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In previous chapters, we have shown how to determine the position and size of the Gaussian image of an object. However, we did not consider the sizes of the imaging elements or the apertures in the imaging system. Accordingly, no effort was made there to determine the cone of object rays that enters or exits from the imaging system. Such calculations are essential for the determination of the image intensity in terms of the object intensity, or the image irradiance in terms of the object radiance.
We begin this chapter by introducing the concept of an aperture stop and its images, called the entrance and exit pupils in the object and image spaces of an imaging system, respectively. The light cone from a point object that enters the system is limited by the entrance pupil. Similarly, the light cone that exits from the system and converges to the image point is limited by the exit pupil. Certain special rays, such as the chief and marginal rays, are defined. The chief ray from the edge of an object determines the location of the exit pupil and the height of the image. Similarly, the marginal ray from the axial point of the object determines the size of the exit pupil and the location of the axial image point. Vignetting or blocking of the rays from an off-axis point object by the aperture stop and/or other elements of the system, thus changing the effective shape of the stop and pupils, is explained. A telecentric stop is defined, and its advantages are briefly discussed. The field stop and its images, the entrance and exit windows, and the angular field of view of a system are also described.
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