Laser focal point manipulation

[Stanley514]

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?

 

[Danger]

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.

 

[Drakkith]

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?

 

[Stanley514]

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?

 

[Andy Resnick]

<snip>

1. Is there something wrong with my assumption?

Yes, it violates conservation of momentum.

 

[Drakkith]

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?

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.)

 

[davenn]

Yes, it violates conservation of momentum.

hi Andy

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

Dave

 

[Stanley514]

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.)

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.

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]

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.

 

[Drakkith]

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.

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.

 

[Stanley514]

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.

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.

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.

 

[Drakkith]

The diagram you provided is complicated and I do not understand what is the "object" and what is it needed for.

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.

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.

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.

 

[Stanley514]

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.

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.

 

[Danger]

So what prevents lasers to have the same property?

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.)

 

[mfb]

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.

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.

 

[Danger]

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.

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.

 

[mfb]

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.

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.

While that might not necessarily cause blindness

Making a laser weaker (if necessary!) is always easy.

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.

Right, it would be a single pixel. Add more pixels and you can get an image.

 

[Stanley514]

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. .)

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.

 

[Danger]

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.

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.

 

[Drakkith]

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.

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.

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.

 

[Stanley514]

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.

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
http://www.eurolaser.com/uploads/pics/eurolaser_Linsen_4_GBR_01.jpg
I think if they will achieve good enough manipulation with focal length 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.

 

[Danger]

You still have to have that image projected onto something tangible for viewing. The only exception to that is the way that a new pair of glasses made by someone (Sony? Nintendo? Whoever...) works. It projects an image directly through the eyeball to the retina. The critical consideration is that the image is generated and projected from a few millimetres away from the viewer's eye. Also, it's projected as an actual image, not a sequence of photons all focused on the same point, as the original laser idea would be. Think of the heads-up displays in fighter jets or even some cars. They are projected onto the windscreen or an intermediary piece of acrylic, not into the drivers' faces.

 

[Drakkith]

Stanley, I'm afraid that your understanding of optics simply isn't sufficient. The way you think image formation occurs simply isn't how it works and I'm afraid that the subject is too complicated for me to explain if you don't know the basic terminology. I highly recommend you get a book on basic optics.

In addition, here's a few links:

https://spie.org/Documents/Publications/00%20STEP%20Module%2003.pdf
http://en.wikipedia.org/wiki/Geometrical_optics
http://en.wikipedia.org/wiki/Ray_(optics)

 

[Stanley514]

So, you believe high quality images floating in air will be never possible?

 

[mfb]

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.

To be outgoing in a broad angle, it also has to come in in a broad angle, which means your image is close to the last optical element.

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.

Why eye damage? Because "Oh my god it is a laser"?
You can choose the laser intensity to be whatever is necessary to be visible without damaging the eye (and that range is huge).

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.

They will not, see the images posted here - the laser beam would be extremely broad both before and after the focal point.

Also, I sure wouldn't want anything powerful enough to plasmacize air anywhere in my house, let alone aimed at my face.

As mentioned, this was not intended for imaging purposes, just to show the statement there is not always true. I saw one of those plasma spots in a lab, it just looks like a bright, tiny point in space.

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.

You can have the image in front of the lens. The allowed viewing angle will be limited to the directions where you can see the lens, of course.

Every microscope and telescope works that way - so yes, of course it is possible. The difference here would just be the initial image generation, using a laser instead of a reflective object or a star.

The floating coin is a very good illusion in that aspect, it even shows the full 3D structure of the object at a place where there is no object.
Video

 

[Drakkith]

You can have the image in front of the lens. The allowed viewing angle will be limited to the directions where you can see the lens, of course.

Every microscope and telescope works that way - so yes, of course it is possible. The difference here would just be the initial image generation, using a laser instead of a reflective object or a star.

The floating coin is a very good illusion in that aspect, it even shows the full 3D structure of the object at a place where there is no object.
Video

Of course. If the OP wishes to do something similar, he's going to need one heck of a mirror/lens or two though, not just a diverging lens. If I understand the OP correctly, he wishes to create an image in midair simply by diverging the laser beam, which of course will not work. A single lens system in front of the laser would only make the image appear at the lens when viewed from a distance. This is just like how the image from a microscope/telescope can still be seen when you back your eye away from the eyepiece. You can still see the image, but it still comes from the eyepiece, not from midair.

 

[Drakkith]

So, you believe high quality images floating in air will be never possible?

Not in a setup like you've suggested, no. As MFB said, you can create an image in midair with specific setups, but these are very different than what you've suggested. For example, the floating coin image is projected between two curved mirrors and requires that you view it from a specific point.

 

[Danger]

Not in a setup like you've suggested, no. As MFB said, you can create an image in midair with specific setups, but these are very different than what you've suggested.

Exactly what I mean. The technology to obtain the effect that he seems to want has existed for decades; it's called a "transmission hologram". The image is absolutely not created by the laser, though. The laser merely allows someone to see the information that is actually stored in the film medium.

 

[mfb]

Not in a setup like you've suggested, no. As MFB said, you can create an image in midair with specific setups, but these are very different than what you've suggested. For example, the floating coin image is projected between two curved mirrors and requires that you view it from a specific point.

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.

 

[Drakkith]

For the two mirrors, you have quite a large possible viewing angle.

The angle isn't important here. 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.

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.

That setup isn't anything like what the OP presented.

 

[Stanley514]

I think, one more unsolved problem of any screenless display and maybe the largest one is creation of a true black colour. Unless they want to use it in a complete darkness, of course.

"Exactly what I mean. The technology to obtain the effect that he seems to want has existed for decades; it's called a "transmission hologram"."

Not exactly. A laser holograms usually have poor quality and are semi-transparent. With use of defocused lasers which beam in viewers direction you may not have this problem if you will succeed to solve problem of a black colour.

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.

 

[mfb]

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.

 

[Drakkith]

It fits to the images shown in post #8.

I really don't see how.

 

[Drakkith]

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?

 

[mfb]

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.

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.

 

[Drakkith]

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?