ASM Metals HandBook Vol. 17 - Nondestructive Evaluation and Quality Control

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• By using a rigid tube with a series of relay lenses

• By using a tube (normally flexible but also rigid) with a bundle of optical fibers

• By using a tube (normally flexible) with wiring that carries the image signal from a charge-

coupled

device (CCD) imaging sensor at the distal tip

These three basic tube designs can have either fixed or adjustable focusing of the objective lens at the distal tip. The distal

tip also has prisms and mirrors that define the direction and field of view (see Fig. 2). These views vary according to the

type and application of borescope. The design of illumination system also varies with the type of borescope. Generally, a

fiber optic light guide and a lamp producing white light is used in the illumination system, although ultraviolet light can

be used to inspect surfaces treated with liquid fluorescent penetrants. Light-emitting diodes at the distal tip are sometimes

used for illumination in videoscopes with working lengths greater than 15 m (50 ft).

Fig. 1

Three typical designs of borescopes. (a) A rigid borescope with a lamp at the distal end. (b) A flexible

fiberscope with a light source. (c) A rigid borescope with a light guide bundle in the shaft

Rigid Borescopes

Rigid borescopes are generally limited to applications

with a straight-line path between the observer and the

area to be observed. The sizes range in lengths from 0.15

to 30 m (0.5 to 100 ft) and in diameters from 0.9 to 70

mm (0.035 to 2.75 in.). Magnification is usually 3 to 4×,

but powers up to 50× are available. The illumination

system is either an incandescent lamp located at the

distal tip end (Fig. 1a) or a light guide bundle made from

optical fibers (Fig. 1c) that conduct light from an

external source.

The choice of viewing heads for rigid borescopes (Fig.

2) varies according to the application, as described in the

section "Selection" in this article. Rigid borescopes

generally have a 55° field of view, although the fields of

view can range from 10 to 90°. Typically, the distal tips

are not interchangeable, but some models (such as the

extendable borescopes) may have interchangeable

viewing heads.

Some rigid borescopes have orbital scan (Fig. 1c), which

involves the rotation of the optical shaft for scanning

purposes. Depending on the borescope model, the amount of rotation can vary from 120 to 370°. Some rigid borescopes

also have movable prisms at the tip for scanning.

Rigid borescopes are available in a variety of models having significant variations in the design of the shaft, the distal tip,

and the illumination system. Some of these design variations are described below.

Basic Design. The rigid borescope typically has a series of achromatic relay lenses in the optical tube. These lenses

preserve the resolution of the image as it travels from the objective lens to the eyepiece. The tube diameter of these

borescopes ranges from 4 to 70 mm (0.16 to 2.75 in.). The illumination system can be either a distal lamp or a light guide

bundle, and the various features may include orbital scan, various viewing heads, and adjustable focusing of the objective

lens.

Miniborescopes. Instead of the conventional relay lenses, miniborescopes have a single image-relaying rod or quartz

fiber in the optical tube. The lengths of miniborescopes are 110 and 170 mm (4.3 and 6.7 in.), and the diameters range

from 0.9 to 2.7 mm (0.035 to 0.105 in.). High magnification (up to 30×) can be reached at minimal focal lengths, and an

adjustable focus is not required, because the scope has an infinite depth of field. The larger sizes have forward, side view,

and forward-oblique views. The 0.9 mm (0.035 in.) diam size has only a forward view. Miniborescopes have an integral

light guide bundle.

Hybrid borescopes utilize rod lenses combined with convex lenses to relay the image. The rod lenses have fewer

glass-air boundaries; this reduces scattering and allows for a more compact optical guide. Consequently, a larger light

guide bundle can be employed with an increase in illumination and an image with a higher degree of contrast.

Hybrid borescopes have lengths up to 990 mm (39 in.), with diameters ranging from 5.5 to 12 mm (0.216 to 0.47 in.). All

hybrid borescopes have adjustable focusing of the objective lens and a 370° rotation for orbital scan. The various viewing

directions are forward, side, retrospective, and forward-oblique.

Extendable borescopes allow the user to construct a longer borescopic tube by joining extension tubes. Extendable

borescopes are available with either a fiber-optic light guide or an incandescent lamp at the distal end. Extendable

borescopes with an integral lamp have a maximum length of about 30 m (100 ft). Scopes with a light guide bundle have a

shorter maximum length (about 8 m, or 26 ft), but do allow smaller tube diameters (as small as 8 mm, or 0.3 in.).

Interchangeable viewing heads are also available. Extendable borescopes do not have adjustable focusing of the objective

lens.

Fig. 2

Typical directions and field of view with rigid

borescopes

Rigid chamberscopes allow more rapid inspection of larger chambers. Chamberscopes (Fig. 3) have variable

magnification (zoom), a lamp at the distal tip, and a scanning mirror that allows the user to observe in different directions.

The higher illumination and greater magnification of chamberscopes allow the inspection of surfaces as much as 910 mm

(36 in.) away from the distal tip of the scope.

Mirror sheaths can convert a direct-viewing

borescope into a side-viewing scope. A mirror sheath is

designed to fit over the tip of the scope and thus reflect

an image from the side of the scope. However, not all

applications are suitable for this device. A side,

forward-oblique, or retrospective viewing head

provides better resolution and a higher degree of image

contrast. A mirror sheath also produces an inverse

image and may produce unwanted reflections from the

shaft.

Scanning. In addition to the orbital scan feature

described earlier, some rigid borescopes have the ability

to scan longitudinally along the axis of the shaft. A

movable prism with a control at the handle

accomplishes this scanning. Typically, the prism can

shift the direction of view through an arc of 120°.

Flexible Borescopes

Flexible borescopes are used primarily in applications that do not have a straight passageway to the point of observation.

The two types of flexible borescopes are flexible fiberscopes and videoscopes with a CCD image sensor at the distal tip.

Flexible Fiberscopes. A typical fiberscope (Fig. 1b) consists of a light guide bundle, an image guide bundle, an

objective lens, interchangeable viewing heads, and remote controls for articulation of the distal tip. Fiberscopes are

available in diameters from 1.4 to 13 mm (0.055 to 0.5 in.) and in lengths up to 12 m (40 ft). Special quartz fiberscopes

are available in lengths up to 90 m (300 ft).

The fibers used in the light guide bundle are generally 30 m (0.001 in.) in diameter. The second optical bundle, called

the image guide, is used to carry the image formed by the objective lens back to the eyepiece. The fibers in the image

guide must be precisely aligned so that they are in an identical relative position to each other at their terminations for

proper image resolution.

The diameter of the fibers in the image guide is another factor in obtaining good image resolution. With smaller diameter

fibers, a brighter image with better resolution can be obtained by packing more fibers in the image guide. With higher

resolution, it is then possible to use an objective lens with a wider field of view and also to magnify the image at the

eyepiece. This allows better viewing of objects at the periphery of the image (Fig. 4). Image guide fibers range from 6.5

to 17 m (255 to 670 in.).

Fig. 3

Typical chamberscope. Courtesy of Lenox

Instrument Company

Fig. 4

Two views down a combustor can with the distal tip in the same position. A fiberscope with smaller

diameter fiber

s and 40% more fibers in the image bundle provides better resolution (a) than a fiberscope with

larger fibers (b). Courtesy of Olympus Corporation

The interchangeable distal tips provide various directions and fields of view on a single fiberscope. However, because the

tip can be articulated for scanning purposes, distal tips with either a forward or side viewing direction are usually

sufficient. Fields of view are typically 40 to 60°, although they can range from 10 to 120°. Most fiberscopes provide

adjustable focusing of the objective lens.

Videoscopes with CCD probes involve the electronic transmission of color or black and white images to a video

monitor. The distal end of electronic videoscopes contains a CCD chip, which consists of thousands of light-sensitive

elements arrayed in a pattern of rows and columns. The objective lens focuses the image of an object on the surface of the

CCD chip, where the light is converted to electrons that are stored in each picture element, or pixel, of the CCD device.

The image of the object is thus stored in the form of electrons on the CCD device. At this point, a voltage proportional to

the number of electrons at each pixel is determined electronically for each pixel site. This voltage is then amplified,

filtered, and sent to the input of a video monitor.

Videoscopes with CCD probes produce images (Fig. 5) with spatial resolutions of the order of those described in Fig. 6.

Like rigid borescopes and flexible fiberscopes, the resolution of videoscopes depends on the object-to-lens distance and

the fields of view, because these two factors affect the amount of magnification (see the section "Magnification and Field

of View" in this article). Generally, videoscopes produce higher resolution than fiberscopes, although fiberscopes with

smaller diameter fibers (Fig. 4a) may be competitive with the resolution of videoscopes.

Fig. 5 Videoscope images (a) inside engine guide vanes (b) of an engine fuel nozzle.

Courtesy of Welch Allyn,

Inc.

Fig. 6

Typical resolution of CCD videoscopes with a 90° field of view (a), 60° field of view (b), 30° field of view

(c). Source: Welch Allyn, Inc.

Another advantage of videoscopes is their longer working length. With a given amount of illumination at the distal tip,

videoscopes can return an image over a greater length than fiberscopes. Other features of videoscopes include:

•

The display can help reduce eye fatigue (but does not allow the capability of direct viewing through an

eyepiece)

• There is no honeycomb pattern or irregular picture distortion as with some fiberscopes (Fig. 7)

• The electronic form of the image si

gnal allows digital image enhancement and the potential for

integration with automatic inspection systems.

• The display allows the generation of reticles on the viewing screen for point-to-point measurements.

Fig. 7

Image from a videoscope (a) and a fiberscope (b). In some fiberscope images, voids between individual

glass fibers can create a honeycomb pattern that adds graininess to the image. Courtesy of Welch Allyn, Inc.

Special Features

Measuring borescopes and fiberscopes contain a movable cursor that allows measurements during viewing (Fig.

8). When the object under measurement is in focus, the movable cursor provides a reference for dimensional

measurements in the optical plane of the object. This capability eliminates the need to know the object-to-lens distance

when determining magnification factors.

Working channels are used in borescopes and fiberscopes to pass working

devices to the distal tip. Working channels are presently used to pass measuring

instruments, retrieval devices, and hooks for aiding the insertion of thin, flexible

fiberscopes. Working channels are used in flexible fiberscopes with diameters as

small as 2.7 mm (0.106 in.). Working channels are also under consideration for the

application and removal of dye penetrants and for the passage of wires and sensors in

eddy current measurements.

Selection

Flexible and rigid borescopes are available in a wide variety of standard and

customized designs, and several factors can influence the selection of a scope for a

particular application. These factors include focusing, illumination, magnification,

working length, direction of view, and environment.

Focusing and Resolution. If portions of long objects are at different planes, the

scope must have sufficient focus adjustment to achieve an adequate depth of field. If

the scope has a fixed focal length, the object will be in focus only at a specific lens-

to-object distance.

To allow the observation of surface detail at a desired size, the optical system of a borescope must also provide adequate

resolution and image contrast. If resolution is adequate but contrast is lacking, detail cannot be observed.

In general, the optical quality of a rigid borescope improves as the size of the lens increases; consequently, a borescope

with the largest possible diameter should be used. For fiberscopes, the resolution is dependent on the accuracy of

alignment and the diameter of the fibers in the image bundle. Smaller-diameter fibers provide more resolution and edge

contrast (Fig. 4), when combined with good geometrical alignment of the fibers. Typical resolutions of videoscopes are

given in Fig. 6.

Illumination. The required intensity of the light source is determined by the reflectivity of the surface, the area of

surface to be illuminated, and the transmission losses over the length of the scope. At working lengths greater than 6 m

(20 ft), rigid borescopes with a lamp at the distal end provide the greatest amount of illumination over the widest area.

However, the heat generated by the light source may deform rubber or plastic materials. Fiber-optic illumination in scopes

with working lengths less than 6 m (20 ft) is always brighter and is suitable for heat-sensitive applications because filters

can remove infrared frequencies. Because the amount of illumination depends on the diameter of the light guide bundle, it

is desirable to use the largest diameter possible.

Magnification and field of view are interrelated; as magnification is increased, the field of view is reduced. The

precise relationship between magnification and field of view is specified by the manufacturer.

The degree of magnification in a particular application is determined by the field of view and the distance from the

objective lens to the object. Specifically, the magnification increases when either the field of view or the lens-to-object

distance decreases.

Working Length. In addition to the obvious need for a scope of sufficient length, the working length can sometimes

dictate the use of a particular type of scope. For example, a rigid borescope with a long working length may be limited by

the need for additional supports. In general, videoscopes allow a longer working length than fiberscopes.

Direction of View. The selection of a viewing direction is influenced by the location of the access port in relation to the

object to be observed. The following sections describe some criteria for choosing the direction of view shown in Fig. 2.

Flexible fiberscopes or videoscopes, because of their articulating tip, are often adequate with either a side or forward

viewing tip.

Circumferential or panoramic heads are designed for the inspection of tubing or other cylindrical structures. A centrally

located mirror permits right-angle viewing of an area just scanned by the panoramic view.

The forward viewing head permits the inspection of the area directly ahead of the viewing head. It is commonly used

when examining facing walls or the bottoms of blind holes and cavities.

Fig. 8

View through a

measuring fiberscope with

reticles for 20° and 40° field-

of-view lenses.

Courtesy of

Olympus Corporation

Forward-oblique heads bend the viewing direction at an angle to the borescope axis, permitting the inspection of corners

at the end of a bored hole. The retrospective viewing head bends the cone of view at a retrospective angle to the

borescope axis, providing a view of the area just passed by the advancing borescope. It is especially suited to inspecting

the inside neck of cylinders and bottles.

Environment. Flexible and rigid borescopes can be manufactured to withstand a variety of environments. Although

most scopes can operate at temperatures from -34 to 66 °C (-30 to 150 °F), especially designed scopes can be used at

temperatures to 1925 °C (3500 °F). Scopes can also be manufactured for use in liquid media.

Special scopes are required for use in pressures above ambient and in atmospheres exposed to radiation. Radiation can

cause the multicomponent lenses and image bundles to turn brown. When a scope is used in atmospheres exposed to

radiation, quartz fiberscopes are generally used. Scopes used in a gaseous environment should be made explosionproof to

minimize the potential of an accidental explosion.

Applications

Rigid and flexible borescopes are available in different designs suitable for a variety of applications. For example, when

inspecting straight process piping for leaks rigid borescopes with a 360° radial view are capable of examining inside

diameters of 3 to 600 mm (0.118 to 24 in.). Scopes are also used by building inspectors and contractors to see inside

walls, ducts, large tanks, or other dark areas.

The principal use of borescope is in equipment maintenance programs, in which borescopes can reduce or eliminate the

need for costly teardowns. Some types of equipment, such as turbines, have access ports that are specifically designed for

borescopes. Borescopes provide a means of checking in-service defects in a variety of equipment, such as turbines (Fig.

9), automotive components (Fig. 10), and process piping (Fig. 11).

Fig. 9

Turbine flaws seen through a flexible fiberscope. (a) Crack near a fuel burner nozzle. (b) Crack in an

outer combustion liner. (c) Combustion chamber and high pressure nozzle guide vanes. (d) Compressor

damage showing blade deformation. Courtesy of Olympus Corporation

Fig. 10 In-service defects as seen through a borescope designed for automot

ive servicing. (a) Carbon on

valves. (b) Broken transmission gear tooth. (c) Differential gear wear. Courtesy of Lenox Instrument Company

Fig. 11 Operator viewing a weld 21 m (70 ft) inside piping with a videoscope. Courtesy of Olympus Corporation

Borescopes are also extensively used in a variety of manufacturing industries to ensure the product quality of difficult-to-

reach components. Manufacturers of hydraulic cylinders, for example, use borescopes to examine the interiors of bores

for pitting, scoring, and tool marks. Aircraft and aerospace manufacturers also use borescopes to verify the proper

placement and fit of seals, bonds, gaskets, and subassemblies in difficult-to-reach regions.

Visual Inspection

Optical Sensors

Visible light, which can be detected by the human eye or with optical sensors, has some advantages over inspection

methods based on nuclear, microwave, or ultrasound radiation. For example, one of the advantages of visible light is the

capability of tightly focusing the probing beam on the inspected surface (Ref 1). High spatial resolution can result from

this sharp focusing, which is useful in gaging and profiling applications (Ref 1).

Some different types of image sensors used in visual inspection include:

• Vidicon or plumbicon television tubes

• Secondary electron-coupled (SEC) vidicons

• Image orthicons and image isocons