Light-Field Technology for Practical 3D Displays

While most inter­action with digital content is still constrained to keyboards and 2D touch panels, augmented and virtually reality (AR/VR) tech­nologies promise ever more freedom from these limitations. AR/VR devices can have their own drawbacks, such as a tendency to induce visual motion sickness or other visual distur­bances with prolonged usage due to their stereoscopy or auto-stereo­scopy based designs. One promising solution is to adapt holo­graphy or light field tech­nology into the devices instead. This, however, requires addi­tional optics that would increase the size, weight, and cost of these devices – challenges that have so far prevented these devices from achieving commercial success.

A fabricated digitally designed holographic optical element (DDHOE) lens array (1a); The 3D display system consisting of a 2D projector and a DDHOE, 1b); A computer-modeled 3D scene of depth 6 cm (1c); A 3D recon­struction of a modeled scene captured by camera looking into the DDHOE. (1d, Source: B. J. Jackin)

Now, a group of researchers in Japan and Belgium has begun to explore a combination of holography and light field tech­nologies as a way to reduce the size and cost of more people-friendly AR/VR devices. “Objects we see around us scatter light in different directions at different intensities in a way defined by the object’s characteristic features – including size, thickness, distance, color, texture,” said researcher Boaz Jessie Jackin of the National Institute of Infor­mation and Communi­cation Tech­nology in Japan. “The modulated light is then received by the human eye and its charac­teristic features are reconstructed within the human brain.”

Devices capable of generating the same modulated light are known as true 3D displays, which includes holo­graphy and light-field displays. “Faith­fully reproducing all of the object’s features, the modulation, is very expensive,” said Jackin. “The required modu­lation is first numeri­cally computed and then converted into light signals by a liquid crystal device. These light signals are then picked up by other optical components like lenses, mirrors, beam combiners and so on.” The addi­tional optical components, which are usually made of glass, play an important role because they determine the final per­formance and size of the display device.

This is where holo­graphic optical elements can make a big difference. “A holographic optical element is a thin sheet of photo­sensitive material that can replicate the functions of one or more addi­tional optical components,” said Jackin. “They aren’t bulky or heavy, and can be adapted into smaller form factors. Fabri­cating them emerged as a new challenge for us here, but we’ve developed a solution.” Recording, or fabricating, a hologram that can replicate the function of a glass-made optical component requires that parti­cular optical component to be physically present during the recording process. This recording is an analogue process that relies on lasers and recording film; no digital signals or infor­mation are used.

“Recording multiple optical components requires that all of them be present in the recording process, which makes it complex and, in most cases, impossible to do,” said Jackin. The group decided to print/record the hologram digi­tally, calling the solution a “digitally designed holographic optical element” (DDHOE). They use a holo­graphic recording process that requires none of the optical components to be physi­cally present during the recording, yet all the optical components’ functions can be recorded.

“The idea is to digi­tally compute the hologram of all the optical functions [to be recorded and] reconstruct them together optically using a LCD and laser,” said Jackin. “This recon­structed optical signal resembles the light that is other­wise modulated by all of those optical components together. The recon­structed light is then used to record the final holographic optical element. Since the reconstructed light had all optical functions, the recorded hologram on the photo­sensitive film will be able to modulate a light with all of those functions. So all of the addi­tional optics needed can be replaced by a single holographic film.”

In terms of appli­cations, the researchers have already put DDHOE to the test on a head-up light field 3D display. The system is see-through, so it’s suitable for augmented reality applications. “Our system uses a commer­cially available 2D projector to display a set of multi-view images onto a micro-lens array sheet, which is usually glass or plastic,” said Jackin. “The sheet receives the light from the projector and modu­lates it to reconstruct the 3D images in space, so a viewer looking through the micro-lens array perceives the image in 3D.”

One big difficulty their approach overcomes is that light from a 2D projector diverges and must be made colli­mated into a parallel beam before it hits the micro-lens array in order to accu­rately reconstruct the 3D images in space. “As displays get larger, the colli­mating lens should also increase in size. This leads to a bulky and heavy lens, the system consuming long optical path length and also the fabri­cation of the collimating lens gets costly,” said Jackin. “It’s the main bottle­neck preventing such a system from achieving any commercial success.”

Jackin and colleagues’ approach completely avoids the require­ment of colli­mation optics by incor­porating its function on the lens array itself. The micro-lens array is a fabri­cated DDHOE, which includes the colli­mating functions. The researchers went on to create a head-up, see-through 3D display, which could soon offer an alter­native to the current models that use the bulky colli­mation optics. (Source: OSA)

Reference: B. J. Jackin et al.: Digitally designed HOE lens arrays for large size see- through head up displays, Present. FTh3E.1, Frontiers in Optics Conference, Washington 2018

Link: Expertise Center for Digital Media, Hasselt University, Hasselt, Belgium • National Institute of Information and Communication Technology, Tokyo, Japan

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