Projection of an hologram on a screen ?

Holography related topics.
pierro787
Posts: 12
Joined: Tue Aug 22, 2017 5:47 am

Projection of an hologram on a screen ?

Post by pierro787 »

Hello,

Is it possible to project a transmission or reflection hologram to a screen with a white LED as light source and with a lense to focus the picture on the screen ? (like a slides projector but with hologram instead of slide)

If not, what is the best setup to project an hologram on a screen / wall ?

Is it possible to use multiplex hologram so that viewer can see the animation by moving in front of the screen / wall ?

Thanks for all your informations !

Pierre
lobaz
Posts: 280
Joined: Mon Jan 12, 2015 6:08 am
Location: Pilsen, Czech Republic

Re: Projection of an hologram on a screen ?

Post by lobaz »

The image on the screen should be 3-D? What do you mean by "screen" - a plain wall, or something elaborate?
pierro787
Posts: 12
Joined: Tue Aug 22, 2017 5:47 am

Re: Projection of an hologram on a screen ?

Post by pierro787 »

I was thinking about a blank screen or a mirror. It seems it can not been done with a white screen (or a wall) but with a mirror.
lobaz
Posts: 280
Joined: Mon Jan 12, 2015 6:08 am
Location: Pilsen, Czech Republic

Re: Projection of an hologram on a screen ?

Post by lobaz »

It is difficult to give a meaningful reply as I do not know what are you trying to achieve.
I understand you want some flat surface ("a screen") that shows an animation - as an observer walks around, the picture changes. Right?
Why do you need to use any kind of projection instead of watching a hologram directly? To magnify the image?
What is the target application, image size, intended viewing environment?

Yes, it is indeed possible to "project a hologram on a screen", for example holographic cinema developed by V. Komar does exactly this. There are holographic projection screens that allow you to see different images depending on the observation angle; and so on.

Your original question makes some assumptions, and it is not clear if you really need all of this: "to project a transmission or reflection hologram to a screen with a white LED as light source and with a lense to focus the picture on the screen"
- Do you really need a hologram as a source of image information?
- Do you really need to use LED?
- Light has to be white?
- Do you need any lenses?
- Do you assume 2-D picture?

For example, you make a small transmission hologram of a flat picture, say 1 m from the hologram. If you illuminate the hologram with a laser pointer and place a sheet of paper 1 m from the hologram, the picture appears. Thus, some projection was achieved, but with different type of light and without lenses.
Din
Posts: 402
Joined: Thu Mar 12, 2015 4:47 pm

Re: Projection of an hologram on a screen ?

Post by Din »

Yes, it's possible. But, you have to reconstruct a real image, as opposed to a virtual image.

When you look at a real object in the real world, light emanates from the object, ie the light diverges outward from the object to your eyes. When you look at a mirror image of some object, light appears to diverge from behind the mirror, therefore the object appears to be behind the mirror. However, there is no actual object behind the mirror. Divergent light from the object hits the mirror, is redirected to your eyes, and continues to diverge. Your eyes "backtraces" the (redirected) divergent light to a point behind the mirror, and your brain assumes that, because light is diverging from some point behind the mirror, there really is an object behind the mirror. The light from the mirror causes a Virtual Image."Virtual" because there is no actual object behind the mirror from which light is actually diverging.

When you place a (converging) lens, ie a magnifying glass, between your eyes and an object, the (magnified) image appears to be coming from behind the lens. This is because, if you placed the object at some point less than the focal length of the lens, the light from the object diverges towards the lens, and continues to diverge. Again, as in the mirror example, your eyes "backtraces" the divergent light to some point behind the lens, and assumes there is a (magnified) object somewhere behind the lens. The lens creates a virtual image. However, if you placed the lens at some point greater than the focal length, the diverging light from the object hits the lens, then converges, and diverges again. So, the light comes to a real point, then diverges from that real point. Since the light is actually diverging again from a real point in space, there actually is an image of the object, as if it were the real object. This is called a Real Image, "real" because there really is divergent light from some point in space. A real image can be projected on some surface. So, if you placed a lens against a window, where there was a tree some distance away, you can place a piece of paper at the focal length of the lens, and you'll see a clear focused and projected image of the tree.

In a hologram, you record divergent light from an object, ie the light from the object diverges towards the hologram plate. You're actually recording the divergent wavefront itself. Now, there are two ways you can reconstruct the hologram. One way is to reconstruct it with the exact shape and position of the original reference. If you do this, you reconstruct the original divergent light. The image is now virtual, and cannot be projected. But, if you reverse the hologram, so you reconstruct with the exact opposite of the original reference beam, the hologram reconstructs the opposite of the original divergent light, it reconstructs convergent light. Now, the image is real, and can be projected on some surface.

So, to keep things simple, let's say you make a laser transmission hologram of a porcelain cat. You place the cat at,say, 8in (20cm) from the plate, and reference it in same direction as the cat, ie the ref and object light hits the same side of the plate. Now, you develop the hologram, replace it, and re-illuminate the plate. You'll see the porcelain cat behind the plate. This is the virtual image. If you now flip the plate (side-to-side for a side referenced ref, top top-to-bottom for a top ref) and re-illuminate the plate again. Now the cat appears in front of the plate, This is a real image. This real image can be projected on a wall. As Petr says, if the original object 8in from the plate, the real object focuses at 8in away. But, this is possible only if you laser illuminate. If you use white light, each wavelength component of the white light will create it's own image, slightly displaced, so you'll get many, many cats, all displaced slightly, creating a colour blur.

If you take the hologram of the cat, and re-illuminate it from the opposite side (the opposite of the original ref is called the 'conjugate' of the ref), but with an unexpanded laser beam, a raw laser directly from the laser, then you'll get a flat real image of the cat that can be projected at any distance, but magnified. This is because you're only reconstructing a tiny portion of the hologram, the area of the laser beam, and you see one single 2D perspective of the image.
pierro787
Posts: 12
Joined: Tue Aug 22, 2017 5:47 am

Re: Projection of an hologram on a screen ?

Post by pierro787 »

The purpose is to magnify the image to be able to show it to several persons at once, in a large room.

Idealy, a white light (some kind of LED) would be easier to show the hologram, instead of using a laser but if the laser is required for the projection, I can use it.

I suppose lenses are required if i want to magnify the image from the hologram.

Thanks again for your support (Iobaz, Din and others members from the forum).

Pierre
lobaz
Posts: 280
Joined: Mon Jan 12, 2015 6:08 am
Location: Pilsen, Czech Republic

Re: Projection of an hologram on a screen ?

Post by lobaz »

OK, you need to magnify the image.

Before we delve into technical details, you should know there is an important optical concept called "space-bandwidth product". Informally, it says that the product of the image size and its viewing angle is constant. So for example, if you have a 3-D image 50x50 mm that can be viewed in angle 60 degrees, and you enlarge it tenfold to 500x500 mm, then the viewing angle will be much smaller, approximately tenfold.

So the first question is: do you expect wide viewing angle of the enlarged image? If yes, then you must build quite complicated optical setup - for example many holograms are magnified simultaneously and perfectly aligned to that you have both large size and viewing angle. Space-bandwidth product is hard to cheat!
Prof. Lunazzi

Re: Projection of an hologram on a screen ?

Post by Prof. Lunazzi »

You can do it with a halogenous lamp 15V-150W (not anyone) in a holographic screen 1m x 1m, but the diffraction efficiency being the product of that of the hologram and that of the screen, I only could show it to two-three persons in a dark ambience. I did a special hologram 35 mm format, it could be better with a larger one, I guess, but am not sure, because you always need the closer equivalent to a point source.
Refs.: https://arxiv.org/pdf/0905.1157.pdf
https://www.researchgate.net/publicatio ... ic_screens

I hope that better efficiency in holograms may give better results, I did my best with silver halide material.
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