NDE_VisualInspectionAndHolography

Introduction

Visual inspection is probably the most widely used among all non-destructive testing methods. It is simple, easy to apply, quickly carried out and low in cost. A good visual inspection is necessary even though the component will be inspected using other NDT methods. A simple visual test can reveal gross surface defects, thus leading to an immediate rejection of components saving lot of time and money, which would otherwise be spent on more complicated methods of testing. With the advent of microprocessors and computers, visual examination can be carried out reliably and with minimum cost.

Principle
The basic procedure used in visual inspection involves illumination of the test specimen with light, usually in the visible region. The specimen is then examined with eye or by light sensitive devices such as photocells. Adequate illumination is essential for visual inspection. The surface of the specimen should be thoroughly cleaned before being inspected. When direct visual inspections cannot be made, sophisticated optical instruments can be used to provide a remote viewing of the critical areas.

Optical Aids for Visual Inspection

1. Borescope: Borescopes are optical instruments designed for remote viewing of objects. In certain cases borescopes are essential because it is impossible to get close to objects that are to be inspected, such as internal parts of jet engines. In other cases it is dangerous to get too close to the objects that need inspection because of heat or radiation. The diameter of the borescope determines the size of the minimum opening into which it can be inserted.

Borescope magnification is determined by its three main optical components: the objective lens system, the relay lens system and the eyepiece. The objective lens is located at the end of the borescope and it acts similar to a camera lens. It forms the primary image of an object on the back of the lens. Relay lenses reform or relay the primary image every few inches along the length of the borescope. Generally several relay lenses are used in a long borescope. The last set of relay lenses produce the final image at the eyepiece of the borescope. The eyepiece lens enables the human eye to see the final image formed. Each of the three components produces a magnification (which can be < 1) and the total magnification is the product of each magnification in accordance with the equation below:

$$ Mb = (Mo) (Mr) (Me) $$

Where,

\(Mb= \) total borescope magnification
\(Mo = \) magnification of the objective lens
\(Mr = \) magnification of the relay lens (usually 1)
\(Me = \) magnification of the eyepiece

THe manufaturer determines the relay lens and eyepiece magnification are determined by the manufacturer. The magnification of the objective lens is complicated by the fact that it varies with distance from the object to the objective lens. Therefore, in specifying the magnification of the borescope it is essential to specify the magnification at a specific distance. The accuracy of defect image measurement depends on the probe to object distance and the operator’s ability to focus the instrument. Borescopes find application in jet engine inspection and nuclear inspection systems.

2. Projection microscope: The development of projection microscopes led to the incorporation of microscopes into the inspection lines of many industries. The optical projector reduces eye strain and enables the operator to visually inspect large numbers of small parts at reasonable production rates. Cameras can be attached to virtually any projection microscope and many of them can project their images on the walls of darkened rooms. Projection microscopes are applied in the inspection of precision mechanics and examination of electronics and cables. A typical projection microscope is shown below:

Figure 4: Projection Microscope

3. Long Distance Microscope (LDM): This equipment enables the user to obtain microscopic inspection of objects at relatively long distances. The LDM can provide sharp images of objects at 31.5 inches and beyond, and also large microscopic magnifications. LDM provides a 42-mm unobstructed light path and is able to tune away spherical aberration and astigmatism at any distance. A variable aperture control at the rear of the instrument allows the user to precisely adjust the depth of field and image contrast. LDM has excellent resolution compared to a conventional microscope. LDMs can also be used with photographic and video cameras. Applications of LDM are for long distance studies of stresses in hot plastics, glass and ceramics. LDM is extremely useful for viewing potentially harmful or dangerous subjects. A long distance microscope is shown in Figure 5.

Figure 5: Long Distance Microscopes

4. Holographic Interferometry: Holographic interferometry became possible after the invention of the laser, a monochromatic light source in 1960. Holography is the name given to the method of obtaining an accurate three-dimensional image of a given object. First, a two dimensional interference pattern is obtained on a photographic plate by means of a laser beam. The three dimensional image is then obtained from the two dimensional record by again using a laser beam. A HeNe (helium neon) laser is generally used. A variable attenuator beam splitter is installed in the laser beam that divides the beam into an object beam and reference beam. The object beam illuminates the object while the reference beam is projected directly on the film. After exposure, the film is processed using standard photographic techniques, in about 20 seconds. The processed film is re-illuminated by the reference beam and the light is diffracted into exact waveforms that had originally been reflected by the object. The holographic image of the projected object contains all visual information about the original object. When this image is superimposed on the object, real time holography is produced. Any differences between the image of the original and the present object can immediately be seen. Holography is used for NDT of highly complicated and precision components.