VISUAL TESTING
VI. ADVANTAGES AND LIMITATIONS
11. Lack of fill
12. Excessive reinforcement
Specific acceptance criteria can be found in various codes, specifications, and standards.
In many cases, they will be specified as part of the contract requirements.
Records of inspections or examinations should also include statements indicating part acceptance (if applicable), and include appropriate documentation to support findings. In- spection report forms should be complete, legible, concise, and signed and dated. If cor- rections are necessary, they should be made with a single line through the entry error, ini- tialed, and dated. All reports, results, and/or drawings, sketches, photos, electronic files, digital video discs (DVD), or videotapes should be listed and retained in accordance with contractual requirements or codes and included with the data sheet.
transferred to light-sensitive paper as a positive image. The result is a positive paper- based photograph. The American and international standard for film speed is expressed in ASA numbers. The European or German standard is expressed in the DIN number. Color film comparisons are listed in the Table 3-2.
The three most common film speeds used for nonprofessional photographs are ASA 100, 200, and 400. The finest grain and slowest speed combination for snapshot photogra- phy with ASA 100 is considered as medium speed in the range of available film. Mi- crophotography requires more light than is available from a consumer type flash attach- ment and a much finer-grain film than that which is used by the amateur photographer.
CCD Based Records
Section III discussed the concept of the charged coupled device (CCD). This is reviewed briefly in the following paragraph.
Photons (packets of energy that behave like particles), also known as light, are reflect- ed from an object. They then pass through the lens of the camera and fall into the collec- tion region of the picture element or pixel. Here electrons are freed from the semiconduc- tor’s crystal lattice. Thus, the more light that falls on the pixel, the more electrons that are liberated. Electrical fields trap and isolate the mobile electrons in one pixel from those of other pixels until the electrons can produce an indication of the amount of light that fell on the pixel. Electrons are passed from one CCD to another. This is analogous to a buck- et brigade moving water. A tiny color filter over each pixel allows photons of only one color to pass through into the pixel’s collection region. This pixel is assigned that color during the reconstruction of the image on the LCD monitor. The array of light-sensitive picture elements, or pixels, each measuring five to 25 microns across, make up the array matrix. The low-end digital camera for nonprofessional use typically has an array of 640
× 480 pixels. At the time of writing, arrays up to 1536 × 1024 are commercially available.
Top of the line cameras are available for professional use. These comprise arrays that have millions of pixels.
Photographic Techniques
Whether the recording medium is emulsion-coated acetate-based film or CCD matrix ar- rays, the lens creates an image by focusing the light rays from an object on a plane behind the lens. An important concept to understand is depth of field. Depth of field can be de- fined as the overall range of focus apparent in a photograph. The principal plane of focus is the single plane through the subject that is actually in focus. The distance in front of and behind the principal plane of focus that is in focus is known as the depth of field.
When working at higher magnifications, this effect becomes even more significant. For this application, the lens diaphragm is used to control the principal plane of focus or depth of field.
If the lens diaphragm opening is reduced, portions of the image that were blurred ap- pear sharper because areas of the object both in front and behind of the principle plane be- come focused. If the opening is further reduced, this effect is greater. Increasing or reduc- ing the diaphragm opening controls the depth of field. The offsetting factor when increasing the depth of field in this manner is that there is less light reaching the film. A
TABLE 3-2 Comparative Color Film Speeds
Slow Medium Fast
< 32 ASA (16 DIN) 64–125 ASA (19–22 DIN) 160 > (23 DIN)
balance between adequate light and optimum depth of field must be achieved. When us- ing a typical 55 mm lens, the depth of field or area of focus extends farther behind the principal plane of focus than it does in front of it. Therefore, the best depth of field can be obtained by focusing at approximately one-third into the region or area of the object or area of interest. This way, the two-thirds region behind the optimum focus will also be in focus. As magnification is increased (by using 90 mm or 120 mm lenses), the reverse is true. This is why macrophotography, e.g., of a bee on a flower, may yield a sharply fo- cused image of the flower or the bee while the background may appear out of focus.
Fiberoptic Cameras and Video Borescopes
Video Borescopes with charged coupled device (CCD) cameras are available with diame- ters as small as a quarter inch and pixel arrays with 410,000 pixels coupled with mi- crolens technology. The monitor can provide 470 lines visible in the super-video home system (SVHS) format. The four-way articulating tip is available in up to 20 feet lengths.
Without the articulating tip, lengths of 45 feet can be achieved. The camera control unit (CCU) can fit under the 7-inch diagonal color monitor and can control light intensity, gain, white balance, and shutter speed. The light source can be provided with 150 Watt halogen lamps or optionally with 300 Watt Xenon lamps.
This instrument is excellent for examining the inside surfaces of tubing found in heat exchangers, feedwater heaters, condensers, and steam generators. It would be especially useful for short distances in small-orifice access scenarios, such as with pumps, valves, engines, small-diameter pipes, and short-distance views of turbines. It would be a poor choice for large-space inspections, such as tanks, vessels, large-diameter pipes, or any- where where detail at some distance was desired and an additional light supply could not be provided. The small diameter of the lens can yield near-infinite focus, but inadequate light would limit the view.
Miniature Cameras
Camera equipment specifically developed for boiler tube inspection application (See Fig- ure 3-12) utilizes a half-inch color CCD with 420,000 pixels, a micro lens, a 460 horizon- tal video line (HVL) monitor for SVHS video, and a light source made from various light head options. These light attachments can produce from 9.6 Watts to 80 Watts of illumi- nation. The elimination of the fiber bundle to transmit light to the site makes the electrical wire and frequency of the electric current the limiting factor for length rather than the lim- its of how long a fiber bundle can be manufactured. The half-inch diameter cable can be manufactured in 100 foot long lengths. Depending on light head diameters, tube diame- ters down to a half inch can be accessed. This type camera is best suited for boiler tubes but can do an adequate job on larger-diameter pipes or small vessels. The larger pipe di- ameters reduce the ability of the camera head to rest anywhere but on the bottom of the pipe, restricting the viewing options. Additionally, pushing a cable does not work well for any distance if the space is sufficient to allow snaking and curling of the cable. Therefore, centering devices and push rods are necessary to alleviate these problems. There remains the problem of articulation. The wide angle of view, as would be achieved with a “fish- eye” lens, allows for a large downfield view and limited sidefield examination. This makes it suitable for many applications where examination for general conditions is the goal. Drain lines, oil lines, steam and process lines, headers, and vessels can all be inter- nally inspected with the miniature camera with light head to lengths of 100 feet.
Miniature Camera with Pan, Tilt, and Zoom Lens
The pan, tilt, and zoom (PTZ) camera, as illustrated in Figure 3-13, is designed for multi- purpose applications that require objects at major distances to be brought under closer ex-
amination. A one-third inch CCD chip is joined with a 12:1 or 24:1 lens train for zoom capabilities. The 12:1 lens can yield greater than 460 horizontal video lines of image. The 24:1 lens can yield more than 380 horizontal video lines of image. The 12:1 lens and the 24:1 lens both utilize variable lens diameters from 0.216 inches (5.4 mm) to 2.6 inches (65 mm).
A spot or flood lamp is needed for adequate light at the greater distances for which these cameras are employed. The standard cable length is 100 feet, but lengths up to 500 feet can be manufactured. A pan range of ± 175° is available. A tilt range of 253° is achievable. This type of camera and light arrangement can be mounted on a tripod, tractor crawler, push pole, or any other suitable platform to examine a wide variety of surface configurations.
This visual examination system is particularly useful in hazardous environments and confined spaces. Tank and vessel applications are probably the most common. Inspec- tions of steam headers, sumps, manifolds, pipe system supports and large-diameter piping (including sewer and water utility lines) are also common, as are petrochemical, refinery, power generation, and engineering services industry applications. The transportation in- dustry requires infrastructure inspection of structures such as bridges, shafts, and tunnels.
The nuclear power and medical industries may need contaminated or hostile environment areas surveyed. Other industrial surveillance and law enforcement applications may in- clude security and terrorist bomb investigations using remote means.
Other applications in the nuclear industry include the inspection of vast surface areas of reactor containment structures made of concrete or steel. A single location of a camera with a 24:1 zoom lens can inspect concrete walls and roofs for spalling, cracking, defolia- tion, and other concrete deterioration anomalies much more efficiently than placing an in- spector hundreds of feet in the air and covering thousands of square feet of surface manu- ally.
It is interesting to note the difference between 12:1 and 24:1 zoom capability. Moving the lens assembly optically performs the zoom effect up to 12:1 magnification. The zoom effect from 12:1 up to 24:1 is achieved by computer enhancement. Portions of the image are expanded and digitally magnified to create enlargements, providing greater detail for viewing and interpretation. When observing the zoom effect of this camera, the transition is smooth and seamless up to 12:1. Beyond 12:1 magnification, a slight “jiggle” in the im- age may be observed. From 12:1 to 24:1 the appearance of squares for each pixel are barely visible, but detectable. This is due to the digital magnification process.
Delivery Systems
Previous discussions of remote visual systems have made some reference to delivery sys- tems. The following paragraphs list these by category. Some typical means of moving a camera from one point to another include fiberglass push rods, small-, medium-, and large-wheeled tractors, mobile platforms with tracks, and various crawlers with retrieval attachments. Applications can vary considerably with regard to the access size or open- ing, length of travel, environment, magnification, and lighting requirements. The basic el- ements of a remote visual delivery system are the camera, light, tractor, cable, mobility control unit, pan, tilt, and zoom controls, and monitor.
A small camera can be affixed to push rods. Limitations may be encountered when multiple changes in direction are required; i.e., the push rod soon binds up against multi- ple turns in the pipe, shaft, or opening.
A steerable-wheeled tractor can negotiate many turns with a camera attached for showing the way. The smaller the opening in which the tractor must operate, the greater the limitation to the payload that can be delivered. For example, a two-inch diameter pipe-crawling tractor may only weigh 3.5 pounds, thereby gaining access to a three-inch
nominal pipe size. But it may only be able to drag along 100 meters of quarter-inch diam- eter cable. Multiple friction drag or contact points may reduce this to a much shorter dis- tance. Additionally, inclined angles of the shaft, pipe, or structure would increase the load and decrease the access distance. Remote-controlled delivery systems may eliminate the need for cable but would require radio contact at all times.
A larger tractor that would fit into a 4 inch diameter pipe may be able to carry a 16 pound load. This may include 100 to 200 meters of cable. Larger tractors may carry a 24 pound load and 200 or more meters of cable. The advantage of larger tractors and accom- panying larger wheels would be the ability to traverse adverse terrain and steeper inclines.
The unmanned rovers that landed on Mars in the mid 1990s demonstrated that multijoint- ed and complex designs can be designed to fit into small packages. They may appear frail but they were able to cross over rocks approaching a 2:1 ratio. Generally, the heavier the structure, the more friction that can be generated; thus, the heavier the load, the rougher the terrain that can be negotiated.
Any of these remote systems can incorporate a retrieval system. The “three pronged fork and tine” is one approach. Two pronged pincers resembling a crab claw is another.
Each additional function requires more cables, lines, and articulating capabilities.
Monitors
Video monitors are the last link in the video system chain. They are the display devices.
In simple terms, the monitor reverses the electronic coding of the video camera and re- turns the signals to visible displays. The standard monitor since the invention of TV has been the cathode ray tube (CRT). Solid-state and liquid crystal displays (LCD) have mostly replaced the CRT, allowing for thinner and smaller monitors. In particular, the miniature camera with lens and CCD designed for boiler tubes has been outfitted with LCDs. The detail and nature of the indications being sought in tubes, pipes, and small vessels do not necessarily require higher resolution. The LCD display is less expensive and more rugged in the field.
Conclusion
Generally, the main limitation to visual testing is access. The image of the object must be delivered to the eye. That image is always of the surface of an object. Visual testing is ca- pable of examining the surface of an object unless the material is translucent. Remote vi- sual testing advances are being driven today, as in recent years, by consumer demand and improvements in video technology. The challenge remains to understand fully “what” the inspector is examining and “how” the image is delivered to the eye. As designers make the image-gathering package smaller and smaller, the limitations of access will be further reduced. Applications in the field of medicine have been influencing the industrial field for years. Military applications including drones and robotic devices should continue to bring innovations to the technology of remote visual testing.