Due to the influence of science fiction movies ("Iron Man" and "Star Trek", etc.), people have been glaring at the emergence of holographic optical lenses. So what is the working principle of the augmented reality display? Game developer Aaron Yip answered the question on Quora (a famous foreign question and answer website). The following is the text.
Let us start with the basics. We have these partially transparent displays that mix digital images with the real world. Light needs to bounce on something to redirect to your eyes. In the real world, we have got redirected light. For the digital world, we need to create artificial rays (for example, through LEDs, OLEDs) and then redirect them. An optical device that combines the generated computer image with the real world is called a "combiner." Basically, the combiner works like a partial mirror, redirecting the display light and selectively passing light through the real world. this is very simple.
Optical hardware solutions can be divided into two categories: conventional HMD optical combiners and emerging waveguide combiners. Both are very different and have very different tradeoffs.
Perspective displays have appeared since the 1960s. Thus, this produces many different optical techniques, but is basically a trade-off between resolution, field of view, eye box, image quality, hardware weight/adaptation, shape parameters, and other features. Ideally, everyone wants stylish, lightweight glasses with a 200 x 100 degree field of view (matching the human eye) and the perfect image quality invented by Iron Man's protagonist Tony Stark. However, due to physical and optical limitations of head-on/near-eye displays and the like, this has become an unrealistic fantasy in the foreseeable future. So we need to think about the trade-offs mentioned above.
Optical hardware is completely weighed
Traditional combiners produce reasonable perspective and image quality, consistent performance and affordable materials thanks to decades of supply chain development. The following figure shows two common implementations: a polarized beam combiner as an example of a planar combiner (upper left); an off-axis combiner (upper right) as an example of a curved combiner.
Examples of polarized beam combiners include Google Glass, as well as smart glasses from Epson, Rockchip, and the Taiwan Industrial Technology Research Institute. The beam splitter can be polarized using an LCOS (liquid crystal on silicon) microdisplay, such as Google Glass; or just a simple halftone mirror. Unfortunately, due to the weight and size limitations of the combiner, the field of view of a display based on a polarized beam combiner is typically small and there may be additional blurring caused by beam splitting, resulting in lower resolution. The field of view of Google Glass is 13 degrees FOV, while the Epson BT-300 is 23 degrees and the resolution is 1280×720. Both are at the low end of the consumer display's acceptable range. However, larger FOV and/or resolution will require larger and heavier hardware.
Advantages: light, small, relatively affordable ($500-$700)
Disadvantages: limited field of view and resolution, difficult to improve.
The best modern example of an off-axis, hemispherical combiner is me ta 2. Unlike other small and light combiners, me tends to have a larger FOV and display resolution. They introduced a single OLED panel to support "almost 90 degrees FOV" and 2560 x 1440 pixels. However, their hardware is huge, similar to VR heads (such as Oculus and HTC Vive ). Additional issues include lower angular resolution (less detail/images are not clear enough) and how the plastic material of the combiner maintains its mass (for example, slight jitter over time will be enhanced, possibly leading to final vision) Illusion). But this is the choice they make to reduce costs. Another earlier example of a curved combiner is the Advanced Helmet Mounted Display.
Advantages: Wide field of view and high resolution, relatively affordable (around $900)
Disadvantages: large and bulky, low angular resolution, material quality risk.
As you can see, trying to improve traditional combiners in FOV and resolution means smaller eyeboxes, thicker combiner optics, larger combiners, and/or worse image quality. It has nothing to do with computing performance limitations, but rather with the performance characteristics of the hardware.
To address this hard trade-off, new technologies are using unconventional technologies such as holography and diffractive optics. These techniques use a so-called waveguide grating or waveguide hologram to gradually extract a collimated image guided by total internal reflection (TIR) ​​in the waveguide. A waveguide is a sheet made of glass or plastic in which light is reflected. In fact, you can think of a waveguide as a router that transmits images in front of your eyes.
Waveguides are the most technically complex see-through optics and they are equally difficult to design. However, these are not new concepts. Optical waveguides have been explored since the early 1980s. Since then, companies such as Sony (Figure 2), Konica Minolta (pictured above), Nokia/Microsoft (pictured below), and Magic Leap have been studying various waveguide combiners.
For example, a sub-wavelength grating with a tilted surface is typically a hypothetical implementation for Microsoft Hololens. Here, the waveguide has a series of very fine structures (close to the wavelength of light) in a linear array. The diffraction grating bends the light like a lens until it is directed at the eye. The pleasant result of this process is "pupil expansion", where the exiting light can be slightly diffused to increase its FOV.
All in all, the most advanced waveguide technology may approach 32Hx18V FOV at 1920x1080 resolution, which may not be as bulky and weighty as traditional combiner solutions. The Magic Leap patent shows that its technology is eager to approach a horizontal FOV of 120Hx80V, but perhaps it is ultimately close to a FOV of 50-55 degrees. Compared to traditional methods, Magic Leap's technology may be more promising, or at least there may be more hype. But they have not given too much proof of demonstration so far. In addition, waveguide combiners are also challenging.
First, waveguides require very high precision, while fine volume holographic media such as photopolymers, dichromated gelatin, silver halide, etc., can vary depending on ambient temperature, humidity, and/or pressure. Second, the angular resolution decays with more diffusion (ie, the tradeoff between FOV and imaging detail). Finally, the supply chain of related technologies has not yet been established, so mass production is difficult and costly. Not to mention the two companies up to $1 billion + ongoing research and development costs.
Advantages: It is possible to achieve higher field of view and resolution on medium sized devices.
Disadvantages: Expensive (expected at $3,000 or more), technology still needs to be actively improved.
In summary, the main topics discussed in this paper are traditional techniques that have been proven and relatively extensively explored, as well as experimental techniques with a lot of hype. Personally, I think it's reasonable to have a distrust of the waveguide technology. After all, there are still no public demos demonstrating that the effect is better than the traditional combiner. On the other hand, I also think that these huge investments are very reasonable.
Consumers affected by sci-fi works are indifferent to most/all of the traditional combiner hardware. In the optical development of the past fifty years, AR has only been used as a niche product. There may be some interesting improvements in traditional combiners (eg ODG glasses), but for Microsoft and Magic Leap, Wave Technology is AR's moon landing project and hopefully will be accepted by the mass market.
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