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Are there two different lasers that can be invisible when not overlapping, but visible when overlapping? I desire to build a hologram-ish thing, but with lasers. The system would use many lasers that intersect each other at points to build an image in midair. Is this possible?

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  • $\begingroup$ Yes, this is possible, just not practical (at least not with air, as far as I know). See "3d volumetric laser displays" like "FELIX: A Static Volume 3D-Laser Display" Knut Langhans*, Christian Guill, Elisabeth Rieper, Klaas Oltmann, Detlef Bahr. The physics of these ideas is kind of solid... the engineering challenges are enormous, though, and the resulting effect maybe not as impressive as one would think, since the method starts with a wrong idea of what a 3d image actually is. $\endgroup$ – CuriousOne Jan 22 '16 at 6:03
  • $\begingroup$ Can you elaborate on what you mean when you say wrong idea of what a 3d image is? Is this the team's fault, a consequence of the mathematical complexity? Please elaborate. $\endgroup$ – djax1234 Jan 22 '16 at 6:08
  • $\begingroup$ A true 3d hologram is not made from voxels but from a light field that behaves like a 2d picture which emits different amounts of light in different directions from each pixel. The practical implementation of such a display would require a sub-wavelength (around 100nm) resolution phase modulated 2d display and a digital hologram signal processing engine. In comparison to a real 3d display voxel displays are extremely crude. Having said that, the lacking progress of these gas based displays shows that they are not that simple to implement as the basic physical idea suggests. $\endgroup$ – CuriousOne Jan 22 '16 at 6:57
  • $\begingroup$ I see, I will have to do more reading on what makes them so hard to implement. $\endgroup$ – djax1234 Jan 22 '16 at 18:59
  • $\begingroup$ For one thing, look at the absorption spectra of nitrogen and oxygen molecules. You need excitation lasers the are in the UV and the infrared to avoid interfering with your optical light output. $\endgroup$ – CuriousOne Jan 22 '16 at 19:31
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It's depend what you call "visible" and "invisible" and "air". :-) In air with a little bit of mist, one laser could just be dim, while the crossing superimposition of several could make the added value more visible in the intersecting region: this is a question of contrast rather than on-off.

Also, there exist a technology where this principle is used to "crack" (ionize) air at the aimed position. It works (3D points in air), but what you see is not the laser per se.

PS: I would avoid calling "hologram" what you really mean to be 3D: real hologram produce a 3D impression from a non-3D plate, since the image change with the view angle (and this is often done with lasers, which thus adds to the confusion).

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