PURPOSE: To show what a field lens does.
DESCRIPTION: The object consists of an illuminated transparency containing a field of o's and x's. Light from the object is focused symmetrically by a 10 cm converging lens to an intermediate focus, which in turn is focused onto a distant screen by a 20 cm convex lens. A second 10 cm convex lens, the field lens, can be rotated into the beam at the position of the intermediate image. With the field lens out, the image is clear and bright near the center but very dim near the periphery of the image, a phenomenon known as vignetting. When the field lens is inserted, light illuminating the entire image is kept within a small enough angle that it passes through the lens system. Vignetting is virtually eliminated, and the image is uniformly well lighted.
Many types of projectors use field lenses to obtain the greatest illumination for the projection bulb used.
REFERENCES: (PIRA unknown.) See Demonstration L7-32: OVERHEAD PROJECTOR - DISSECTED, for an example of a projector using a field lens.
EQUIPMENT: Bright point source with condenser lens, object transparency, 10cm focusing lens, 10cm field lens, and 20cm imaging lens.
SETUP TIME: 5 min.
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