Focusing light through frosted glass leads to new 3D display technology

Chris Lee
We're not at the hand-held hologram projector state yet, but we're getting there.
Sometimes it amazes me how fast physics goes from fundamental ideas to producing a new toy. The latest example comes from a bunch of experiments and theory on how opaque materials affect light passing through them, a topic that we have covered extensively in the past. The work had the catnip qualities of being cute and simple and exploring some pretty fundamental physics ideas.
The idea behind the research was simple. Scattering materials, like white paint or sugar cubes, turn light into a chaotic jumble. But if we could control how they scatter light, we could turn them into useful things like focusing devices. I know the researchers who pioneered this idea, and they were all rather conservative about possible applications. And that was appropriate; the ideas that they had—medical imaging, high-resolution imaging, and security applications—have all (with the exception of security) proved to be possible but really difficult.
So I was a bit surprised to see the ideas applied to holographic displays. I have to admit, I never even thought of it, but once you see the idea it is like being slapped silly by Captain Obvious.

Why holographic displays suck

Holographic displays suck. This is a fundamental truth. How fundamental? It's actually a physical law.
Perhaps the best way to illustrate this is to compare a 2D projection system with a holographic projection system. In a 2D projection system, a distorted light field carries the information about the image. A lens system recreates that image at some distance, where it falls on a white screen. The white screen scatters the light in every direction, so no matter where you are in the theater, you can see the image (after craning your head to avoid that dude). The size of the image only depends on the magnification of the imaging system—there is no connection between the size of the image and the viewing angle.
In holography, the first part is the same: a distorted wavefront carries the information to recreate a 3D image. But that recreation occurs at the focus of the imaging system. To view it, you have to stand in the path of the projection system. So, an imaging system with a really tight focus (a short focal length lens) creates a tiny image that can be viewed from a wide range of angles. An imaging system with a looser focus (a long focal length lens) generates a larger image, but it can only be viewed by that dude—crane your head, and you are out of the viewing angle.
This is actually a fundamental geometrical principle. Sure, you can beat it with multiple lenses that project at different angles or some other horribly complicated idea. But the added complexity only serves to make for an expensive and unreliable projector.

Scattering to the rescue

Scattering by itself is not helpful. Imagine setting up a projection system that incorporated your bathroom window: the light makes it through, but it turns your beautiful movie into the world's least interesting set of blurry blobs. This is because the glass is full of randomly oriented interfaces and air bubbles, which scatter the light in all directions.
If you shine a single color of light on a scattering medium like this and examine the light after it has passed through the scattering medium, you won't find a homogeneous, even blur. Instead, it will be quite bright in some places, and in others it will be exceptionally dark. By accident, the light fields that took a whole lot of different paths through the scattering medium will add up in such a way that they create bright spots and dark spots.
The surfaces that scatter within the glass are fixed—they don't change. So if we control the input light field carefully, we can ensure that it goes through paths that add up exactly the right way. This is what allows researchers to use scattering materials as a lens, creating a sharp focus at an arbitrary location behind the scattering medium.
What a group of Korean researchers realized is that this means that the image size and viewing angle are no longer coupled. And by rapidly scanning a focus, images can be drawn in the air and viewed from a wide range of angles.
In an ordinary imaging system, the contributions from each spatial location of the object being imaged stay together. As a result, they all have similar angles as they exit the image volume and cannot be viewed widely. The scattering medium spreads all that information out over the whole area of the scattering medium, so contributions to every part of the image come from all over the exit surface of the scattering medium. As a result, all light from all possible angles contributes to each part of the image.
Effectively, the scattering medium acts like many different lenses at many different positions at once. Instead of a single image being created, many are created, each with a narrow viewing angle. Because all these images are overlapping, they produce a single image that can be viewed from many angles. The viewing angle for the image is now determined by the size of the image volume and the width of the scattering medium. Therefore, to increase the image size and viewing angle, one needs a bigger plate of scattering glass, correspondingly bigger optics, and a brighter set of lasers.
Optical setup consists of a deformable mirror, the scattering medium with two successive holographic diffusers.
Optical setup consists of a deformable mirror, the scattering medium with two successive holographic diffusers.
Lurking behind all this awesomeness is something even more awesome. To get any of this to work, the researchers need to understand their scattering medium well enough that they can set up the incoming electromagnetic wave to focus at exactly the right place in a 3D volume. But they know it so well that they can also scan that focal point around the volume in a fast and controllable way. Images can be created by doing a 3D raster scan. With a three-color projection system, they will be able to create full-color images (in the paper, they only demonstrated red and green separately).
In their demonstration, the team used spiraling dots and letters that are projected at various distances to show that both viewing angle and image size of the hologram are increased. The image size is still pretty small—a cylindrical volume that is 2cm long and just under 1cm in diameter. Despite them calling it HD, I would like to see something less hazy.
But the viewing angle is huge relative to the image size. Just using the standard optics in the imaging system—meaning no scattering—an image with a 0.5mm size has a viewing angle of just over 0.5 degrees. The product of viewing angle and image size remains a constant, so to get a 30 degree viewing angle, the image size would be reduced to just a few micrometers.
By introducing scattering, the viewing angle was increased to 60 degrees, and the image size was increased to 4.2mm, corresponding to increasing the product of viewing angle and image size by about 1,000. In tests with a different scattering medium, they increased the product to over 2,000.
Now, I should note (as the authors do), that there are other holographic display systems out there that achieve similar combinations of viewing angle and size, but they all suffer from combinations of flaws. Some are not dynamic, so only pictures are displayed. Some have no prospect of full color display. Others are horribly complicated. The projection system here isn't perfect, but considering the simplicity of the system, it's a remarkable result.

What’s next?

Clearly, the researchers are hoping that this will become a commercial display technology, but there are problems to overcome. First of all, the resolution of any image depends on the pixel density of the controller. In this version, the relationship between pixel number and image resolution is complicated. The researchers used a deformable mirror with an array of 64×64 pixels. In an ordinary imaging system, this would be laughable. But this is just to control the light propagation through the scattering medium—the actual light source could be higher resolution. These 4,000 pixels control how well defined the raster point is and how precisely it is positioned. It is not clear to me that this is good enough yet; more pixels may be required.
The second problem is that I know these systems to some extent, and they are very sensitive to minor changes in alignment. This cannot be changed, but it can be accounted for by having an automatic calibration system. As far as I know, no one has done anything like that before, and it is unclear how often it would need recalibration in a house where the temperature changes a lot on a daily basis. A tall order, I feel.
Nature Photonics, 2017, DOI: 10.1038/NPHOTON.2016.272
Focusing light through frosted glass leads to new 3D display technology Reviewed by Bizpodia on 16:45 Rating: 5

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