What “true 3D” would mean
Most 3D imagery on flat screens is a trick of the eyes: two slightly different pictures create a sense of depth, but the picture stays on the surface. A truly three-dimensional display would instead put light points at positions in space, so that the image really occupies a volume. Walk around it and you would see it from different sides, and you could focus on near or far parts of it as with a physical object.
Two related ideas go by names that are often confused. Volumetric displays build the image from points of light located within a volume. Holographic displays reconstruct the light waves that an object would have produced. Films and marketing often call many things holograms that are neither, such as images projected onto a screen or a sheet of glass.
Volumetric approaches
Several concepts have been explored in laboratories and in niche products.
Swept surfaces. A flat screen or a rotating arrangement of light sources moves quickly through a space, perhaps spinning or oscillating. Light is flashed at the right moments so that, because of persistence of vision, the eye merges the points into a solid-looking shape. This resembles how a spinning sparkler can draw a circle.
Stacked layers. Many thin, switchable layers, each able to show a slice of the image, are placed one behind the other. Turning the right pixels on in each layer builds a depth stack. Each layer must be transparent enough to let the ones behind it be seen.
Points in air or in a medium. Concentrated energy can make light appear at chosen positions in a gas, fluid or solid, or small particles can be moved and lit. These methods exist as demonstrations and often face limits in brightness, safety considerations and size.
A volumetric image can often be viewed from many sides, which is its great appeal. A limitation is that each point of light normally emits in all directions, so it is hard to make solid-looking objects that block what lies behind them. A hand in front of a dim point does not hide it the way a real object would.
Holographic ideas
A traditional hologram is a recorded pattern that, when lit properly, recreates the light waves from the original object. Because the waves are recreated, different viewing positions see different perspectives, and the eyes can focus at different depths. Static holograms exist, such as those on some cards and in some art, but they are fixed pictures.
A moving, electronically controlled version is much harder. To shape light waves, the display would need to control the light at a scale comparable to the wavelength of light, which is far smaller than the pixels in current screens. That implies enormous numbers of tiny elements and huge amounts of computation for each frame.
Researchers have explored approaches based on devices that modulate the phase or brightness of light with fine control. Typical issues are small image size, narrow viewing angle, speckle noise from laser light, and heavy computation. Claims about large, bright, full-color, real-time holographic video should be viewed skeptically unless they come with clear demonstrations.
The data problem
Even if the optics worked, there is a quantity problem. A normal frame has a certain number of pixels. A volumetric image needs a value for each position in a volume, so the data grows rapidly with each added depth layer. A holographic image needs far more still. Storing, computing and transmitting this at video speed is a serious obstacle, and it goes beyond what is needed for even very high screen resolution on a flat panel.
Other difficulties
- Brightness and contrast. Light points in space must stand out against ambient light. In a bright room, faint volumetric points can wash out.
- Resolution. Dividing the available light control among depth positions reduces detail in each, much like the sharing of pixels in glasses-free 3D.
- Speed. Swept systems must move parts at high speed and synchronize light flashes, which raises noise, safety and wear concerns.
- Size and cost. Larger images need larger, more complex hardware.
- Content. Creating and capturing true 3D content is itself a challenge.
- Occlusion. Making parts of an image hide others, as real objects do, is difficult.
Neighboring ideas
Several things that look similar are actually different:
- Pepper’s ghost style effects use a clear angled sheet to reflect a screen into a scene, giving a floating look without any real depth.
- Projection onto fog, mesh or transparent screens makes images that appear to float, a topic related to transparent displays.
- Headsets add depth by giving each eye its own picture, as in virtual reality optics.
These can be effective, but they are not volumetric or holographic in the strict sense.
Possible uses and cautions
People have discussed possible uses in design visualization, scientific data, entertainment and exhibits such as museum exhibit screens, where a floating object might draw interest. For medical or engineering uses, any decision-relevant display would be a matter for qualified specialists, and nothing in this article assesses suitability for such tasks.
What remains open
No one can say whether true volumetric or holographic video will reach everyday devices. The main obstacles, namely the extreme amount of information and the fine control of light, are fundamental. Progress may be gradual and may come in small, specialized forms before, or instead of, any general-purpose version.