically involve only a hollow tube, eyepiece, a light source, and one or more relay lenses. Lenses used in rigid borescopes are typically achromats, GRIN lenses, or Harold Hopkins designs. While rigid borescopes are economical and effective, their grave disadvantage is that their viewing is effectively limited to a straight line. When inspecting certain entities, such as straight pipes, engine cylinders, and fuel injectors, rigid borescopes are effective.
A broken down rigid borescope, showing the components mentioned above.
Image credit: Olympus IMS
Flexible borescopes are much more versatile than rigid types. Because they require additional means for transmitting an image to the eyepiece, they are more expensive to manufacture and purchase. Flexible types include fiberscopes and borescopes, as described below.
Specialty borescopes called endoscopes are used to examine cavities within the human body. While these are typically flexible devices, some rigid ones are produced.
Relay optics determine a borescope's image quality.
Rigid Relay Types
Rigid borescopes may use one of three different relay types for image transmission.
Achromats are double lenses (doublets) which automatically correct chromatic and spherical aberration. These are cost-effective options but are typically used only in large-diameter borescopes.
Hopkins design is a system devised by Harold Hopkins, a British physicist, which involves the precise placement of numerous polished glass tubes in place of tiny lenses. Hopkins relays result in excellent image quality for medium-diameter borescope applications but are tedious to produce and maintain, given the delicate nature of the optics involved.
Gradient index (GRIN) relays operate in a similar fashion to Hopkins designs, but are simpler to produce and use. For example, a single GRIN rod is capable of performing the same function as 24 separate Hopkins tubes. Therefore, gradient index optics result in the best combination of quality and cost. This relay type is typically used for small- to medium-diameter borescopes.
A visual comparison of GRIN vs. Hopkins relays. Note that both relays produce the same optical output by different means.
Image credit: Quality Digest
Flexible Relay Types
When selecting borescopes for a specific application, specifications about the device's tube and field of view must be considered.
Borescope manufacturers typically provide the tube's diameter and length. These two values must be carefully considered after determining the size and depth of the object or space to be inspected.
Viewing Angle and Field of View
A borescope's viewing angle (or direction of view [DOV]) and field of view (FOV) are both specified in degrees. The viewing angle describes the device's view relative to an imaginary longitudinal line through the scope's tube. A borescope which views straight-on has a 0° viewing angle, while a scope which provides a view out the side of the tube is said to have a 90° angle. Borescopes with the latter viewing angle are able to inspect piping and other narrow cavities with greater detail than a straight-ahead scope.
Field of view can be thought of as a cone extending beyond the tip (or distal end) of the tube. As shown below, a 30° field of view results in a narrowly-focused image, while a 120° view is much broader but is capable of less magnification. Selecting an FOV is dependent upon the inspected object's distance from the tip of the tube; as distance from the tip increases, the field of view angle should decrease (and therefore magnification and image quality will increase).
Depth of field (DOF) is a specification closely related to FOV. DOF describes the distance from the tip of the tube to the furthest object which appears clearly focused.
The image below shows both of these angles for a side-view borescope. This particular device is adjustable between 45° and 115° DOV, with a possible FOV of 120°. Note that, as marked, a 90° DOV points straight up, and a 115° DOV points somewhat backwards from the distal end of the tube at left.
Image credit: Olympus IMS
Borescopes may be produced, tested, and used based on various standards or specifications. Some common standards include:
SAE ARP4078 - Storage, care, and use of borescopes and fibrescopes
A-A-59883 - Borescopes
TM 9-4933-200-35 - Pullover gages/borescopes
Gradient Lens Corporation - Choosing a borescope
Olympus - What are industrial rigid borescopes?
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