An aspheric surface is considered a special optical surface. It is special because it can do more than a spherical surface with regard to aberration correction. But it is also special because it is much more expensive to manufacture, especially if the surface is produced by means other than diamond turning. A diffractive surface with its grating profile engraved into it to act as a thin lens belongs in this category of special surfaces as well. Both will be discussed in detail.
Because windows, filter substrates, and other plane-parallel elements such as beam splitters and Dewar windows are often ignored by the user and not considered to be optical elements, we treat them here as elements with special surfaces. These elements in converging or diverging light contribute to aberrations just as a lens or a curved mirror does. In addition, they shift and displace image locations. Knowing the behaviors of plane-parallel plates allows us to correct for their aberration contributions or take advantage of that knowledge in balancing the system's aberrations.
Another element surprisingly often ignored in the layout stage is the concentric dome. It contributes to the aberrations and has focusing power. The details of these characteristics will be addressed.
Ball lenses and gradient index lenses are frequently used as coupling elements with optical fibers. Their features and performances are included as well, even though their applications are limited to the visible (VIS) and near-infrared (NIR) spectra.
5.2 The Plane-Parallel Plate
By definition, a plane-parallel plate is flat and parallel. If this is not the case, it is a lens with convex or concave spherical or cylindrical surfaces. The surfaces can also be toroidal or general aspheres on either or both sides. The plate can also be flat on both sides, with the surfaces not parallel to each other. In that case the element is a prism.
If a plane-parallel plate is inserted into an image-forming optical system, with its surfaces perpendicular to the optical axis, the image plane is shifted toward the right, assuming the light is coming from the left (see Fig. 5.1).
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