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Chapter 6:
Measurement of Imperfections by Obscuration
Since an imperfection can occur anywhere over the surface of a component of any size or even within an assembled system, measurement cannot take place until it has been found and located in the measurement area. Inspectors often prefer to retain inspection methods for locating imperfections that they themselves have developed. This is inevitable due to the subjective nature of visual inspection. This chapter describes the currently favored methods for the objective measurement of imperfections. Many different proposals have been made for the rapid inspection of optical elements for imperfections. A prime consideration is comfort for the observer, but with sufficiently high levels of illumination to permit detection of the smallest imperfections of interest. Clean laminar airflow over the specimen will minimize dust deposits. Dark-field viewing, with the aid of a magnifier if necessary, with the eye positioned to collect radiation scattered over low angles, is generally regarded as the preferred arrangement. A typical inspection station is shown in Fig. 6.1. The component under inspection, shown here, is being viewed in transmitted light by magnifier. A component to be inspected in reflected light is placed in the bottom corner of the box and tilted so that the specular light just misses the eye. Due to the directional effect of light scattered by a scratch, the component may need to be rotated in its plane so that the eye can receive as much scattered light as possible.
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