Laser focal point manipulation

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Discussion Overview

The discussion revolves around the feasibility of using modern optical technologies to create 2D images in mid-air through the manipulation of laser beams. Participants explore concepts related to optics, image formation, and the potential limitations of such a system.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant questions whether a laser beam can defocus over a distance to create images in mid-air, suggesting it might be theoretically possible using destructive wave interference.
  • Another participant expresses skepticism about the practicality of the idea, citing that once light leaves the laser, it cannot be controlled, comparing it to a dog off its leash.
  • Some participants emphasize the importance of understanding basic optics, noting that typically light needs to be focused onto a surface to create an image.
  • Concerns are raised about the violation of conservation of momentum in the proposed system.
  • Discussion includes the mechanics of how images are formed in traditional optical systems, highlighting the need for multiple points of light to create a full picture.
  • One participant argues that expanding a single laser beam will not create an image, as it would only appear as a dot when focused by the eye.
  • Another participant suggests that lasers could potentially have properties similar to pixels in a TV, questioning what prevents this from happening.
  • Clarifications are sought regarding the operation of CRT televisions and how they differ from the proposed laser system.

Areas of Agreement / Disagreement

Participants generally do not reach a consensus on the feasibility of creating images with defocused laser beams. Multiple competing views and uncertainties remain regarding the underlying principles of optics and the practicality of the proposed concept.

Contextual Notes

Limitations include a lack of consensus on the fundamental principles of optics relevant to the discussion, as well as varying levels of understanding among participants regarding optical systems and image formation.

  • #31
Drakkith said:
The angle isn't important here.
Well, you said "requires that you view it from a specific point."
This is similar to a telescope eyepiece. You can see a small part of the image from a good distance away, but to see the full image you need to stick your eye very close to the hole in the mirror.
Right.
That setup isn't anything like what the OP presented.
It fits to the images shown in post #8.

@Stanley514: It will still be semi-transparent, but you can control the light in your optics to make the background reasonably dark.
 
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  • #32
mfb said:
It fits to the images shown in post #8.

I really don't see how.
 
  • #33
Stanley514 said:
This dispute went ridiculous. I was thinking about future yet unknown technology and only attempted to give some clue how approximately it could work. While someone here blame me that it is not going to work very well. Of course, some breakthrough is required, I didn't assume it is going to work right away.

You presented and stuck to a specific idea which was shot down multiple times. Had you asked, "Is there a way to create a screenless image", then this thread would have gone differently. There is no "breakthrough" needed here because your idea doesn't even make any sense in the way you've presented it.

What is concerning to laser focus, everything depends at least in two things: the focal length and the angle of defocusing. Maybe someday it would be possible to make it defocus 360 degree wide, so it will radiate light in all sides completely evenly, and viewing angle will be practically unimportant.

The focus of the laser is entirely irrelevant. Defocusing the laser simply turns it from a coherent beam which can only be viewed from directly in front of the laser to something similar to a normal light bulb which can be viewed from a wide angle. An array of lasers can easily make an image similar to how a TV screen works, except that unless you defocus each beam, you'd need to stand directly in front of the lasers.

The amount of divergence of a cone of light entering your eye determines how your eye focuses it, not how it forms an image. For example, the light from a very distant object like a star enters your eye with practically zero divergence. It is essentially composed of parallel rays. In contrast, the light from you TV is diverging when it enters your eye. The lens of the eye alters its shape in order to bring light from objects at different distances to a focus.

Using a spotting scope with an adjustable focus, you can easily adjust the divergence of the light from an object at a set distance by moving the focus back and forth. Generally the most comfortable focus position for viewing an object through a scope is one where the light from the object enters the eye with very little divergence. That way the lens of the eye can completely relax and you don't have to expend any effort to bring the light into focus on your retina. Moving the scopes focus will cause the light to diverge before entering your eye instead of being composed of parallel rays, so your lens will flex in order to bring the light into focus on your retina. No matter how the focus is set, the image appears the same once it is focused on the retina. As you can see, whether the light is diverging or not doesn't have any effect on the image, it only affects what needs to happen to bring the image into focus.

When the light enters your eye with no divergence, it is acting like a laser beam. A laser beam has little to no divergence over a long distance. Adjusting the focus to make the light diverge before entering the eye is similar to the light coming from your TV. The light leaves each pixel in a diverging cone which means that you don't have to stand right in front of the TV to see it, but now your lens has to flex in order to bring the part of the cone entering your eye into focus.

Does all that make sense?
 
  • #34
Drakkith said:
I really don't see how.
I don't see how not. The discussed system is exactly the result of the geometry in the lower two images, where the green beam corresponds to one pixel.

Stanley514 said:
This dispute went ridiculous. I was thinking about future yet unknown technology and only attempted to give some clue how approximately it could work. While someone here blame me that it is not going to work very well. Of course, some breakthrough is required, I didn't assume it is going to work right away.

What is concerning to laser focus, everything depends at least in two things: the focal length and the angle of defocusing. Maybe someday it would be possible to make it defocus 360 degree wide, so it will radiate light in all sides completely evenly, and viewing angle will be practically unimportant.
No, certainly not. This is not an engineering issue, it is a fundamental physical limitation.
 
  • #35
mfb said:
I don't see how not. The discussed system is exactly the result of the geometry in the lower two images, where the green beam corresponds to one pixel.

Are we talking about the two-mirror system?
 
  • #36
No. That thread started with your quote of
Sure, the lens (better: use a mirror) would have to be larger than the image, the viewing angle would not be so good and there is no real point in it anyway... but it is possible.
which refers to the setup of post 8.
 
  • #37
mfb said:
No. That thread started with your quote of which refers to the setup of post 8.

Ah, I see. I misread your post. My mistake.
 
  • #38
davenn said:
hi Andy

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

Dave

Sure- changing the propagating radiant field requires interaction with a material ("scattering"). Scattering without anything to 'scatter off of' is a violation of the conservation of momentum.
 
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  • #39
An example of a sreenless display from a sci-fi movie.

 
  • #40
Stanley514 said:
...a sci-fi movie.
Exactly the point. If you care to check out the gorgeous cartoon chick (Jessica Rabbit) from a neighbouring video, you'll note that she is fictional as well.
 
  • #42
Stanley514 said:
Some companies claim, they already started to develop cell phones with holographic displays.
I don't know whether that's even a real video or a fake one, but in either case that particular technology is nothing new. There have been virtual keyboards available for years which work by projecting the keys and then measuring where your fingers are in relation to them. The display in that case, as in yours, is not holographic, since it's not 3-D; that's just a hype term to use for a simple projected image. In both cases, it has to be projected onto the desk surface rather than into thin air.
 
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  • #43
mfb said:
For the two mirrors, you have quite a large possible viewing angle.

Sure, the lens (better: use a mirror) would have to be larger than the image, the viewing angle would not be so good and there is no real point in it anyway... but it is possible.
Yeah, in order to make it practical they need to achieve that image would be much larger than a mirrors and make it look like it floats in dozen of inches above mirrors. But I do not think it have to be 3d image, I would be satisfied with a 2d image.
I suggest you do not need to use a real physical object like a coin to generate image and everything could be gone with help of a lasers simply? Mirrors could work with lasers as well?
 
  • #44
Stanley514 said:
I would be satisfied with a 2d image.
I suggest you do not need to use a real physical object like a coin to generate image and everything could be gone with help of a lasers simply? Mirrors could work with lasers as well?
Okay, 2D is not as unreasonable as 3D; I merely mentioned that aspect since your phone clip says "holographic" when it clearly isn't.
I honestly can't see how you could use lasers instead of a physical object in the instance that you refer to. With that double parabolic mirror cavity "floating coin" effect, you would have to have your source in the exact same focal point of the device that the coin currently occupies. That means that the illusion would be of the projector with a red dot on it floating above the dish, not just the dot.
 
  • #45
Stanley514 said:
Yeah, in order to make it practical they need to achieve that image would be much larger than a mirrors and make it look like it floats in dozen of inches above mirrors. But I do not think it have to be 3d image, I would be satisfied with a 2d image.
That does not work. At least not in the way you describe it.
 
  • #46
Then I have question to specialists in optics: What is maximal possible angle of defocusing and what is dependence between the focal length and angle of defocusing?
 
  • #47
Stanley514 said:
Then I have question to specialists in optics: What is maximal possible angle of defocusing and what is dependence between the focal length and angle of defocusing?

I believe it's about 180 degrees for a mirror, and a little less for a lens. There is no dependence between focal length and angle of defocusing. For a laser beam and a simple optical system, the larger the beam radius, the longer the lens/mirror focal length can be.
 

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