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The term borescope is frequently applied to a family of optical instruments, designed for a variety of applications. The term no doubt originates from those instruments that were intended for the inspection of the bore of a firearm. Today that family of instruments is used for inspection of many otherwise inaccessible cavities within a variety of mechanical assemblies. A separate branch of the borescope industry has evolved relating to various fields of medicine. In these cases, the medical instrument is used to visually observe and diagnose certain conditions, permitting observation inside the human body by the medical team so that these conditions can be repaired.
In a most general sense, the borescope is a low-magnification optical instrument, typically quite small in diameter (<20 mm), and relatively large in terms of its overall length, often 50 to 100 times its diameter. The optical design of a borescope will be driven by these dimensional limitations, by the precision requirements of the optical components, and by the high level of image quality that is usually required.
The borescope will traditionally consist of three major optical components. These are: the objective lens, the relay optics, and the eyepiece. The objective lens configuration will be dictated by final instrument specifications such as: field of view, lens speed (f/number), distortion, and image quality. The relay optics configuration will be driven by allowable lens diameter, required instrument length, allowable vignetting, and image quality. The eyepiece selection will usually be based on required overall magnification, apparent field of view, and eye relief. Figure 8.1 shows a thin lens layout of a typical borescope optical system. The remaining sections of this chapter will deal with details of the design procedures involved in generating a final optical system that is consistent with the parameters shown in this thin lens layout.
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