A borescope (or boroscope) is an endoscope that fulfills the same function as the corresponding medical devices, but which is applied primarily in workshops and in the automotive, naval and aeronautical industry to inspect inaccessible areas, such as the interior of engines, turbines, machines and facilities. The basic principle of a borescope and an endoscope are identical since it is a matter of directing a clear and cold light through the instrument to internal cavities that are difficult to access to enable a clear vision and, at the same time, objective documentation by transferring images.
In World War II, primitive boroscopes were used to examine the interior holes of large firearms for defects. Over time, the technology was perfecting the instrument as we know it today.
In the general sense, the borescope is an optical instrument of low magnification (3x to 4x), usually of small diameter (<20 mm) and relatively large regarding total length, often 50 to 100 times its diameter. The optical design of a borescope is influenced by these dimensional limitations, by the precision requirements of the optical components and by the high level of image quality that is generally necessary.
Traditionally, a borescope consists of three large optical components: the lens of the lens, an optical system of retransmission of the image (known as “relay” ) and the eyepiece, as we see in the figure below. The objective lens forms an internal image of the illuminated object and the eyepiece magnifies that image that it presents before the eyes of the observer.
The lens configuration is determined by the instrument’s final specifications, such as field of view, lens speed (f / number), distortion and image quality. The permissible diameter of the lens gives the configuration of the image relay system, the required length of the instrument, the allowable reduction of the brightness or saturation in the periphery of the image ( “vignetting” ) and the image quality. The selection of the eyepiece is usually made by the general magnification required, the apparent field of vision and comfort for the eye.
In the current market, there are several types of borescope, from the simplest and most economical to the most sophisticated, equipped with image or video devices that offer the best resolution.
Let’s see in detail what are the types of the borescope and its main uses.
They are comparable to a periscope, with a stem or sheath and a special glass tip that in addition to providing the borescope with great resistance to damage and wear, guarantees the great protection of the optical system.
Depending on the way in which they transmit the image, rigid borescopes are subdivided into two types:
- Borescopes with oscillating prism: use a system of lenses for the transfer of images and consist of a prism oscillating at the tip of the stem. With the help of a steering handle, the visual direction can be adjusted progressively between 0 ° and 140 ° and the stem can be turned 400 °. It is the one shown in the figure above.
- Semi-rigid boroscopes: they use a fiber optic bundle housed in a rigid rod and, therefore, its diameter is smaller than that of borescopes with oscillating prisms. The advantage is that, unlike the previous one, they allow the inspection of winding areas.
The rigid borescopes oscillating prism have the limitation that access to what is to be observed must be straight, therefore, are better suited for certain tasks such as inspection cylinder automobile fuel injectors bodies multiple hydraulic and weapons manufacturing.
The criteria for selecting a borescope are generally image clarity and access. If we have two instruments of similar quality, the rigid borescope of larger diameter and shorter length will provide the best image.
A flexible borescope includes a bundle of optical fibers housed in a flexible sheath that divides the image into pixels. The difference with the semirigid borescope mentioned above is that it lacks a rigid stem. It is also known as a fiberscope and can be used to access cavities surrounding a curve, such as a combustion chamber, to observe the state of compressed air inlets, turbine blades and seals without the need to disassemble the motor.
The flexible borescopes experience pixelation and crosstalk due to the image guide fiber. Depending on the number of fibers and the construction used in the fiber imaging guide, the image quality varies widely among the various models of flexible borescopes.
The components of the articulation mechanism, the amplitude of the articulation, the field of vision and the viewing angles of the objective lens are important parameters in a flexible borescope, as is the flexible system of retransmission of the image to offer the best possible resolution to the observer. The minimum quantity for this is 10,000-pixel beams, while in larger-diameter boroscopes, the best images with a greater number of fibers in bundles of 15,000 to 50,000 pixels are obtained. The ability to control light at the end of the insertion tube allows the user to make adjustments that can greatly improve the clarity of the video or still images.
A video borescope is similar to the flexible borescope, but at the end of the flexible tube uses a miniature video camera that makes it possible to capture video or still images in the depth of equipment, motors and other dark spaces.
As a remote visual inspection tool, the possibility of capturing video or still images for further inspection is a great benefit. A screen on the handle shows the camera’s display and the display position can be changed through a joystick or similar controls. Since the optical guide complex is replaced with a low-cost power cable, video boroscopes can be much less expensive and, depending on the specifications of the camera, can have better resolution.
The costs of high-end video boroscopes can range between 10,000 and 50,000 dollars, depending on the manufacturer, the options and the specifications. The video borescopes to stack, easy to use, with LCD screen 3 – inch, 320 x 240 pixels, or more, ranging from 100 to 400 dollars. In many of these models, the video camera and the flexible tube are submersible.
Other uses of the borescope
The borescopes have also found application in non – destructive testing techniques in welding, for example, for recognition of surface defects, imperfections, pinholes, cracks, corrosion, delamination, swelling, leaks, loss of protective coatings, deformation and wear.
Boroscopes are also used in the fabrication of machined or cast parts to inspect critical interior surfaces to check burrs, surface finish or complete through holes. Other common uses include forensic applications in the enforcement of laws and the inspection of buildings, as well as the manufacture of weapons to inspect the interior hole of a firearm.
The wide range of boroscopes of all types is completed with a series of accessories not only to adapt them to monitors, computers, digital cameras, conventional cameras and camcorders but also to optimize performance. In addition to these adapters and the corresponding cables (USB, composite video or S-video, etc.) the variety of accessories ranges from light sources, mounting tripods and instrument clamps to the angular vision and x2 magnification accessories for a better vision
All this is complemented with various types of software to combine, when necessary, visual inspection with computer analysis, which facilitates the work of monitoring and planning thanks to the storage of information and images in databases. The teams that collect and analyze the data to update the standards and expand the certification programs allow the visual inspector to concentrate only on the data collection, leaving all the calculations to the computer.