VISUAL TESTING
VII. GLOSSARY OF KEY TERMS
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.
Angle of Field—the greatest angle in between two rays coming from the object through the objective lens and into the optical system.
Angstrom—unit of length, equal to 0.1 nanometer.
Aspect Ratio—in television, the ratio of the frame width to the frame height.
Borescope—an industrial scope used to transmit images from inaccessible interiors for visual testing. Borescopes are so called because they were originally used in machined apertures and holes such as gun barrel bores. There are both flexible and rigid, fiberop- tic, and geometric light (lens optic) borescopes.
Brightness—the attribute of visual perception in accordance with which an area appears to emit more or less light.
Burned-in Image—an image that persists in a fixed position in the output signal of a camera tube after the camera has been turned to a different scene.
Charged Coupled Device (CCD)—a solid state image sensor. CCDs are widely used in- spection systems because of their accuracy, high speed scanning, and long service life.
Semiconductors, when struck by photons, accumulate electrons that are passed on from one charged coupled device to another. Each pixel represents one color or another.
Each pixel and its color is reconstructed on a monitor to form an image.
Code—a standard enacted or enforced as a law.
Color—sensation by which humans distinguish light of different intensities (brightness) and wavelengths (hue).
Cone—in biology, a retinal receptor that dominates the retinal response when the lumi- nance level is high and provides the basis for the perception of color.
Contrast—the range of light and dark values in a picture or the ratio between the maxi- mum and minimum brightness values.
Corrosion—loss or degradation of metal as a result of chemical reaction.
Crevice Corrosion—a kind of galvanic corrosion caused by differences in metal ion con- centrations in neighboring portions of the corrodent.
Depth of Field—in photography, the range of distance over which an imaging system gives satisfactory definition when its lens is in the best focus for a specific distance.
(Also see Depth of Focus.)
Depth of Focus—the region in front of and behind the focused distance within which ob- jects still produce an image of acceptable sharpness or resolution when viewed by an observer having normal vision, or an observer whose vision is corrected to normal. In the case of a fixed-focus system, this parameter is often called the Depth of Field (DOF).
Direct Viewing—viewing of a test object in the viewer’s immediate presence. The term is used in the fields of robotics and surveillance to distinguish conventional from re- mote viewing.
Distal—in a manipulative or interrogation system, of or pertaining to the end opposite from the eyepiece and farthest from the person using the system. Also objective or tip.
Dustproof—so constructed or protected that dust will not interfere with successful opera- tion.
Dust-tight—so constructed that dust will not enter the enclosing case.
Endoscope—device for viewing the interior of objects. From the Greek words for inside view, the term endoscope is used mainly for medical instruments.
Erosion—loss of material or degradation of surface quality caused by friction or abrasion from moving fluids or particles.
Explosion-proof, Intrinsically Safe, Purged and Pressurized—constructed in a man- ner to prevent the surrounding atmosphere from being exploded by the operation of, or the results from, operating the item so classified.
Fiber Optics—an array of flexible glass or plastic fibers that has the capability of trans-
mitting light (random array) or an image (coherent array) axially through the fiber bun- dles.
Flux (luminous)—the intensity of light per unit area of its source.
Foot-candle—a unit of illuminance when the foot is taken as the unit of length. It is the illuminance on a surface one square foot in area on which there is a uniformly distrib- uted flux of one Lumen, or the illuminance at a surface, all points of which are at a dis- tance of one foot from a uniform source of one Candle.
Fovea Centralis—a small depression near the center of the retina, constituting the area of most acute vision.
Frame (in Television)—the total area, occupied by the picture, that is scanned while the picture signal is not blanked.
Geometric Distortion—any aberration that causes a reproduced picture to be geometri- cally dissimilar to the perspective-plane projection of the original scene.
Horizontal (Hum) Bars—relatively broad horizontal bars, alternately black and white, that extend over the entire picture. They may be stationary or may move up or down.
Sometimes referred to as a “Venetian-blind” effect, they are caused by the approximate 60-cycle interfering frequency or one of its harmonic frequencies.
Image—a reproduction of an object produced by light rays. An image-forming optical system gathers a beam of light diverging from an object point and transforms it into a beam that converges toward another point. If the beam converges to a point, a real im- age is produced, which can be projected upon a screen, a film plane, video camera tube, or semiconductor array.
Interlaced Scanning—a scanning process in which the distance from center to center of successively scanned lines is two or more times the nominal line width, and in which the adjacent lines belong to different fields.
Lumen—the unit of luminous flux. It is equal to the flux through a unit solid angle (stera- dian) from a uniform point source of one Candle, or to the flux on a unit surface, all points of which are at unit distance from a uniform point source of one Candle.
Object—the figure seen through or imaged by an optical system that may contain natural or artificial structures, or may be the real or virtual image of an object formed by an- other optical system.
Peripheral Vision—the seeing of objects displaced from the primary line of sight and outside the central visual field.
Pixel—a lighted point on the screen of a digital image. The image from a conventional computer monitor is an array of over 256,000 pixels, each of which has a numerical value. The higher the number for a pixel, the brighter it is. Formerly called picture ele- ment.
Raster—a predetermined pattern of scanning lines that provides substantially uniform coverage of an area.
Reference Standard—work piece or energy source prepared according to precise in- structions by an approved agency for tests and calibrations requiring precise and con- sistent measurements. (In the case of photometry, the standard is a light source.) Reflection—a general term for the process by which the incident flux leaves a surface or
medium from the incident side, without change in frequency. Reflection is usually a combination of regular and diffuse reflection.
Remote Viewing—viewing of a test object not in the viewer’s immediate presence. The word remote previously implied either closed-circuit television or fiberoptic systems remote enough so that, for example, the eyepiece and the objective lens could be in dif- ferent rooms. High-resolution video and digital signals can now be transmitted around the world with little loss of image quality. Compare with Direct Viewing.
Resolution—an aspect of image quality pertaining to a system’s ability to reproduce ob-
jects, often measured by resolving a pair of adjacent objects or parallel lines. See also Resolving Power.
Resolution (Horizontal)—the amount of resolvable detail in the horizontal direction in an image. It is usually expressed as the number of distinct vertical lines, alternately black and white, that can be seen in a distance equal to image height. This information usually is derived by observation of the vertical wedge of a test pattern. An image that is sharp and clear and shows small details has good or high, resolution. If the picture is soft and blurred and small details are indistinct, it has poor or low resolution. Horizon- tal resolution depends upon the high-frequency amplitude and phase response of the system, the transmission medium, and the image monitor, as well as the size of the scanning spots.
Resolution (Vertical)—the amount of resolvable detail in the vertical direction in a pic- ture. It is usually expressed as the number of distinct horizontal lines, alternately black and white, that can be seen in a test pattern. Vertical resolution is fundamentally limit- ed by the number of horizontal scanning lines per frame. Beyond this, vertical resolu- tion depends on the size and shape of the scanning spots of the pickup equipment and picture monitor and does not depend upon the high-frequency response or bandwidth of the transmission medium or picture monitor.
Resolution Threshold—minimum distance between a pair of points or parallel lines when they can be distinguished as two, rather than one; expressed in minutes of arc.
Retina—the tissue at the rear of the eye that senses light.
Resolving Power—the ability of vision or other detection system to separate two points.
Resolving power depends on the angle of vision and the distance of the sensor from the test surface. Resolving power is often measured using parallel lines. Compare with Resolution.
Retained Image (Image Burn)—a change produced in or on the target that remains for a large number of frames after the removal of a previously stationary light image and yields a spurious electrical signal corresponding to that light image.
Rod—retinal receptor that responds at low levels of luminance, even down below the threshold for cones. At these levels, there is no basis for perceiving differences in hue and saturation. No rods are found in the fovea centralis (see definition).
Simple Magnifier—a device having a single converging lens.
Specification—a set of instructions or standards invoked by any organization to govern the results or performance of a specific set of tasks or products.
Standard—document to control and govern practices in an industry or application, ap- plied on a national or international basis and usually produced by consensus. See also Reference Standard.
Tip—in casual usage, the distal or objective end of a borescope.
Trace—line formed by electron beams scanning from left to right on a video screen to generate a picture.
Video—pertaining to the transmission and display of images in an electronic format that can be displayed on a cathode ray screen.
Videoscope—jargon for video borescope. See Borescope, Video.
Visibility—the quality or state of being perceivable by the eye. In many outdoor applica- tions, visibility is defined in terms of the distance at which an object can be just per- ceived by the eye. In indoor applications, it usually is defined in terms of the contrast or size of a standard test object, observed under standardized viewing conditions.
Vision—perception by eyesight.
Vision Acuity—the ability to distinguish fine details visually. Quantitatively, it is the re- ciprocal of the minimum angular separation in minutes of two lines of width subtend- ing (extending under or opposite to) one minute of arc when the lines are just resolv- able as separate.
Visual Angle—the angle subtended by an object or detail at the point of observation. It usually is measured in minutes of arc.
Visual Field—the locus of objects or points in space that can be perceived when the head and eyes are kept fixed. The field may be monocular or binocular.
Visual Perception—the interpretation of impressions transmitted from the retina to the brain in terms of information about the physical world displayed before the eye. Visual perception involves any one or more of the following: recognition of the presence of something (object, aperture, or medium); identifying it; locating it in space; noting its relation to other things; identifying its movement, color, brightness, or form.
Visual Testing—method of nondestructive testing using electromagnetic radiation at vis- ible frequencies.
Waterproof—so constructed or protected that water will not interfere with successful op- eration. This does not imply submersion to any depth or pressure.
Watertight—provided with an enclosing case that will exclude water applied in the form of a hose stream for a specified time as stated in the following note.
Note:a common form of specification for watertight is: “So constructed that there shall be no leakage of water into the enclosure when subjected to a stream from a hose with a one-inch nozzle and delivering at least 65 gallons per minute, with the water directed at the enclosure from a distance of not less than 10 feet for a period of 5 minutes, dur- ing which period the water may be directed in one or more directions as desired.”
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