Remote Visual

Whenever the eye cannot obtain a direct , unobstructed view of the specimen test surface without use of another tool, instrument, or device , a “remote visual�?examination is performed. Recall that a direct visual examination is an examination that can usually be made when access is sufficient to place the eye within 24 inches(610mm) of the surface to be examined and at an angle not less than 30°to the surface to be examined. Most codes permit the use of mirrors and magnifying lens to assist with direct visual examinations. A remote visual examination can be defined as an examination that uses visual aids such as mirrors, telescopes, borescopes, fiber optics, cameras , or other suitable instruments.
Borescopes
One of the oldest applications of remote visual examination is the inspection of gun barrels. The failure of a gun barrel, be it the earliest mortar siege gun of medieval times or the precision machined barrel of a modern rifle, is catastrophic to the gunner at the very least. Two obstacles had to be overcome in the inspection of gun barrels; access to the area to be inspected , and a provision of a light source to provide adequate illumination in order to see the conditions of interest on the inside surface. Small rigid borescopes containing a series of lenses provided the answer.
Small lamps were used at the far end to provide sufficient light. Since the original applications utilized a lens train to access the bore of a rifle, the term “borescope�?was the original and lasting term for the device. From this technology evolved the glass fiber bundle(referred to as a fiberoptic borescope). This device transmits both the light to illuminate the inspection surface as well as the light reflected off the object back to the viewing end.
Both the rigid and the fiberoptic borescopes are capable of providing access to small openings. The rigid borescope utilizes a lens train; this is called the lens optic technique. The fiveroptic borescope utilizes a fiber bundle with a lens at each end; this is called the fiber optic technique.
The lens optic technique brings the object image to the eye using an objective lens, sometimes a prism, a relay lens, and an eyepiece lens. The eyepiece lens allows each inspector to adjust the focus as needed with the use of a “focus ring.�?br /> The advent of miniature lamps the size of a grain of wheat was the reason that the light sources used in these devices were called �?wheat lamps.�?These wheat lamps provided limited light and burned out quickly. A more practical means of transferring light to the examination surface is with the use of a light source transmitted through a fiber bundle. The reflective image from the object is returned to the eye via a second “image�?bundle. Both bundles are made up of individual fibers that are �?clad�?with a glass material with a different refractive value sufficient to reflect the light from the outside diameter of each fiber back through the core and down the length of the glass fiber.
Both methods of transmitting the light and image down and back in a bore result in variations in the direction that the view can be delivered. Looking straight ahead is known as the direct view. An angle off the straight-ahead but still forward-looking is known as the fore-oblique view. A sideways look is known as the side view. Anything more than 90° past the straight-ahead view is known as the retrospective view. Different manufacturers will designate the forward , straight-ahead view as 0° or alternatively as 90°. In any case, the included angle of the resultant view is the “field of view.�?br /> A recent development is the combination of the lens-optic and fiberoptic techniques. The object image is transmitted to the eye along a rigid tube containing the lens-optic train. But the light is transmitted to the object through fiberglass surrounding the tube, bringing light from the external source of light to the object.
Yet another version of the borescope is the miniborescope. A single solid fiber diffuses ions in a parabolic are from the center of the rod to the periphery of the rod, with a graded index of refraction . this still captures the light within the rod and passes it down the length of the rod. But the light actually bends down the length of the rod, forming an image at specific intervals. Since the lens aperture is so small. The lens has an infinite depth of field , eliminating the need for a focusing mechanism.
An interesting physical fact can be observed when using bore scopes. A wide field of view reduces the magnification but results in greater depth of field. This can be observed when looking down a great distance during a tube inspection. The image less than a quarter inch away will be magnified. But the image from a quarter inch to one foot away, and theoretically to infinity, may be in focus but greatly reduced in image size. Conversely , a narrow angle field of view produces higher magnification but results in a shallow depth of field. The image at one point may be in focus, but at a short distance in front and behind that point, the image is out of focus.

 

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