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Laser focal point manipulation

  1. Oct 29, 2014 #1
    Is it possible with modern optic technologies to achieve the following effect:
    A relatively thing laser beam passes 30-50 cm and after that completely defocuses?
    It should defocus under very broad angle.
    What I exactly mean is creation of a 2D screenless images just in a thin air with help of a lasers.
    For example a few lasers could beam in viewer's direction from a narrow point. After their beams
    pass few tens of inches they completely defocus and create a picture. Is such a thing possible?
     
  2. jcsd
  3. Oct 30, 2014 #2

    Danger

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    I honestly don't know anything about it, but it seems sketchy. It might be theoretically possible using destructive wave interference or something similar. Achieving it in real life is probably a no-go. The closest thing that I've ever seen is a system that creates a virtually-invisible mist from water sprayers and projects an image onto it.
    What I see as the fatal stumbling block is that once the light leaves the laser and associated equipment, it's totally out of your sphere of influence. It would be kinda like letting a deaf dog off of its leash, with no way to call it back.
    Don't stop thinking about it, though. It's good exercise for your brain, and you might actually come up with something revolutionary.
     
  4. Oct 31, 2014 #3

    Drakkith

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    Do you know how images are created in a normal optical system? How much do you know about optics in general? Do you have any experience uses lenses or mirrors to create images?
     
  5. Oct 31, 2014 #4
    1. What do you mean as a "normal optical system"?
    2. I do not know a lot about an optical systems.
    3. Is there something wrong with my assumption?
     
  6. Oct 31, 2014 #5

    Andy Resnick

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    Yes, it violates conservation of momentum.
     
  7. Oct 31, 2014 #6

    Drakkith

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    Well, it helps to know the basics of optics before coming up with an optical system like you have. Typically you need to focus light onto a surface and the light then bounces off of that surface and enters the eye, allowing you to see an image. I don't know of any way to create an image from defocused laser beams in the way you've presented. (Though that doesn't mean it isn't possible. I'm definitely not an expert in optics.)
     
  8. Oct 31, 2014 #7

    davenn

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    hi Andy

    for my own learning
    could you please elaborate on that response ? :)

    Dave
     
  9. Oct 31, 2014 #8
    If you will look at this picture, you will see that laser light defocusing could happen after it exits a defocusing lens. You could imagine a viewer to the right of the picture.
    24857d1261844132-dx-green-laser-focusing-lens-green-focusing.jpg
    https://www.google.ca/search?hl=en&site=imghp&tbm=isch&source=hp&biw=1097&bih=504&q=focus point&oq=focus point&gs_l=img.1.0.0l10.1347.6211.0.7312.11.8.0.3.3.0.112.737.6j2.8.0....0...1ac.1.58.img..0.11.785.760NeqkOueE#hl=en&tbm=isch&q=focus laser&facrc=_&imgdii=_&imgrc=dtm0n4DY_zW8qM:;5zaCK4Rwe6qbxM;http://laserpointerforums.com/attachments/f44/24857d1261844122-dx-green-laser-focusing-lens-green-focusing.jpg;http://laserpointerforums.com/f44/dx-green-laser-focusing-lens-46599.html;700;449[/URL] [Broken]

    What I wish to know if how large could be a distance between a point where it exits the last lens and defocuses, and how large angle of defocusing could we achieve. And similar details.
     
    Last edited by a moderator: May 7, 2017
  10. Oct 31, 2014 #9

    Drakkith

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    I'm afraid I don't follow what you're trying to do. Expanding a single laser beam will not form a picture of anything. It will just show up as a single colored dot once it is brought back into focus by the eye.

    image2.gif

    Look at the picture above. Notice that light is emitted (or reflected) from the top and bottom of the arrow as a diverging cone (they only show part of the full cone here. In reality the cone would expand in every direction away from the point and the lens would capture a part of the cone equal to its full aperture, not half). The lens then takes this cone and causes it to converge to a focal point. Note that every single point on the arrow is actually emitting its own cone of light. They only show the very top and very bottom for clarity.

    Your laser beam is the equivalent of a single point emitter, and as such the image will be a single, focused spot of light. Nothing more.
     
  11. Oct 31, 2014 #10
    The diagram you provided is complicated and I do not understand what is the "object" and what is it needed for.
    Obviously, a lasers suppose to move vertically and horizontally like in a laser TV in order for a "single focused spots of light" as you said will create a full picture.
     
  12. Oct 31, 2014 #11

    Drakkith

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    The object is simply an arrow. The light comes from the arrow, goes through the lens, and is focused at the focal plane where it forms an image of an arrow. The object could be a tree, a person, or whatever else you want to create an image of.

    A TV works that way because the light from each pixel emanates from different positions in your field of view and each pixel is typically a different color/shade.

    Honestly if you don't understand the very simple diagram I linked above, then you seriously need to study basic optics before trying to design some sort of optical laser system to project images.
     
  13. Nov 1, 2014 #12
    So what prevents lasers to have the same property? If laser beam defocuses under an angle which is broad enough, it should be seen from a different angles from side of a viewer.
     
  14. Nov 1, 2014 #13

    Danger

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    I understand what you are trying to figure out, but your "jumping off point' seems to be midway between 2 different technologies that aren't compatible at present.
    I suspect that you don't actually understand how a CRT television works. You are not seeing the scanning electron beams when you watch a show. Those beams hit thousands of individual phosphor dots on the interior of the screen, in a 30Hz scanning raster. In a black and white set, there is one beam. In a colour set there are 3. Each is electrostatically steered by a sawtooth generator and a flyback generator that move it around. It turns on and off according to the signal input. If it is on when aimed at a particular pixel (phosphor), that pixel lights up and you see it. Even if that beam managed to escape through the screen, you wouldn't be able to see it; only the translation of the electrons to photons by the phosphors allows for that.
    As for laser "light shows" such as at rock concerts, that is simply a single beam that you see reflected from the viewing surface as it scans around in a predetermined pattern that traces out an image. (ZZ Top's Eliminator Tour, by the bye, demonstrated the most egregious waste of a laser that I've ever seen.)
     
  15. Nov 1, 2014 #14

    mfb

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    Well, many dots form an image, if you can make them fast enough.
    This would require the laser beam to move extremely fast, and the viewing angle would be quite small.
     
  16. Nov 1, 2014 #15

    Danger

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    I think that the point that Drakkith is trying to make is the same one that I am: if you simply fire out a laser with no intervening physical medium, the only way that it can be perceived at all is for it to impinge upon one's retina. While that might not necessarily cause blindness, it most definitely will not cause the impression of a picture. It'll just be a large red (or whatever colour) blotch in your vision.
     
  17. Nov 1, 2014 #16

    mfb

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    Right, with exceptions - there are lasers so intense they can transform air to plasma, and that is visible. Let's ignore that exception, it is not practical for an image.
    Making a laser weaker (if necessary!) is always easy.
    Right, it would be a single pixel. Add more pixels and you can get an image.
     
  18. Nov 1, 2014 #17
    I think, you've completely missed the idea of laser defucusing. What is a difference between beam which heats a phosphorus covered shining dot and and a laser which is programmed by a lens to defocus at some point of the space? If it defocuses, then I suspect, it means it will shine light radiation in all sides under very broad angle. I'm not certain but quite possible someday a technology will allow it to defocus under 180 degree angle and to a viewer it should look similar to a phosphorous dot or a mini LED lamp in OLED displays which radiates light under 180 degree angle.
     
    Last edited: Nov 1, 2014
  19. Nov 1, 2014 #18

    Danger

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    Okay... one at a time now. (And that's exactly the point, even though I meant that as a joke until I actually saw it on the screen.) Adding more pixels would just increase your eye damage, not create an image. What you don't seem to be understanding is that they will all hit the same part of your retina, just sequentially rather than all at once as in the case of non-coherent photons from normal vision. (If someone repeatedly stabs you in the arm, in the same hole, in the dark, you will not be able to say what the knife was trying to picture. If someone gently traces out a shape with a Q-Tip in the same vicinity, you will be able to identify the image.)
    Also, I sure wouldn't want anything powerful enough to plasmacize air anywhere in my house, let alone aimed at my face.

    edit: Stanley, your post just appeared when I submitted this one. The point that you seem to be overlooking is that a lens can't "program" a photonic stream to do anything. Its action is immediate and rigidly defined by its geometry. I made the mistake once of buying one of the most remedial optics textbooks available for reference in designing a laser weapon. The whole damned book seemed to be one honkin' horrendous equation from beginning to end. I set it aside and made a gun instead.
     
    Last edited: Nov 1, 2014
  20. Nov 1, 2014 #19

    Drakkith

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    I think everyone's missing the key point to this discussion. It isn't about the power of the laser or anything like that, it is about how light forms an image.

    Sure. A laser is just a focused beam of light. Defocusing it to a very large degree is almost equivalent to having a normal light bulb/LED/pixel. But by defocusing the laser beam, you have simply changed the "screen" from the laser itself to the lens. Any image you project will appear to come from the lens, not from midair, so you won't have a "screenless" image.
     
  21. Nov 1, 2014 #20
    There is such thing as a focal length. This is the distance between a lens and a focus.
    http://www.eurolaser.com/uploads/pics/eurolaser_Linsen_1_GBR_01.jpg [Broken]
    http://www.eurolaser.com/uploads/pics/eurolaser_Linsen_4_GBR_01.jpg [Broken]
    I think if they will achieve good enough manipulation with focal lenght and make it long enough the images may look like they float in the air.

    Currently they achieved very high level of manipulation with light. For example they could create bended light rays and even make a light circles.
    http://en.wikipedia.org/wiki/Airy_beam

    For practical screenless images creation they would have to manipulate at least with a focus lens dynamic change. I think it could be done with help of some optoelectronic materials.
     
    Last edited by a moderator: May 7, 2017
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