Faculty Publications


Projecting virtual hologram images onto a screen

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Proceedings of SPIE - The International Society for Optical Engineering



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Many elementary discussions of real and virtual images assert that virtual images cannot be projected directly onto a screen; using a special classroom transmission hologram, we demonstrate the contrary. Virtual and real images can be defined as images located respectively on the opposite side and the same side of the viewing system from the observer. These definitions appear to preclude the possibility of any virtual image appearing on a screen, assuming that the screen must be at the location of the image. This dilemma is resolved, however, by recognizing the similarity between projected virtual hologram images and pinhole images. Pinhole images are formed by a diverging ray bundle from an object point spreading by an amount controlled by the pinhole diameter. In the case of projected virtual hologram, a diverging ray bundle spreads from a virtual image point; the spread in the ray bundle is limited by the hologram aperture stop diameter. Thus projected virtual images model pinhole images in three ways: First, in both cases diverging ray bundles form the image; for the pinhole the diverging ray bundle originates from a real object point whereas for the hologram the diverging ray bundle appears to originate from the location of a virtual image point. Second, the virtual hologram image acts in a manner directly analogous to the real pinhole object. The third similarity is that, in both cases, successful projection of the image is possible only if the the ray bundle angular size is sufficiently limited. The ray bundle angular size is controlled by an aperture stop; for the pinhole image the pinhole edge is the aperture stop while for the projected virtual images the boundary of the illuminated portion of the hologram is the aperture stop. Reducing the size of the illuminated portion of the hologram is analogous to reducing the diameter of the pinhole. We show photos of projected virtual images for several aperture stop diameters; as the hologram aperture is reduced in successive steps the images becomes increasingly well-resolved. This process leads to the well known example of a hologram illuminated by an undiverged laser beam; a screen placed nearly anywhere beyond the hologram will display all hologram images. Although this procedure is familiar to many persons the associated images appear not to have been carefully analyzed. Because of the widespread assertion that virtual images cannot appear on a screen, it is likely that many persons viewing these projected images incorrectly interpret them as exclusively "real." Recognizing that they are projected virtual images permits a useful analysis to be developed.

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