Display technology in AR glasses: virtuality is beyond imagination, all-encompassing but based on reality

In recent years, the metaverse has been like a hot wind, blowing countless people's imagination about "technology, dreams and the future". With the popularity of the "Metaverse", AR and VR devices as its hardware carrier have become one of the important areas of technological innovation. Augmented Reality (AR) refers to a technology that allows the virtual world on the screen to combine and interact with real-world scenes by calculating the position and angle of camera images and adding image analysis technology.

AR technology mainly includes four parts: hardware, software, content and platform. In this article, we focus on the light source used to display the main hardware part. Before getting into the official content, let’s think about it first. What characteristics should we have if we want AR glasses that are as realistic as the science fiction in movies? First of all, it is necessary to achieve complete integration of reality and reality technically, and secondly, it should be no different from ordinary glasses in appearance. Achieving these two points involves dozens of factors such as weight, ergonomics, and high performance. Display technology is the key breakthrough among these difficulties.

What are the display technologies in mainstream AR glasses?

Currently, the mainstream display technologies used in AR glasses can be divided into passive micro-display technology, active micro-display technology and scanning display technology.

1. Passive microdisplay technology

   
   

Passive microdisplay technologies include traditional LCD, DLP, LCOS, etc., which require the use of RGB LEDs or RGB lasers as light sources when working. Passive microdisplay technology is already quite mature in the market. This technology can achieve the advantages of high brightness and high color gamut. However, the optical machine volume is relatively larger than other microdisplay technologies, and the light etendue is limited.

2. Active microdisplay technology

Active microdisplay technology includes display technologies using Micro OLED and Micro LED. Micro OLED, also known as silicon-based OLED, has self-luminous properties and is more suitable for use in VR glasses. If Micro OLED displays are used in AR devices, the display effect will be greatly reduced in bright scenes. The main reason is that the brightness of the current mainstream Micro OLED display technology can only reach 1000-6000 nits, and the final eye brightness may only be 200-300 nits. Micro LED has better performance in terms of efficiency, brightness, and color gamut contrast. However, because RGB integration is very difficult, the application of this technology still has many challenges.

3. Scanning display technology

Scanning display technology (LBS) uses RGB lasers as light sources and uses MEMS for scanning imaging. It combines the advantages of small size, high efficiency, high color gamut and high contrast, but the system design is relatively complex, and the interference effect of the laser can cause speckle phenomena, so the image quality of LBS technology also needs to be improved.

AR optical-mechanical design requires “trade-offs”

In AR virtual information display, the displayed information needs to be continuously adjusted according to the movements of the glasses wearer and superimposed on what the user actually sees in the real world. The computer needs to detect the environment through cameras, GPS positioning or sensor data and select the information that needs to be displayed. Therefore, when designing, engineers have to consider many factors including weight, ergonomics, display brightness, cost, etc. Various factors interact with each other. Under our current technical level, it is difficult to fully meet all requirements. We need to set different priorities based on needs and decide on relevant display solutions (i.e., light sources and optical solutions).

As a global leader in optical solutions, ams Osram has a variety of LEDs that provide light sources for AR optical machines. Among them, in the dichroic mirror solution, ams-OSRAM provides red and blue two-in-one LED-LE BR Q7WM.02, single green LED-LE T Q8WM, and converted green LED-LCG H9RM. In the light guide column solution, the LED-LE RTB N7WM is provided, which integrates three RGB chips into one package and uses the light guide column to realize the lighting scene.

In AR, dichroic mirrors and light guide pillars are commonly used light combining solutions. Generally speaking, the dichroic mirror solution can charge more light energy, so it has better color uniformity and can achieve higher display brightness. However, the dichroic mirror solution requires more optical components, which will lead to a larger size of the optical machine and also has strict requirements for assembly accuracy. The light guide column solution does not require many spectroscopes, so the assembly accuracy is low and the size of the optical machine is relatively small. However, due to the arrangement, the LED light energy that can be utilized by the display is low. At the same time, due to the arrangement position The difference will also lead to poor color uniformity.

In order to improve color uniformity, ams-OSRAM has launched a "field"-shaped LED-MOSAIC based on the original "one"-shaped arrangement of three RGB chips. It includes a version with four RGGB chips and six RRGGBB chips. version of. Compared with the original "one"-shaped arrangement, this arrangement not only improves the uniformity of color, but also further reduces the distance between the chip surface and the package surface (from the original 0.44mm to 0.15mm), which means The optics are closer to the chip, making it easier to achieve light collection and more uniform color.

So what kind of display brightness can this solution achieve?

Taking the example of AR display brightness based on RGGB MOSAIC, when the electrical power of the LED is 1W, the output luminous flux is about 50lm. After passing through the front-end optical system, it can output 10% to 20%, which means that it will be maintained before reaching the optical waveguide lens. Luminous flux of 5 to 10lm. Matching different optical waveguide types, eye brightness from 350nits to 6500nits can be achieved.

Using MOSAIC LED with LCOS or DLP can reduce the optical machine volume to 3-5 cc (cubic centimeters). This is compared to the 5-10cc optical machine volume of the traditional LED + dichroic mirror solution in terms of size and weight. All aspects have been significantly reduced. Despite this, for ordinary consumer AR, such a volume is still not an ideal state, and the size needs to be further reduced. As a result, ams Osram has developed an RGB integrated laser suitable for laser beam scanning (LBS) technology. Using this laser with a MEMS solution, the size of the entire optical machine can be reduced to less than 1cc, which is suitable for ordinary consumers. AR-like glasses have a greater promotion effect.

New R/G/B laser module

The three most important elements in the LBS solution are RGB three-color laser, beam shaping optics and scanning mirror(s). The principle is that after the RGB three-color laser is emitted from the laser module, it reaches the MEMS mirror after being collimated and combined by the optical element, and then reflected through the MEMS mirror and coupled into the optical waveguide. The optical waveguide is just like the lens of ordinary glasses. The image will be transmitted inside the optical waveguide and then finally projected to the user's eyes.

LBS technology itself is not a brand-new display technology. The three separate R/G/B TO38 lasers used in the early days had a larger optical machine size, about 1.7cc. Based on the three-in-one RGB laser launched by ams and Osram, (VEGALAS™ RGB) designed optical engine can further reduce the size to 0.7cc. The size of this laser is only 7×4.6×1.2 (mm3), and it can be directly used for SMD patches. And the use of an airtight packaging design can prevent the blue laser, especially the blue laser, from being affected by the external environment and greatly improve the reliability. One point that needs to be emphasized is that since this laser does not have integrated beam shaping optics, beam collimation and beam combining need to be implemented outside the package.

How does the brightness of the optical-mechanical display based on VEGALAS™ RGB correspond to the laser power? ams Osram made such a simple estimate. Taking the target eye brightness of 1500nits as an example, the conversion rate of the optical waveguide is about 150nits/lm, so the luminous flux needs to be about 10lm before entering the optical waveguide. After the laser is shaped and combined by optical devices, it can generally achieve a light collection efficiency of more than 50%. We can calculate that the output luminous flux of the required laser is 17lm, and then convert it into the required optical power of the three colors. The total optical power required is approximately 78mW, and then based on the electro-optical conversion efficiency that each chip can currently achieve. Calculated, approximately 0.8W electrical power input is required.

Through parameters such as the wavelength of the RGB laser, the target white point, and the equivalent white light flux@target white point, it can be calculated that the red chip needs to output an optical power of 39mW, the green one needs an optical power of 25mW, and the blue one needs an optical power of 14mW. This is the source of the previous 78mW total optical power requirement.

The future: Multi-Beam Scanning

In order to make AR glasses smaller, thinner and lighter, they can reach consumer-level technology. In addition to the VEGALAS RGB three-in-one laser currently under development, the beam scanning solution can also be expanded to multi-light speed scanning (Multi-Beam Scanning, referred to as MBS) . For example, we can create multiple emission points based on one emission point of the green laser, thereby obtaining higher and denser scanning point pixels, which can effectively improve the resolution and uniformity of the entire display. But at present, it is relatively difficult to realize multi-light speed scanning technology, and there is still a long way to go before it can be truly commercialized. However, ams and Osram are fully prepared for this and are committed to bringing consumers an "all-inclusive" visual experience in the virtual and real world.