Facebook Presents A Prototype Of Flat VR Glasses With A Holographic Screen
Developers from Facebook's research division presented a prototype of VR glasses with flat screens. They use a holographic multi-layer film instead of conventional lenses to focus the image. In the current form, the prototype, made in the shape and size of sunglasses, works with an external laser emitter and shows monochrome pictures. But engineers have already created more massive screens with a similar design, working in color mode. An article about the development will be presented at the SIGGRAPH 2020 conference, and the authors also talked about it in the company's blog.
Almost all serial VR helmets are similar to each other — they are large blocks that are applied to the eyes and nose, and fixed on the head. They are large and can be uncomfortable to wear because the center of gravity is shifted forward from the face. The reason for this lies in the classic optical scheme with a screen and a thick lens removed from the screen to focus the image into the eye. To solve this problem, engineers have to invent alternative optical circuits. For example, recently developers from Intel showed a prototype of a compact VR-helmet with a field of view of 180 degrees horizontally. This result was achieved by replacing conventional lenses with an array of the microlens.
Andrew Maimone and Jun Ren Wang from Facebook Reality Labs presented a prototype of VR glasses with a record-breaking compact optical design, which is 8.9 mm thick. Two main new elements in the development have made it possible to achieve these characteristics.
The first of them is an optical scheme that allows you to significantly shorten the distance from the display to the outer layer of the optical system. It uses several holographic films. First, the light from the display passes through a circular polarizer, then onto a beam splitter, which passes half of the light and reflects the other half to the display. The half that passes through the beam splitter is first reflected on the output layer, which is a reflecting polarizer, then it is reflected from the beam splitter and loses half the intensity, and on the second interaction with the external polarizer passes through it and is directed to the user's eye. All these reflections cause the source beam from the display to exit the optical system closer to its center than if a conventional single-lens were used.
Holographic elements have a disadvantage: they are much stronger than conventional lenses scatter light, that is, they refract it differently depending on the wavelength. When light is reflected several times in an optical system and passes through several holographic plates, this results in noticeable visual artifacts that are unacceptable in a virtual reality helmet. This problem can be almost eliminated if you use monochromatic radiation from a laser. But in this case, the radiation from the need to reflect on the liquid crystal display panel, using a large optical system.
Maimon and van solved this problem by also using a holographic plate, creating a directional display light from it. They recorded in it a holographic scheme that takes light from the end of the plate and refracts it in a perpendicular direction. This allows you to bring the laser not from behind the display, but from the side, and with the help of a waveguide.
The engineers assembled three different prototypes. Wearable prototypes use monochrome displays in the form of regular glasses, as they work with a single green laser. The resolution of each of the two displays in the wearable prototype is 1600 by 1600 pixels, but not all pixels are used, and the actual resolution is approximately 1000-1200 pixels on each side. The field of view of the glasses is 92 by 69 degrees. The developers also assembled a large desktop prototype with three different lasers, capable of displaying a color image and covering a color space exceeding sRGB.
There are also unusual developments in the field of augmented reality devices. For example, last year, engineers from NVIDIA showed a prototype of AR glasses in which they implemented foveal rendering on the hardware level. For this purpose, glasses use separate screens: one is responsible for forming the main field of view, and the second works only in a narrow part of it but displays a high-resolution image in it.