VISUAL TESTING
III. EQUIPMENT AND ACCESSORIES
when the object is brought close to the eye and a large visual angle is obtained. There is a limitation to distance; the eye cannot sharply focus if it is nearer to 10 inches (250 mm) to an object. Therefore, direct visual tests are best performed at a distance of 10–24 inches (250–600 mm). Also of importance is the viewing angle between the line of site that the eye makes with the test site and the plane of the test surface. As de- scribed above, two pencils can become one as the viewing angle of the eye changes. For practical consideration, this angle should not be less than 30° of view off the plane of the surface under inspection, as shown in Figure 3-1.
To summarize, the eyes are the most important and essential components in the entire VT system. It would be impossible to perform examinations without them. They also play a key role in all other methods of NDT, including evaluation prior to, during, and most importantly, after completion of the performance of the examination.
Direct Visual Aids
The eye may need assistance when visualizing the detail of the vast variety of surfaces that are normally accessible to the direct view. Enhancement of the view can be achieved through magnification. A change of angle can be achieved through reflective mirrors.
Both of these aids can enhance views, and their removal restores direct view again.
Hence, their use and the option to return to unaided viewing results in “direct viewing with an aid.”
Magnifier
The magnifier can increase the image size of the viewed object. The power of the magni- fication is expressed as follows:
Magnifying power = 10 ÷ focal length (in inches)
The following is an example of determining the magnification power of a lens: A piece of paper is held in one hand and a magnifying lens in the other. The image of a light source (e.g., a candle) is visible on the paper. After focusing the image on the paper by moving the lens toward and away from the paper, measure the distance from the center of the lens to the paper. This distance is the focal length of the lens. Divide ten by the focal length distance in inches. The resultant quotient is the magnifying power of the lens. For exam- ple, assume the focal distance was 2 inches; the power of the lens is ten divided by 2 inch- es, which equals a magnification power of five (5×).
Another unit of measurement for the magnifier is the diopter. This is a measurement of the refractive power of lenses equal to the reciprocal of the focal length in meters. A “five diopter” lens has the magnification power of five times. The principal limitation to the amount of magnification is the depth of field. As magnification increases, the depth of field decreases. This is why a microfocused view of a small object such as a bee on a flower yields an image with the bee in focus and an out of focus background. It may be so out of focus that it is sometimes blurred out and not distinguishable.
Two common visual magnification devices are hand-held lenses and pocket magni- fiers or microscopes. Hand-held lenses with a frame and handle may contain one lens or multiple fold-out lenses. They are generally plastic (acrylic) or glass. Normal sizes range from one half inch to six inches. The other common magnifying device is the pocket mi- croscope. Small diameter tubes (usually 6 inches in length) are fitted with half-inch diam- eter lenses. Light is available through cut outs in the tube or through translucent tube ends. Due to the higher magnification ranges of 25× to 60×, the depth of field and field of view are extremely limited. The larger the diameter of the lens, the lower the magnifying power.
Light Sources (Direct)
When employing magnification devices, additional light sources are usually required.
Several lighting devices are available that permit light to be concentrated on a small spot.
The most common of these is the hand-held flashlight. This light source is usually held at some angle to, and within inches of, the specimen to be examined. The actual light inten- sity (foot candles or lumens) at the surface to be examined is dependent on distance, light
angle, light bulb wattage, and battery strength. Another common source of auxiliary light is the “drop light,” which is usually a 100-watt light bulb encased in a protective cage with a reflective shield connected to a length of electric cable. Again, the actual light in- tensity is dependent upon angle, distance, and wattage. A light meter can be placed on the specimen surface and the actual light intensity measurement can be made. As discussed, when the distance from the light source to a surface is doubled, the light intensity is de- creased to one fourth of the original intensity. This is seen in the following equation, which is expressed differently than Equation 3-1:
= (3-2)
where:
I1= intensity at first location I2= intensity at second location D1= distance to first location D2= distance to second location Measuring Devices
There are a multitude of measuring devices available for different applications (See Fig- ure 3-8). For the sake of brevity, only a few will be discussed here. The direct visual in- spection method is frequently augmented by the use of several common tools used to measure dimensions, discontinuities, or range of inspection. Among these are linear measuring devices, outside diameter micrometers, ID/OD calipers, depth indicators, opti- cal comparators, gauges, templates, and miscellaneous measuring devices.
D22
ᎏD12
I1
ᎏI2
FIGURE 3-8 Various measuring gauges.
The most common linear measuring device is the straightedge scale. Typical scale lengths are 6 inches (15 cm) and 12 inches (30 cm) long. Additionally, 25-foot to 50-foot tape measures are frequently utilized. The 6-inch pocket scale is frequently misused by placing the thumb on one edge of the specimen and abutting the end of the scale against the soft and pliable thumb flesh. This is inherently inaccurate when a movable surface is the beginning point for a measurement. Preferably, the first whole unit of measurement (metric or imperial) should be aligned with one edge of the specimen and the other edge read for total distance. Caution: One must then remember to subtract the first unit of measurement from the total measurement noted.
Micrometers are used to measure outside or inside diameters. They are very accurate measuring devices commonly utilized to measure to the nearest one thousandth of an inch. Micrometers are available that can be used to measure to an accuracy of one ten thousandth of an inch. The outside diameter (OD) micrometer is made up of several parts, including the anvil, stirrup, spindle, sleeve, thimble, internal screw, ratchet, and knurled knob. It is worth noting that the essential component is the internal screw. It has been turned to have 40 threads per inch. One divided by 40 results in 0.025 thousandth of an inch spindle movement per 360° turn of the thimble. The micrometer may be outfitted with attachments for different applications. For example, a 0.2⬙ ball may be attached to the spindle to accommodate the inside radius of a pipe when measuring pipe wall thick- ness. Additionally, a pointed attachment may be utilized when measuring pits or very lo- calized variations in wall thickness.
The vernier caliper is a variation of the basic caliper. A caliper is usually used to meas- ure the outside diameter of round objects. A common application is the measurement of remaining stock on a part in a machine lathe. If transfer calipers are used, the calipers are placed with the two contact points touching opposite sides of a round object. The calipers are removed and compared to the distance between the contact points as measured on a linear scale. Vernier calipers have inside diameter (ID) and OD capabilities, depending on which set of points or jaws are utilized. The handle that forms part of the stationary jaw has one or two inscribed scales. There is also a vernier slide assembly that is integral with the movable jaw. The correct OD or ID set of calipers or jaws makes contact with the specimen. The user reads the vernier scale zero mark upon the stationary scale for the whole unit measurement. The vernier scale marks are matched with the stationary scale marks and the best match-up of lines are utilized to read the linear distance to the nearest one thousandth of an inch or millimeter as the case may be. These two basic forms of calipers have since been replaced with direct read-out dial or digital calipers. Some digital gauges can be interfaced with a laptop or hand-held computer so that a large number of readings may be stored.
Depth indicators (dial indicators) are frequently used to measure surface discontinuity depths. Examples of discontinuities are pits, corrosion, or wastage. Verification of dimen- sions can also be conducted. In either case, zero depth must first be verified. The dial in- dicator or the digital read-out indicator is placed on a flat surface and zeroed. It is then moved over the depression. The spindle movement indicates the depth of the depression.
When using the older dial indicator, the sweep hand rotations must be counted if the movement exceeds one 360° sweep. If a digital indicator is used, one must note the mini- mum and maximum point to which the indicator or its extensions are to range, i.e., 0⬙to 1⬙or 4⬙to 5⬙, etc. It is a common mistake not to count the number of rotations or not to note the beginning and end range of the indicator, resulting in only the last increment of measurement being accurate.
Optical comparators are frequently used in machine shops where close tolerance measurements are desired. A comparator produces a two-dimensional enlarged image of the object on a large smoked (frosted) glass screen. The reflected light or background
lighting is utilized to cast a magnified image onto the glass screen. This image is com- pared with a template of the object to check for dimensional accuracy.
The welding, fabrication, and construction industries use templates extensively to measure fillets, offset, mismatch, weld reinforcement, undercut, and other dimensional at- tributes. A common application is a template with minimum and/or maximum dimensions made from sheetmetal stock. The actual weld or base metal is compared to the template to determine “go” or “no-go” status. Accurate and actual measurements would still require linear measuring devices.
Miscellaneous measuring devices come in many sizes and configurations. Snap gauges, feeler gauges, radius gauges, temperature gauges, pitch gauges and diameter gauges are just a few. Some applications could have a gauge custom made for a specific need or requirement.
Remote Visual (Indirect)
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 per- formed. 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 (610 mm) of the surface to be examined and at an angle not less than 30° to the surface to be examined. Figure 3-1 illustrates this definition. Most codes permit the use of mirrors and magnifying lens to as- sist with direct visual examinations. A remote visual examination can be defined as an ex- amination that uses visual aids such as mirrors, telescopes, borescopes, fiber optics, cam- eras, or other suitable instruments.
Borescopes
One of the oldest applications of remote visual examination is the inspection of gun bar- rels. 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 con- taining a series of lenses provided the answer (see Figure 3-9).
Small lamps were used at the far end to provide sufficient light. Since the original ap- plications 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 bun- dle (referred to as a fiberoptic borescope). This device transmits both the light to illumi- nate the inspection surface as well as the light reflected off the object back to the viewing end (see Figure 3-10).
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 optictechnique.
The fiberoptic borescope utilizes a fiber bundle with a lens at each end; this is called the fiber optictechnique.
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 in- spector to adjust the focus as needed with the use of a “focus ring.”
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.
FIGURE 3-10 Fiberoptic borescope. (Courtesy of Olympus Industrial, with permission.) FIGURE 3-9 Small rigid borescope. (Courtesy of Olympus Industrial, with permission.)
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 manufac- turers 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.”
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 diffus- es ions in a parabolic arc 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 borescopes. 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 quar- ter 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.
Fiber Cameras
Like the fiberoptic borescope, the fiber camera uses light from an external source brought to the test site via the fiberoptic bundle (light guide). This fiber bundle that delivers the ex- ternal light to the object site is limited in its length. An articulated (four-directional tip) bundle can be manufactured up to 40 feet long. Fiberglass bundles up to 45 feet can be made without articulating tips. Unlike the fiberoptic borescope, the light waves from the object pass through the objective lens and strike a charged coupled device (CCD). The light waves are then converted to electronic signals and are transmitted to the processor in the central control unit (CCU). The signals are processed before being viewed on the monitor.
Another name for this equipment is “video borescope” (see Figure 3-11).
Charged Coupled Device (CCD)
The workings of a charged coupled device are illustrated in Figure 3-4. Electromagnetic radiation in the visible light wave spectrum and some infrared radiation reflect off the ob- ject to be examined. The light waves pass through the lens, which can be focused through the mechanical means of moving the lens distance relative to the solid-state silicon chip or CCD. The CCD comprises thousands of light-sensitive elements arrayed in a geometric pattern of rows and columns (a matrix). Each single microchip contains a single point or picture element known as a pixel. Each chip is struck by light waves. The intensity or quantity of light striking the chip determines the amount of electrons that are generated in
the silicon sandwich (chip). This accumulation of electrons in proportion to the amount of light that strikes that pixel is periodically passed on as packets. These packets of electrons are transferred from one pixel site to its adjacent site, just as water is moved from one bucket to the next in a bucket brigade.
The force required to move these electrons is produced in the pixels by means of a voltage that alternately turns on and off. As this “clock voltage” alternates, the electrons move from one site to the other. After the last pixel, the electrons are converted to a volt- age proportionate to their number. This voltage is amplified, filtered, and then applied to the input of a video monitor. The differential in electrical charge is what is processed down the wire at the other end, where the processor is located. By assignment, the pixel is coded as one of the primary colors (red, green, or blue) and is reassembled as electronic signals that are sent to a color monitor. The monitor reads the electronic signals and through electron beams scanning the interior of the vacuum tube (cathode ray tube) caus- es visible light of the appropriate color to be displayed on the screen.
The application of CCD images to flexible endoscopes during the 1970’s permitted development of a solid-state scope that relied on electronics rather than fiber optics for image transmission. The CCD image processor produces a brighter, higher-resolution im- age than does the conventional fiberscope. This advanced microelectronic technology en- ables the CCD to function as a miniature TV camera able to record and display images with great clarity on a video monitor.
Miniature Cameras
Another variation of the fiberoptic camera is the miniature camera developed for the ex- amination of boiler tubes (see Figure 3-12). The limiting factor in fiberoptic cameras is
FIGURE 3-11 Video borescope. (Courtesy of Olympus Industrial, with permission.)