A three-dimensional image of an object made by holography.
A Closer Look To produce a simple hologram, a beam of coherent, monochromatic light, such as that produced by a laser, is split into two beams. One part, the object or illumination beam, is directed onto the object and reflected onto a high-resolution photographic plate. The other part, the reference beam, is beamed directly onto the photographic plate. The interference pattern of the two light beams is recorded on the plate. When the developed hologram is illuminated from behind (in the same direction as the original reference beam) by a beam of coherent light, it projects a three-dimensional image of the original object in space, shifting in perspective when viewed from different angles. Appropriately enough, the word hologram comes from the Greek words holos, “whole,” and gramma, “message.” If a hologram is cut into pieces, each piece projects the entire image, but as if viewed from a smaller subset of angles. The large amount of information contained in holograms makes them harder to forge than two-dimensional images. Many credit cards, CDs, sports memorabilia, and other items include holographic stickers as indicators of authenticity. Holography is used in many fields, including medicine, data storage, architecture, engineering, and the arts.