An optical instrument used for inspection work where the area to be inspected is inaccessible by other means, borescopes can be rigid, flexible and semi-rigid. A rigid borescope has an eyepiece on one end and an objective lens on the other linked together by a lens relay system. Flexible and semirigid scopes use fiber optics, instead of traditional lenses, to relay images and transmit light.
"Borescopes allow users to look inside areas of metal machined parts, castings and welded parts that are not visible with the naked eye," says Dr. Doug Kindred, president of Gradient Lens Corp. (Rochester, NY). "The importance to manufacturers and quality assurance personnel is fast, nondestructive visual inspection that ensures a part or component is free of burrs, cracks, surface finish irregularities and other imperfections.
"Borescopes are particularly useful to manufacturers of small or complex metal machined parts or castings used in mission critical situations, often involving the flow of fluid through the part, Kindred continues. "These parts include fuel injectors, fuel injector nozzles, hydraulic parts used in automotive and aerospace transportation, and medical parts such as valves and needles."
Tube manufacturers frequently use borescopes. "Many tube manufacturers use borescopes to look for internal defects in critical piping applications such as drill casings," says Dan Carnelvale, president of Danatronics Corp. (Danvers, MA). "Your eyes cannot look many feet inside a small diameter pipe. Other applications in manufacturing include looking inside objects such as medical packaging, tires, tubes, pipes and tanks." Explosive environments and large, open areas that can be easily inspected with the naked eye are two situations where borescopes should not be used.
Rigid vs. Flexible
Borescopes are both rigid and flexible. "Flexible scopes can bend and articulate usually up to longer lengths," says Carnevale. "Rigid scopes cannot usually bend unless custom engineered but the image quality is better (than flexible borescopes) and the diameters can be smaller." Flexible borescopes or fiberscopes are primarily used in applications that do not have a straight passageway to the point of observation. They can access cavities that are around a bend, such as a combustion chamber, to view compressed air inlets, turbine blades and seals without disassembling the engine.
Fiberscopes are more expensive, more fragile and harder to handle than borescopes. "The rigid borescope is much more durable, reducing costly repairs," says Richard Parello, product manager at Lenox Instrument Co. (Trevose, PA). Rigid borescopes are better suited to certain tasks such as inspecting automotive cylinders, fuel injectors and hydraulic manifold bodies.
The two types work a little differently from each other. "Rigid borescopes utilize traditional lens relay systems that transmit the image," Kindred says. "These lenses are usually assembled inside a rigid stainless steel tube. Flexible borescopes use fiber optics, which are housed inside a flexible sheath. Flexible scopes have inherently lower resolution because of the fiber optics."
Semirigid borescopes combine elements of both rigid and flexible borescopes. They usually are small- less than 1 millimeter in diameter-and use fiber optics housed in a thin stainless steel tube. "It is not practical to construct these small diameter scopes using traditional lens optics," Kindred says. "These borescope are somewhat flexible only due to their construction and small diameter, however, they are not intended to bend more than approximately 20 degrees. They are used to inspect very small, often submillimeter bores, tubes and catheters."
Other specialized borescopes can resist high temperatures, high- or low-pressure situations and harsh chemical environments. There are charge-coupled devices (CCDs) ranging in diameters as small as 3.9 millimeters to 8 millimeters as typical sizes.
The eyepiece is a standard part of any borescope to help the operator align his vision. The eyepiece uses a lens system to match the desired magnification and view with the series of lenses or fiber optics within the borescope
Not all eyepieces are created equal. "There is an international standard for all medical and industrial borescopes, 31.75 millimeters to 32 millimeters," says Kevin Sebley, sales and business development manager at Moritex USA Inc. (San Jose, CA). "This allows all C-mount adapters to work. There are a few specials around, especially videoscopes, designed to couple directly to CCD cameras."
Eyepieces can have different focusing methods, which can affect borescope quality when attached to a camera. Jerald M. Ewert, president of JME Technologies (Crystal Lake, IL), says, "The quality of the eyepiece is dependent first on the lens' quality in the eyepiece and the material and consistence of the physical size specification. The clarity of a borescope's view is directly linked to the first lens in the sequence of optics your eyepiece. The highest quality eyepiece will enhance the borescope lens system or fiberoptic bundle to its highest clarity level."
Both rigid and flexible borescopes can be fitted with a magnifying device as a way to illuminate the desired area being inspected. There are multiple types of light sources including, halogen, metal halide and xenon.
Illumination fibers are contained in the borescope's insertion tube. According to Sebley, these light sources really only gain importance in a dark field of view. "There are battery-powered lamps, which are quite low, that work with your eyes. With a camera you may need a cold light source, something from 50-watt halogen to 300-watt xenon."
Portability is important to match the operator's desired application site including battery operation for instant view without available power. Also, "Downtime on borescope equipment due to the light source is critical," cautions Ewert. "It is very important to provide a high-quality product to avoid the loss of all of the borescope's system usage due to a poorly manufactured light source product."
Variables Affecting View
The borescope's field of view affects the image quality. Fields-of-view may be very wide, wide, medium or narrow. "A narrower field-of-view unit of approximately 50 degrees will have a less distorted image than a 70-degree or 90-degree field-of-view, which will give a fish-eye affect to the image," says Frank Menza, president of Titan Tool Supply Inc. (Buffalo, NY).
The direction-of-view is the angle from the longitudinal axis through the tip of the borescope. This view is determined by the technician's requirements. "If the user needs to see a view other than straight forward, they will need an angular viewing borescope such as 90 degrees (perpendicular to the line of site) 65 degrees (forward oblique view to the line of site) or 110 degrees (rear oblique view to the line of site)," says Menza. "This will allow the user to see other areas of interest, such as a cross drilled hole to make sure there are no burrs or other contamination in the part."
Seeing the future
There is a growing feeling that today's borescopes could find themselves outdated when compared against more sophisticated, flexible and smaller diameter fiber-optic systems. Menza disagrees. "There will always be a need for a rigid borescope system, until they can produce an electronic sensor smaller than 3 millimeters. Fiber-optic systems do not give you the necessary resolution like a lens system borescope does. We have a sub 1 millimeter diameter-0.7 millimeter diameter to be exact-borescope. A fiber-optic system is limited to 1-millimeter diameter and resolution is not as great as on the lens system. All lens system borescopes can be hooked up to video or digital cameras for image acquisitions."
Sebley agrees that fiber-optic borescopes cannot produce as high of image quality as rigid borescopes, which use glass-rod lenses instead of fiber-optic bundles. He also predicts "huge developments with ‘chip on a stick' technology. As CCD chips shrink in size, there will be more borescopes with no optics-the camera will be in the end of the scope." Medical companies are reportedly testing 4 millimeter arthroscopes with a 2.5 millimeter chip.
Anticipate continued growth of video and digital capture quality capability at more affordable prices. Expect to see higher resolution images, smaller diameter scopes with better, more intense light sources stressing portability and recording capability. Q
• Flexible and semi-rigid scopes use fiber optics, instead of traditional lenses, to relay images and transmit light.
• The borescope's field of view will affect the image quality.
• Portability is important to match the technician's desired application site including battery operation for instant view without available power.
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