ABSTRACT
Relays are used to relay an image from one place to another. They are used in rifle sights, periscope systems, microscopes, and military infrared imaging systems. (This type of lens is also used for photocopy work.) Generally, a field lens is required at an image plane to image the exit pupil of the previous group into the entrance pupil of the next group. The designer should keep in mind that a computer program, if allowed to vary the refractive index of this field lens, will select a glass of high index and low dispersion. Very little is gained by using glasses of refractive index greater than that of N-SK5. Also a material must be selected with a low bubble code. Because this field lens is at an intermediate image, dirt, scratches, and other imperfections are imaged onto the final image. It is thus advisable to slightly move the field lens away from the intermediate image.
