Due to the complexity of required calculations, digital holograms are also often associated with slow, off-line calculations. However, eye-tracked approximate holograms provide correct image focus and best image quality.
The ability to change the image focus is desirable in a near-eye display: It addresses the accommodation-convergence conflict, allows image focus to match eye focus in a see-through display, and adds realism. Unlike vari-focal, multi-focal and light field displays, a holographic display is able to provide per-pixel focus control with virtually no discretisation, enabling smooth and natural focal cues to imagery.
Another desirable quality of a display is vision correction—the ability to fix defects in the user’s vision. Such a display allows users to view the display without their glasses. Recent holographic displays can correct simple vision problems such as near- and far-sightedness as well as higher-order vision problems like astigmatism.
Holographic displays are also capable of aberration correction—encoding optical corrections to the display optics in software. Such a capability allows use of simpler optics and enables new optical architectures. The optical correction capability of holograms allows arbitrary optical corrections on a per-pixel basis.
One of the most important considerations for a near-eye display is form factor, especially for see-through, mixed-reality devices. Lightweight, eyeglasses-like displays are needed to facilitate viewer comfort and all-day use.
In the past, researchers interested in holographic display systems proposed or focused on methods for overcoming limitations in the combined spatial resolution and speed of commercially available, spatial light modulators. Representative techniques included space-division multiplexing, time-division multiplexing and combination of these two techniques.
Implementing a viewing window design demands close attention to the optical image system. Holography is being advanced through development of structure materials at the nanoscale. These allow devices to achieve new optical properties that go beyond the properties of natural materials.
With a tabletop display, a viewing window can be created by using a magnified virtual hologram, but the plane of the image is tilted with respect to the rotational axis and projected using two parabolic mirrors. As parabolic mirrors do not have an optically-flat surface, visual distortion can result.
This problem can be solved by designing an aspheric lens, which allows multiple viewers to observe 8cm (3.2-inch) holograms from any position around the table without visual distortion. Building on these advances, researchers hope to implement a key design feature of strategically sizing the viewing window, so that it is closely related to the effective pixel size of the rotating image of the virtual hologram. Watching through this window, observers’ eyes are positioned to accept the holographic image light field because the system tilts the virtual hologram plane relative to the rotational axis.
To enhance the viewing experience, the team hopes to design a system in which observers can see 8cm holographic 3D images floating on the surface of the parabolic mirror system at a rate of 20 frames per second. Test results of the system using a 3D model and computer-generated holograms were promising—though right now still in a monochrome green colour. Next, the team wants to produce a full-colour experience and resolve issues like undesirable aberration and brightness mismatch among the four digital micromirror devices used in the display.
In an attempt to assimilate holographic technology with modern electrical devices, a team of researchers has created the world’s thinnest hologram. The team claims the thickness of holograms has typically been limited to the optical wavelength scale, something which seems to have reduced integration with thin electronic devices. However, researchers were able to surpass this limit and achieve holograms 60nm in size using a topological insulator material—a material which seems to behave as an insulator while also having conducting states upon its surface.
An advantage of this new nano-hologram is the way users may observe holographic projections outside of 3D glasses requirements. This means consumers may potentially witness nano-holograms built into electronic devices such as smartphones, computers or TVs in the future as they require additional peripherals necessary for operation. With this breakthrough in technology, the research team claims the next step for the technology is the development of a new method to adapt holography for LCD screens.
A graduate student at MIT (USA) Media Lab has developed an inexpensive optical chip that, when combined with a prototype display, “renders colour holographic video at resolutions equal to that of a standard-definition TV. Besides the low cost of the chip, the true advance is the way the colours are handled. In the past, red, green and blue light had to be handled separately, resulting in lower image resolution. This chip handles these lights simultaneously, allowing for both higher resolutions and more efficient processing.”