The eyepiece is quite similar in function to the magnifier discussed in the preceding chapter. The major distinction is that the eyepiece is used to view a real image that has been formed by optical components located between the eyepiece and the object being viewed (foreoptics). As a result, those foreoptics will define the size, shape, and paths of the ray bundles that will pass through the eyepiece.
While the final design of an eyepiece may be fine-tuned to function with a specific set of foreoptics, the eyepiece will generally be designed as an independent optical device, based on the specifications of the final system. For the examples to be presented here, it will be assumed that all eyepieces have a focal length of 28 mm (approximately 9Ã magnification). The exit pupil diameter will be set to 4 mm (f/7.0), and the field of view will be set to an appropriate value, consistent with the eyepiece type. As with most other visual designs presented in this book, equally-weighted spectral wavelengths of 0.51, 0.56, and 0.61 microns will be used.
Each eyepiece has a unique set of characteristics that will determine its applicability to a particular task. We will cover nine eyepiece types in this chapter and present design and performance data for each. Figure 5.1 contains tabulated design data on these nine eyepiece types, making meaningful side-by-side comparisons possible. The focal length, entrance pupil size, and exit pupil location (chief ray angle) have been held constant for all designs, while the field of view, eye relief, and image distance will vary depending on eyepiece type. In general terms, the lens or lens group that is located close to the eye is referred to as the eye lens, while the lens or lens group that is located close to the image is referred to as the field lens. Note that for all eyepiece design work presented here the optical layout is such that the object is assumed to be at infinity and light rays are traced through the exit pupil of the eyepiece, through the lens, and then on to the eyepiece image surface.
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