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http://www.youtube.com/watch?feature=player_detailpage&v=jfySIeS1XZE#t=118s

If so, how are they created (roughly speaking). Earlier in the video he says that a laser scanned the surface of the object, which suggests that no physical object needs to be present at the time of viewing the hologram, like what is required for a Mirage toy (seen here: http://www.youtube.com/watch?v=lFqlQiTTHRs). Is this right?

I am aware that this is a fictional show, so what they are saying should not be taken too seriously and the hologram effect could be added by CG effects, but I am interested in the actual workings of this type of hologram (as opposed to a "flat" hologram).

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Sorry, you're confused. As a particular technical realization, a hologram is always flat - including the normal ones as well as this one - but the object seen in the hologram is always 3-dimensional - including well-known holograms as well as this one. More typically, we deal with 3D "virtual images" (behind the plane of the hologram where the interference pattern is located, possibly overlapping with a the bulk of a wall) while Leonard showed a "real image" - that is apparently located in the air. But this is just a minor difference. Normal holograms induce both virtual and real images. –  Luboš Motl Oct 31 '12 at 7:56
    
I guess I should have said more precisely that I mean holograms not attached to a physical medium like a holographic card or surface. I appears that in this particular instance the hologram is projected rather than encoded in a surface. –  Ron Volt Oct 31 '12 at 9:21
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I think you'd describe the display in that clip as a volumetric display rather than a hologram. As the Wikipedia will tell you, there have been basic examples of such displays created but at the moment nothing like the display in the clip exists.

The problem is how to get some medium in your display to emit light. The example in the Wikipedia article uses a laser to heat air until it forms a plasma. However this is only monochrome and of limited resolution. Try a Google image search for "volumetric display" for a selection of current attempts, but be prepared for disappointment!

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3d-display-info.com/files/3d-display/images/… ← How can one be possibly disappointed by this? –  Claudius Oct 31 '12 at 10:39
    
:( It seems you are right. Everything that looks decent relies on a solid of some sort (like a rotating mirror). I hope true volumetric displays are achieved in my lifetime. –  Ron Volt Nov 1 '12 at 1:29
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I believe an image similar to the one shown in the video can be created by focusing a laser beam in vapour. One can use three colours to achieve full colour images. I would say, it is rather a technological than fundamental problem at the moment.

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