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Enabling Objectives

1.5Unaided, the participant will correctly identify and describe the use of optical aids.

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part i 1.5

Identify and Describe the Use of Optical Aids

Visual inspection is rarely done by the eye alone. There are many optical aids which may be used to assist an inspection including:

1. Magnifiers and microscopes

2. Mirrors

3. Bore scopes and endoscopes

4. Fiber optics

5. Video cameras

6. Special equipment, including imaging and computer-based systems

MAGNIFIERS AND MICROSCOPES

Magnifiers range from I.5x magnifying glasses/lens to the limit of optical microscopy. Hand-held magnifiers normally cover the range up to 10 x magnification. Above this magnification the short working distance becomes a problem and low-powered microscopes and macro-scopes are used. These may he ocular or binocular, wide field and or stereoscopic.

Low-powered microscopes often have one or two objectives to give two magnifications up to 40x.

Medium-powered microscopes may have two or more objectives with magnification between 20x and 100x.

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High-powered microscopes have a number of objectives, often up to six, which give a magnification range of 50x to 2000x. With these microscopes specially prepared surfaces, sections or replicas are required. Often these microscopes have the facility for polarization, phase contrast and interference examinations.

Stereoscopic microscopes have a typical magnification range from 7x to 150x, with a useful upper limit of about 60x. This type of microscope allows the specimen to be moved around and gives a three-dimensional view.

The microscopes may be either of a transmitted light or a reflected light type. The former is used for transparent samples; hut opaque samples require reflected light.

Transmitted light microscopes operate with the light source behind the specimen with light passing through the transparent sample. Reflected light microscopes pass light through the objective by a light reflector on to the surface of opaque samples which reflects the light hack through the objective and to the eyepiece.

Hand-held Lenses

Low-powered hand-held lenses, up to about 10x magnifications, are used to magnify fine small detail to enable a better assessment to be made.

The hand lens is moved close to the surface to be inspected and then slowly moved away until the surface is in focus. The distance from the lens to the eye will he variable so and should be around 300 mm, the distance for near vision. Continuous adjustment is often required to focus specific parts of the detail.

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The uses of hand-held lenses are infinite with most visual inspection of fine detail benefiting from their use. They are widely used in many industries, from metal components to fabric inspection.

Angle-poise Mounted Magnifiers

With magnification of about 10 x, the equipment often incorporates a light source, typically a circular fluorescent tube producing a uniform illumination in the inspection zone.

Note: ES 5165- Guide to the selection of low-power magnifiers for visual inspection

Components are transported to the test bench and either manipulated on the bench with the magnifier adjusted to produce the desired focus or manipulated under the magnification at a fixed distance. The working distance of the eye from the lens varies hut with most magnifiers should be about the normal reading distance of approximately 300 mm.

Angle-poise magnifiers would be used in a fixed inspection station within a machine shop, test house or inspection department. Typical inspection includes machine tools, small components, and fabric inspection.

Low-power Microscope

These groups of microscopes cover a large range from low-power stereoscope of up to 20x magnification to monocular and binocular equipment up to 50x magnification. The majority of equipment is bench mounted and is also portable, with some types capable of mounting on large components. In most cases the microscopes consist of a stage, objective lens, eyepiece lens and a light source.

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Mode of Operation

The working distance between stage and objective lens is restricted and may therefore require small sections to be taken from large pieces. The sample may require special preparation, sections from translucent material for mounting on to glass slides for examination by transmitted light. Opaque materials can only be examined by reflected light and may require some form of preparation depending on the features requiring examination.

Once a sample is prepared it is placed on the microscope stage, the light source switched on, the eyepieces adjusted (binocular type) and the stage slowly racked up by means of the coarse focus until an image is obtained whilst viewing through the eyepiece lens. Critical focusing is achieved by the use of the fine focus.

Applications

These microscopes are used for routine testing/inspection of component surfaces, structure, metals, ceramics, plants, tissues, electronic component materials, fabrics, liquid, fractures, fibers, etc.

Measuring Microscopes and Special Microscopes

These are microscopes used to measure specific parameters and are used for small detail which requires accurate measuring, e.g. from surface finish to fabrics and hardness impressions of Brinell and Vickers hardness testing. Our illustrative example is the Brinell microscope used to measure the diameter of the indentation made by a steel ball on the surface of the test piece. The microscopes incorporate a measuring scale which can normally he adjusted to

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obtain a sharp focus of the scale of the surface to be measured.

A light source is usually incorporated in the form of a small tungsten filament bulb operating from a small battery and should be checked for illumination by switching on prior to the operation. The microscope is then checked against the calibration scale to ensure the measurements obtained from the test are correct and within any specified limits. The Brinell microscope is used to measure the ball impression diameter in two directions after the above checks have been made. The dimensions obtained are then converted by means of a chart to give a Brinell hardness number, which is an indication of the material’s hardness.

Special Microscopes and Magnifiers

Surface comparator magnifiers - used to check surface finish.

Measuring magnifier - this is a magnifier incorporating a measuring scale available in a range of units.

Shop microscope - about 40 x magnification. Used for a range of inspection from plated and painted surfaces to defective components and surface wear.

Laboratory microscope - this is a conventional compound microscope. A great range of magnification, field coverage, and resolution is available. Magnification can range from 100 to 2000 x. It is designed principally for transmitted light as it tends to be used with transparent or semi-transparent materials.

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Metallurgical microscope - this is very similar to a laboratory microscope but will have vertical illumination so that opaque samples can be viewed using reflected light.

Brinell microscope - See example above.

MIRRORS

Inspections can be made with mirrors allowing viewing behind or underneath objects or components with flexibility to obtain optimum viewing angles.

Mirrors are available in various shapes, sizes and curvature configurations (convex, concave, parabolic), with adjustable and telescopic handles.

The use of mirrors requires a degree of practice to reflect the light and obtain the desired reflection.

BORE SCOPES and ENDOSCOPES

The bore scope, which is a self-illuminated telescope, originates from an early development of equipment to explore inside human bodies without having to operate. The original equipment was called an endoscope, derived from the Greek words for inside view and this is the term now used for flexible bore scopes.

Cysto-scopes (a tube incorporating a lens and light source) were developed for examination of the human bladder and are the basis for bore scopes used in visual inspection.

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Types of Bore Scopes

Rigid Bore Scopes

The rigid bore scope was originally developed to inspect the bores of rifles and gun barrels. The image at the eyepiece is produced by an objective lens, prism, relay lenses and eyepiece and may have either fixed or adjustable focusing, the latter having a greater advantage over the fixed focus type.

Figure 1Rigid Bore Scope

Figure 2Rigid Bore Scope

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Figure 3Typical Bore scope Lens System

A rigid mini-bore scope contains a single solid fiber to replace the lenses. The fiber is about 1 mm in diameter and the lens aperture equal to a pin-hole camera, resulting in an infinite depth of field. Focusing adjustments can help in overcoming and compensating for variability in eyesight and expands the depth of field, therefore producing sharper images

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Figure 4Available Angles of View

The following is a list of special bore scopes used for special examination:

Panoramic Bore Scopes - this has a scanning mirror mounted in front of the objective lens system. This gives a wide range of vision and allows the rapid inspection of the 60 insides of cylinders, pipes etc.

Water- or gas-proof Bore Scopes - for high-temperature applications inside engines etc. Can be used in liquid or gas environments.

Angulated Bore Scopes - having various bends, permitting inspection of areas not normally accessible by a rigid, straight bore scope.

Right-angled Bore Scopes - used for looking around corners. Wide-field bore scopes - up to 1200 field of view. Miniature Bore Scope - down to 1.75 mm diameter. Periscope bore scopes - used to see above or over objects.

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Ultraviolet Bore Scopes - used in fluorescent inspection (e.g. magnetic particle inspection and penetrant testing). Comes complete with UV light source, filters etc.

Calibrated Bore Scopes - used in special examinations. The external tube is calibrated in order to indicate depth of insertion during tests.

Figure 5Calibrated Bore Scope

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Figure 6Calibrated Bore Scope

Setting Up a Rigid Bore Scope

1. Place a protractor on a board and position the bore scope parallel to the 00 line with the lens directly over the center mark.

2. Ensure that the protractor center is behind the lens window between 25 to 50 mm away.

Sight through the instrument and, using marks on the edge of the protractor, mark the field of view, left and right hand edge and center. Readings from the protractor give direction and field of view. The angle of view ranges from 20° to 360°.

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Figure 7A Rigid Bore Scope Set-up

ENDOSCOPES or FIBERSCOPES

Endoscopes are flexible systems using fiber optics, which are used in a similar manner to bore scopes. They are used extensively in medicine and many engineering applications.

FIBER OPTICS

Fiber optics use very thin flexible glass fiber filaments between 9-30 microns in diameter. These filaments are capable of transmitting light within the boundaries of the fiber by internal reflections, the light following the path of the fiber irrespective of its shape. This property allows the light or image to he transmitted around bends and curves without additional optical equipment.

The fiber consists of a core of high quality optical glass with a case of glass of different refractive index, which acts as a mirror. The fibers are very small in cross-section and transmit very little light; therefore the fibers are grouped together in bundles, many

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thousands at a time, to produce the required level of illumination.

Max Dia. (mm)

Working length(mm)

Direction of view(DOV)

Field of view(FOV)

Depth of field(mm)

4.10 292333

Lateral (90°)Direct (0°)

50°60°

2 – Infinity4 - Infinity

76161247333

Direct (0°) 10°, 30°,

60° ,or 80°

10° FOV 80 – Infinity30° FOV 20 – Infinity

50°/60° FOV (direct)

4- 4 Infinity50°/60°

FOV (Lateral) 2 – Infinity

80° FOV 2- Infinity

121207292379

Direct (0°) 30° or 50°

5.48 290440590

Direct (0°) 55° 5 - Infinity

290440590

Fore-oblique (45°)

290440590740

Lateral (90°)

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7.98 240440650

Direct (0°) Infinity

240440

Fore-oblique (45°)

Infinity

2403404405507609601280

Lateral (90°)

Infinity

340 30° 10 - Infinity340 Retro

(110°)55° 5 - Infinity

9.98 240440

Direct (0°) 55° 5 - Infinity

440 Fore-oblique (45°)

240340440550760960

Lateral (90°)

340 330° 10 - Infinity

11098 340 Lateral (90°)

330° Infinity

The application of transmitting light and receiving images requires the use of two separate bunches of fiber, one to transmit (the light guide) and one to receive (the image guide). The fiber filaments for light guides are about 30 micron in diameter, and are used in bundles, the light guide bundle. The fibers for the image guide have a diameter of about 9-17 microns, smaller than the light guide fibers, because the diameter of the fibers is one of the factors which will affect the resolution. An objective lens is attached to one end of the fibers to focus the picture, which is

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transmitted by fiber optics to the eyepiece and can then be adjusted to produce a sharp image.

The many variants of the bore scope are used to inspect the internal condition and integrity of pipework, combustion chambers, gas cylinders, small tanks, chambers and vessels where unaided visual inspection is not practical.

Bore scopes are widely used in the automotive industry, to examine engine cylinders without having to take the engine apart. In machine shops, they are used to test the internal surface conditions of many components. They are used in the nuclear and chemical industries for remote observation, so that an inspector can remain in a safe area while examining a more dangerous environment. This is particularly important in tube inspection in power stations, chemical plants, etc.

As remote inspection requirements become ever more complex and demanding, the systems design engineer seeks more sophisticated solutions to inspection problems. This often leads to the use of video systems, using either real or virtual images. A real image is composed of real light waves, which can be projected onto a screen or captured on film or video tape. A virtual image is only an apparent image and so cannot be directly captured, but the sophisticated electronics in a video application of bore scopes and endoscopes system allows the virtual image to be converted into a real picture.

Video systems work on the principle that a picture can be thought of as being composed of a large number of very small dots (picture elements or pixels). These dots can be any shade of brightness from white (brightest) through shades of grey to black (darkest).

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Figure 8Video System

When observing the screen from normal viewing distance, these dots merge to form a continuous picture. This technique is used in television, where information on the degree of brightness of each pixel is sent from the transmitting end to the receiving end, where a reproduction of the original scene is formed.

The basic equipment required for a video system is the video camera, a TV monitor and cables to relay electrical information between them. Additions to the system can include light sources, a control unit, and signal processing/analyzing equipment. An analyzer makes it possible to store or to freeze images. Stored images can be processed to improve upon the real image in order to enhance inspection and detection of discontinuities in the object inspected.

TELEVISION CAMERA

The optical image of the scene to be televised is focused, via a zoom lens, onto the target of the camera tube. The target is coated in photo -conductive or photo -emissive material, and this generates a pattern of electrical voltages at the back

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of the target, with the voltage at any point being proportional to the brightness of the corresponding image point.

The target is scanned by an electron beam generated in the camera tube - moving left to right across the target and rapidly back to left again, then left to right and rapidly back and so on. The beam starts at the top of the target and works down to the bottom, and returns rapidly to the top and begins the process again.

As the target (videcon) is being scanned the voltages representing detail in the image are transferred to the output terminal of the camera tube.

A video camera operates in the same way as a TV camera, but is usually of a much simpler construction.

Photos & Tables courtesy of Olympus C6 series

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Figure 9Olympus Videoscope

Articulation Brake Left & Right

Another method of televising an image is by the use of a semiconductor pick-up device in place of the tube. Each small chip contains thousands of silicon photodiodes, with each diode storing a degree of charge dependent upon the amount of light falling on it. Each diode represents a pixel and can be electronically read and converted to corresponding image in a picture tube.

Picture Tube - Cathode Ray Tube

A cathode ray tube is used to convert the signals from the camera hack to an image. The tube contains an electron beam, which is fired at a screen coated in a material, which emits light when struck by electrons. The beam density is controlled at each point by the picture signal input from the camera, i.e. the voltages representing each point of the scene determine the electron density hitting the screen, and therefore the brightness, at the corresponding point on the screen, and so a reproduction of the original image is built up on the screen.

Image Quality

The television camera tube is a very important component in the system since it must produce high-resolution pictures. As mentioned above, any image can be regarded as a series of very small dots or elements. The best resolution is obtained with the highest number of image picture elements as possible, therefore the higher the number of pixels, the better the resolution on the target. The quality of the picture on the screen is governed by scattered and reflected light within the tube, all of which reduce

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image contrast. The visual interpretation of the images/pictures on the screen is governed by contrast, brightness and the resolution or number of lines in the picture. The greater the number of lines, the better the resolution. Therefore with the screen size, number of lines in the picture and the magnification, it is possible to calculate the smallest resolvable detail.

Effect of Magnification

An increase in the size of the image, which is then projected electronically onto a screen, improves the resolution of the smallest detail without having to resort to improving the resolution of the monitor. The disadvantages are that increasing magnification of the test-piece also magnifies any movement in the camera system and may also affect the depth of field available.

Depth of Field

Depth of field is the range over which the camera/lens produces satisfactory definition which is in focus. It can he expected that the depth of field will decrease with increase in magnification.

Applications

Cameras can operate in a range of diverse applications. They can be used alone, with zoom and telescopic lenses, or in conjunction with optical fibers to produce very small endoscopes. Video cameras can also he fitted to visual equipment such as magnifiers, microscopes etc.

Inspection of pipe work and vessels which may appear difficult, if not impossible, can now be

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performed by remote control video equipment employing small cameras and lighting systems. The equipment is complex, usually consisting of a video endoscope incorporating a camera, fiber optic lighting and control systems, all of which are controlled from a distance. The cameras can be front view, wide angle or side view or a combination of these kinds.

Fiber optics are used to transmit light to the working head from a remote source. In general, cable lengths are limited to 30 meters since electronic problems occur with longer lengths.

The camera is only a small part of the total system, which requires control systems, energy and light sources, monitoring systems and recording systems, the latter often 80 being a video recorder. The camera system can be, pushed through a tube, pulled through a tube or lowered into a tube. If pushed, the cable must be able to carry all the systems to operate the camera and rigid enough to push the system, and yet it has to he flexible enough to negotiate bends up to about 450, Cable reels with hand cranking systems are used to pull inspection systems through pipe work.

Cables for camera systems require sheaves and guides at all changes in section in order to avoid damage or sticking.

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