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If a 2D picture is worth a thousand words, then a 3D image is worth a million. With holography, it is possible to reconstruct 3D images using holograms, and the process is unlike anything found in traditional display technology. Even though it was invented over 70 years ago, holography remains the best candidate for achieving true 3D displays. Here we present six things you may not know about the strange and wonderful world of holography.
1. Tupac is not a hologram
When you see Tupac, Michael Jackson, or anyone for that matter projected in the way you see at concerts or similar, they are not holograms. It’s a trick, and the fundamental hologram has no relation to this whatsoever. The trick that is being used was invented in the 1800s by John Pepper to wow unsuspecting audience members that a ghost had appeared next to the actors on stage (pictured above). In reality, a clever illusion was employed, whereby a piece of glass at an angle placed between the audience. The stage was used to reflect the light from an actor below the stage toward the audience, but still allowing them to see through to the stage ahead. Because the glass is effectively transparent, we think there is a ghost hovering on stage. As such, most “holograms” seen on TV are some variant of this Pepper’s ghost trick.
2. Only a hologram is hologram: it is remarkably different to anything else
Consider you’ve just taken a photo of a scene. You’ve taken your camera, pointed, clicked, and captured some information. From an optics point of view, you’ve stored some time-averaged amplitude of the light-field emanating from that scene using some form of sensor (in separate RGB channels). As a result, a vast amount of information within that light field has just been thrown away. Collecting just this information is effectively capturing a tiny percentage of what’s there. A hologram (invented in 1947 by Dennis Gabor, holography (from the Greek meaning “whole-drawing”) in its most basic sense, is the recording then reconstruction of all the light-field information such that when viewed, the observer is unable to tell the difference from the original scene because the hologram is ‘giving’ the observer all of the original information.
For a really amazing trip into this wonderful world, we recommend the following video:
Now you would naturally ask: how can we do this? Well, if you take the object you want to display, illuminate it with a laser, and interfere this scattered light with another laser (see Figure), a recording of this pattern created is the hologram . It is capturing the amplitude, phase, and wavelength information of the object. Now if we looked at this pattern under the microscope, we would just see these interference fringes, which is uninteresting. However, if we illuminate with the same source, the light is scattered from all fringes simultaneously and interferes with itself to reconstruct the original object’s light-field.
The beauty of this technique is that it is still the only way to truly reconstruct 3D information and achieve real 3D displays. Yet, this technique was originally done nearly 70 years ago to form static holograms. But why can’t we just dynamically change the holograms and effectively create a holographic display? This is discussed in the next section.