Endoscopy

Mascha Krapp, winter semester 2010/11

Artikel auf Deutsch

Endoscopy is a special method of visual inspection which is the oldest and also the most successful method of non-destructive construction testing. Endoscopy is part of every examination of the construction and is easy to carry out yet requires a lot of experience for correct use. Nature has equipped human beings with several powerful “testing devices”. Sight is the most powerful of the five senses. Visual inspection can be done with the naked eye, i.e. to take a closer look at a component or construction and detect and assess quality characteristics. As a rule, visual inspection is the first step in non-destructive testing. It is a superficial or near-surface test, the interpretation of which requires experience and detailed knowledge. This can however, result in inaccuracies and strong fluctuations in quality.

Fundamentals of visual inspection

The following points should be considered with visual inspection:

• Firstly, a good overview should be gained. The component or construction to be examined should be identified and marked, checked for completeness and disposition as well as type of construction.
• Then, particular characteristics should be specifically tested. These can be:
  • Shape deviances, for example, deviations of form in terms of dimensions, geometry or contours of the components which can be seen in deviations of planeness, straightness or parallelism. Shape deviations can also be seen on the surface in the form of ripples, grooves, scratches, flakes or similar.
  • Deflections, for example deviations in parallelism, perpendicularity, positioning, symmetry or similar characteristics.
  • Defects or errors in the form of flaws, inhomogeneities, cracks, pores or similar due to specific features of the production process.
  • Operational damage of the component, for example corrosion, signs of wear and tear and tarnishing

To view defects, angular corners, cracks or cavities more closely, endoscopes can be used for extended visual inspection.

The basic components of each endoscope are a light source, light guide and the endoscope itself, which transmits the image. There are four types of endoscopes:

• rigid endoscope
• articulated arm endoscope
• flexible endoscope
• video endoscope

Rigid endoscopes

Rigid endoscopes are most commonly used in civil engineering to detect defects and have a closer look at them. This sort of endoscope has the best type of image transmission as the image is not stripped down into individual pixels but transmitted via a special compound lens and if necessary magnified. The generated intermediate image can be viewed through the eyepiece. The object under examination can be illuminated with a subordinated optical fibre integrated within the endoscope and connected to a light source. This type of illumination provides a very strong illuminance in contrast to an incandescent lighting that can likewise be integrated into the endoscope. On the other hand, the latter has a very good illumination in the near zone as a very consistent light distribution is generated even though it provides a shorter illumination range.

The diameter of the endoscope can, depending on the application, be 1.5 to 15 mm. Depending on the application, the largest possible diameter should be chosen in order to achieve the best image quality and brightness. Additionally, different perspectives can be chosen: straight, sideways, diagonally straight or backwards. The viewing angle can also be set. The direction of sight can be varied by prism attachments or mirrors. Thus, for example, a cavity can be completely detected and analysed. In most applications, a focusable wide-angle image is preferred. The distance to the object can be in the scale of mm and be scaled up to dm.

Articulated arm endoscope

Articulated arm endoscopes have the same basic structure as rigid endoscopes and are rarely used in civil engineering. With insertion of joints, the endoscope is separated into movable sections. Within the joints are deflection prisms that forward the image to the eyepiece and thus provide a good image quality.

Flexible endoscopes

This type of endoscope offers a larger freedom of movement than for example, do rigid endoscopes. With the exception of the lens and eyepiece, this endoscope is completely flexible and these need a different type of image transmission. Here the intermediate image generated by the lens is transferred via ordered optical fibres to the eyepiece. Every individual optical fibre transmits a pixel, thus the image quality depends on the number of fibres. Thus, it follows that the resolution and the image definition depends on the number of fibres. The diameter can be from 0.64mm to 20 mm. With the handling of this endoscope, special care must be taken that despite the high flexibility, it is not allowed to be folded, otherwise the optical fibres could break and each break of a fibre means the loss of a pixel. To take advantage of the full flexibility, the lens head can be controlled from outside by a manipulator. This is how the view direction or view angle can be set individually.

Video endoscopes

Video endoscopes are miniaturised video cameras in which a CCD camera chip is built into the endoscope’s head. Thus, the image transmission happens electronically. Its construction is very similar to that of the flexible endoscope. Another advantage of the video endoscope is the low-loss image transmission up to 20 meters. Illumination is also provided here via subordinated optical fibres. The digital image generated by the camera chip is typically processed by a processor and forwarded to an output monitor. The current state of the technology allows for the production of very small camera chips, so that video endoscopes with a diameter from 6 to 20 mm are available. In addition, documentation of the test is very easy with this type of endoscopy. Visualised images or video sequences can be recorded and saved on integrated storage media (SD-card) and at the same time, the date, position, tester and further important data can be added.

For almost all endoscopes various lenses and attachments are available that can be interchanged and attached. An attachment with a scaling similar to a crack magnifier can for example serve for the immediate measurement of characteristics.

Literature

• Merkblatt über die Sichtprüfung und Endoskopie als optische Verfahren zur Zerstörungsfreien Prüfung im Bauwesen. Merkblatt B6 der Deutschen Gesellschaft für Zerstörungsfreie Prüfung e.V. Ausgabe 1996.
• Sichtprüfung und Endoskopie. Skript der Deutschen Gesellschaft für Zerstörungsfreie Prüfung e.V. zur gleichnamigen Vorlesung.
• Endoskop. German Wikipedia.