LCOS 和 AR或VR

Companies are delivering different renditions of augmented-reality and virtual-reality  products to the market, including Google Glass, Microsoft Hololens, ODG R-7, Magic  Leap One, Oculus Rift, HTC Vive, and the Dell Visor. All these products have different  specifications and applications. This article will discuss the differences between VR and  AR, and how LCOS microdisplays can play an important role in making these applications  more viable.

VIRTUAL-REALITY (VR) headsets  have existed for more than 30 years, but major technical breakthroughs that provide a new experience to users have only arrived in the past few years. These breakthroughs include high-resolution thin-film transistor (TFT)- LCD and OLED panels, power graphics  processing units (GPUs), cloud computing, and 3D rendering software.

Augmented-reality (AR) devices are cousins to VR devices, but with different DNA. There are several differences, but the main one is that VR provides an immersive experience for users, who experience the  virtual world the system provides and are not able to see the environment around them  (Fig. 1). AR involves a wearable device that allows users to view the surrounding environment with an overlap of digital content. For example, a user can see his friend’s face with a digitally generated hat; an architect is able  to see her newly designed building at the construction site before construction has begun;  a field technician can follow 3D instructions from the other side of the world to fix a copy machine; a doctor can perform heart surgery with directions from an expert in another country (Fig. 2).

AR glasses require a complex optical  module to deliver the digital content overlap. We will briefly introduce these different  optical architectures later in this article. The following are characteristics of AR systems:  

• AR provides a see-through optical device for the user. When the AR glasses are off, the user is still able to see the environment surrounding her. AR glasses are not an immersive experience.

• AR glasses are a stand-alone device. They do not usually connect to any PC or game console. They have enough processing power to render the 3D imagery.

• The user experiences digital content as  an overlap with the real environment. 

• Currently, most AR applications are for enterprises, such as 3D models for  architecture, warehouse management,  and medical and educational applications. But more gaming applications are  coming. In summation, VR provides a virtual world to the user, whereas AR adds digital content to a real-world view. This is the key difference between these two product categories.

Liquid crystal on silicon (LCOS) is a wellknown microdisplay technology that is widely used by AR headset designers due to several key advantages that we will discuss. The  liquid crystal is sandwiched between a layer of glass and a silicon wafer (Fig. 3). The  silicon wafer’s top metal layer has two key functions: First, it is a mirror to reflect the light, and second, the mirror’s voltage drives the liquid crystal, twisting it in order to create an image. When the polarized light reflects from the mirror, the light can project through  the optical system so the user can see the image.

Fig1

Some of the optical architectures use a light guide as the eyepiece, and some of the light-guide designs use polarized light to govern the light direction (e.g., a polarizing beam splitter [PBS]). The LCOS display technology is suitable for polarizedlight optical systems because of the liquidcrystal layer.

An  optical system projects the image onto a  combiner. The user can see the digital content and the outside world through the combiner. The combiner has two functions: It reflects the projected image into the eyes and allows the eyes to see the outside world through the combiner as transparent glasses. Wearables company ODG makes products based on this architecture.

With regard to luminance, FOV, and eyebox issues, all these parameters are dependent on the optical system design. Since LCOS is very flexible for panel size and resolution, LCOS can support most of the optical architectures for different luminance, FOV, or  eye-box requirements. Although the author cannot disclose the specification of current designs, since most of them are customers’ confidential information, it is fair to say that other microdisplay technologies are able to meet some of the results, but not all of them. For example, a typical AR system with an LCOS panel of .37 inches and a resolution  of 720p can produce a 40-degree FOV at 30K nits luminance into an eye box measuring  10 mm × 10 mm.