Three-dimensional (3D) display technologies are an exciting prospect for truly immersive visual experiences. While the world around us is 3D, nearly all our commonly used display technologies are two-dimensional and do not render any depth information.
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Early attempts to render 3D images involved the use of two images that were fused together to create the impression of stereoscopic depth. While the question of exactly what was the first true stereoscopic image is a complex one, the first undisputed example was produced by Charles Wheatstone in 1838.1 Many modern 3D display technologies still rely on similar approaches to this – where slightly different images are projected to the left and right eye to produce an overall 3D image. 3D glasses use color filters to achieve the same type of effect.
There are now several 3D display technologies, including multiview, volumetric, and holographic displays.2 Different approaches to generating 3D images have their own advantages and disadvantages such as the overall viewing angles and the processing power required to generate the separate images for each eye. This can limit the resolution for many types of displays.
Holographic displays are thought to offer the ‘ultimate’ 3D viewing experience.3 However, despite the original development of moving images with holograms taking place over 50 years ago, a holographic television is not seen in our living rooms.
The Data Problem
The development of holographic televisions is challenging due to the sheer volume of data that must be transmitted and rendered to create the final 3D image.
A 3D holographic display requires nearly 10 orders of magnitude of bit rate versus a simple black and white television – coming to nearly 1016 bits per second for a relatively simple 256 color, 60 Hz refresh rate, 70 cm screen.3
However, spatial light modulators may offer a solution to this massive data transfer demand. Liquid crystal on silicon spatial light modulators can process the large pixel counts required to generate high-resolution images as well as having a high diffraction efficiency and being able to do some amount of phase modulation. While the viscoelasticity of the liquid crystals is too high to allow for very high refresh rates, there are current attempts to use microelectromechanical systems (MEMS) devices instead as they can achieve refresh rates that are an order of magnitude higher.
The improved refresh rate does sacrifice some of the ability to perform phase modulation, but improvements in MEMS manufacturing methods may help to reduce the non-linearity of the MEMS devices response and ensure a comparable response across all devices in the full display array.
Generating Holograms
The other challenge the large data volumes pose for holographic televisions is in the generation of the hologram itself. In a standard hologram, a laser beam records a 3D reference image as precisely as possible, which can then be illuminated with a second beam to restore the image. This means that all the generated information or images need to be pre-recorded and encoded.
There are attempts to use machine learning approaches and other efficient computational algorithms to allow the generation of holograms. Displays could be capable of generating random scenes at high resolution as well as for applications such as immersive 3D printing.4 This is carried out using tensor holography to find memory-efficient and inexpensive ways to calculate the holograms required.
An Economic Problem?
While for 3D holography there are still engineering challenges to be overcome to generate 3D displays with good refresh rates and resolutions, ultimately one of the biggest barriers to the adoption of 3D display technologies is cost.
While some household items such as the Nintendo 3DS make use of parallax filters to generate 3D images without the need for glasses, this halves the effective resolution of the screen and has a relatively small ‘sweet spot’ in terms of the viewing angle. Some studies suggest up to 50% of viewers find a degree of visual discomfort with stereoscopic 3D due to factors such as cross talk and inappropriate disparities that are dependent on the viewing angle.5
Another factor hindering the uptake of 3D displays is simply cost. Reliable manufacture of MEMS requires very high precision lithography processes and so truly holographic displays that overcome many of the issues with visual discomfort remain unfeasibly expensive.
3D content itself can be more expensive to produce, and with poor uptake, this has reduced the enthusiasm of consumers for a new generation of 3D displays.
However, with more of the causes of visual discomfort being well understood and significant improvements in multiview and volumetric displays – that currently offer much better display properties than is achievable with holographic displays – there may be a new future for 3D displays.5
References and Further Reading
- Brooks, K. R. (2017). Depth Perception and the History of Three-Dimensional Art: Who Produced the First Stereoscopic Images? I-Perception, 1–22. https://doi.org/10.1177/2041669516680114
- Geng, J. (2014). Three-dimensional display technologies. Advanced Optics Photonics, 5(4), 456–535. https://doi.org/10.1364/AOP.5.000456
- Blanche, P. (2021). Holography, and the future of 3D display. Light: Advanced Manufacturing, 2, 28. http://dx.doi.org/10.37188/lam.2021.028
- Shi, L., Li, B., Kim, C., Kellnhofer, P., & Matusik, W. (2021). Towards real-time photorealistic 3D holography with deep neural networks. Nature, 591(March). https://doi.org/10.1038/s41586-020-03152-0
- Terzi, K., & Hansard, M. (2016). Signal Processing : Image Communication Methods for reducing visual discomfort in stereoscopic 3D : A review. Signal Processing: Image Communication, 47, 402–416. https://doi.org/10.1016/j.image.2016.08.002
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