Domestic researchers are focusing their efforts on finding a solution to the problem of expanding the field of view of holographic displays, which are said to be the crystal of perfect stereoscopic images. It is significant that it presents a new technological method, breaking away from the existing pixel size-based approach to expanding the field of view.

The Electronics and Telecommunications Research Institute (ETRI) announced on the 6th that it has presented a research direction to solve the problem of the narrow field of view of about 3.8 degrees, which is known as the biggest obstacle to commercializing holographic displays, with support from the Ministry of Science and ICT, and that the major research results were introduced in a video published by the Optical Society of America.

Unlike the analog film holograms currently on display in museums, holographic displays use a digital hologram recording device known as a hologram display element to float a three-dimensional hologram image in the air. However, due to issues such as a narrow viewing angle, they have not yet been commercialized.

In order to secure a wide viewing area, i.e. a viewing angle of more than 30 degrees, like an analog film hologram, a holographic electronic recording device with a resolution of several hundred nanometers (nm), which is about the wavelength of light, must be developed.

On the other hand, the pixel size of current commercial devices remains at several micrometers (㎛), making it difficult to avoid the problem of a narrow viewing angle of less than 4 degrees.

There are methods to spatially and temporally multiplex holographic display elements to expand the field of view, or to develop nanometer-level display elements. However, even putting aside the technical difficulties, this is not desirable because it creates another problem of having to process massive amounts of data that are difficult to process with current computers.

The ETRI research team noted that the holographic image field of view is fundamentally dependent on the holographic image resolution rather than the diffraction angle for the digital hologram pixel size.

In other words, the holographic image field of view must be redefined using the numerical aperture of the holographic display system, which determines the image resolution.

This opens up the possibility of securing a sufficient field of view even using existing commercial holographic electronic recording devices.
br /> The researchers found that the larger the numerical aperture, the better the resolution, and the numerical aperture is determined by the size of the hologram and the distance at which the image appears.

This means that the resolution varies depending on where the hologram appears, and the resolution is ultimately related to the viewing angle.

Accordingly, even if it is a holographic electronic recording device with 8 micrometer pixels, if the image is displayed at a sufficiently close distance, the field of view can be increased by 4 or 8 times, and can be made to be more than 30 degrees.

The research team analyzed digital hologram patterns to establish related theories and proved the possibility of expanding the field of view through numerical analysis and optical experiments.

It was revealed that digital holograms have the characteristic of maintaining high-frequency bands without loss even at low sampling, and thus maintain the numerical aperture that determines the hologram image resolution regardless of the pixel size of the hologram display element.

Using this principle, the research team developed an optimization algorithm to create a digital hologram extended to the high-frequency range, and analyzed the created digital hologram numerically. As a result, they discovered that the image field of view could be increased by four times from the existing 3.8 degrees to 13.1 degrees without reducing the hologram pixel size.

The researchers said there are still limitations to this achievement.

When the field of view is doubled from 3.8 degrees, the size of the hologram image must also double, but this causes the problem of the images appearing to overlap, so the higher-order diffraction terms must be filtered.

The research team plans to develop an optical filtering method that efficiently removes high-order optical diffraction terms in the future and to pursue practical research on holographic displays.

Dr. Chae Byeong-gyu of ETRI's Holographic Content Research Lab said, "The technology we developed can be applied to solve the problem of holographic near-eye display eye boxes used in augmented reality."

This study attracted attention when it was introduced as a video of the main research results on the main page of the Optical Society of America website.

Additionally, the research was published in the optical journal 'Optics Express' as 'Study on expanding the viewing angle of holographic displays'.

This research was conducted with support from the Ministry of Science and ICT's Hologram Core Technology Development Project and the ETRI Research and Development Support Project.

Meanwhile, ETRI developed the world's first 360-degree color hologram display in 2012, and in 2020, it received the highest award from the Society for Information Display (SID), the world's largest display society, for its development of a 30-degree viewing angle hologram display technology.