Inversion System

a part of a complex optical system that inverts the optical images of objects that were formed in an earlier part of the optical system. Inversion systems are used because in many cases it becomes necessary to obtain and view erect images of objects, whereas most objective lenses produce inverted images. Inversion systems are widely used in various kinds of optic tubes, such as binoculars and some types of microscopes, periscopes, and projectors.

Inversion systems can be of the prism type, lens type, or mirror type. In prism inversion systems, the most commonly used prism is a right-angle prism with mutually perpendicular edges, which is called a Porro prism. The light rays enter this prism perpendicular to one of the faces of the prism and undergo two total internal reflections from the other faces; the rays leave the prism in a direction opposite to the original direction of entry. The principle of total internal reflection is also used in some other components of inversion systems that are made of optical glass and have flat plane faces; such components are also called inversion prisms, although they are not always prisms in the strict geometric sense. Prism inversion systems make it possible to vary considerably the length of an optical system. Specifically, an inversion system that consists of two Porro prisms (Figure 1) significantly shortens the distance between the eyepiece and the objective lens in instruments intended for visual observations, such as binoculars. In this case, the image of the object is rotated, but its size is not changed.

Figure 1.

A typical lens inversion system (Figure 2) consists of three components: two compound lenses (2 and 3) and an additional plane-convex lens (1), called the field lens. This field lens, located near the focal plane of the objective lens which precedes the inversion system, forms an image of the entrance aperture of the objective midway between lenses 2 and 3. This makes it possible to minimize the transverse dimensions of the inversion system.

Figure 2.

By using a lens inversion system, we can change the size of the image finally obtained of the object; that is, we can change the overall optical magnification (or demagnification) of the system. A smooth adjustment of the distance between the components of an inversion system makes it possible to change the magnification; this is an important feature in some instruments. Moreover, lens inversion systems can be used to increase the total length of the optical system; this feature is a necessity in some instruments, for example, periscopes.

Inversion systems can also be made with fiber optical components provided these components are of a high enough quality to give the resolution required in the optical system.