In movies, holograms can appear to move and recreate entire animated scenes in midair, but today's holograms can only mimic movement. You can get the illusion of movement by exposing one holographic emulsion multiple times at different angles using objects in different positions. The hologram only creates each image when light strikes it from the right angle. When you view this hologram from different angles, your brain interprets the differences in the images as movement. It's like you're viewing a holographic flip book. You can also use a pulsed laser that fires for a minute fraction of a second to make still holograms of objects in motion.
Multiple exposures of the same plate can lead to other effects as well. You can expose the plate from two angles using two completely different images, creating one hologram that displays different images depending on viewing angle. Exposing the same plate using the exact same scene and red, green and blue lasers can create a full-color hologram. This process is tricky, though, and it's not usually used for mass-produced holograms. You can also expose the same scene before and after the subject has experienced some kind of stimulus, like a gust of wind or a vibration. This lets researchers see exactly how the stimulus changed the object.
Using lasers to make three-dimensional images of objects may sound like a novelty or a form of art. But holograms have an increasing number of practical uses. Scientists can use holograms to study objects in three dimensions, and they can use acoustical holography to create three-dimensional reconstructions of sound waves. Holographic memory has also become an increasingly common method of storing large amounts of data in a very small space. Some researchers even believe that the human brain stores information in a manner that is much like a hologram. Although holograms don't currently move like they do in the movies, researchers are studying ways to project fully 3-D holograms into visible air. In the future, you may be able to use holograms to do everything from watching TV to deciding which hair style will look best on you.
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