# Liquid-Crystal-on-Silicon for Augmented Reality Displays
## 4 LCoS for Augmented Reality Displays
### Augmented Reality Head-Mounted Displays
#### 4.1.2 Resolution  
To cover the \(\sim\)100\({}^{\circ}\) field of view (FoV) with human eye acuity 1 arcminute, ideally, a 6K6K resolution is required for each eye [105]. As a reference, the resolution of most commercially available products is \(\leq\)4K2K, which is still insufficient. Increasing the hardware resolution brings several challenges: (1) If the panel size remains the same, then increasing resolution density implies a smaller pixel size, which would result in a stronger FFE [69]. The limit of inter-pixel gap control may lead to a decreased fill factor; (2) the gate lines in an active matrix driving scheme are opened one by one. This scanning update method makes the product of panel resolution and each line's signal time (gate opening + data addressing time) a constant, i.e., the frame time. Keeping the same frame rate, an insufficient charging issue appears if the resolution is increased much; (3) the computational burden increases with the display resolution, which may decrease the source input frame rate.  
Hardware-focused solutions have been proposed to address some of the above issues. In 2004, [PERSON] et al. proposed to integrate four LCDs with a pixel-offset method to achieve 8K4K performance [106]. Besides, in foveated displays, an additional panel provides ultrahigh resolution at the central region (FoV \(\sim\) 20\({}^{\circ}\)) [107]. For the stacked methods, an optical system to accommodate the reflective LCoS without bulkiness needs to be carefully designed. In 2008, JVC demonstrated an 8K4K LCoS projection display, but a smaller pixel pitch is still desired [108].  
To detour the hardware issues, people attempt to shift the burden from the spatial domain to the time domain. The field-sequential color (FSC) method is commonly adopted in LCoS projection displays [109; 110]. That is how the 4K2K resolution is achieved. FSC display is to blink the RGB backlight in time sequence. In other words, each image frame is divided into three monochrome subframes. In this way, the resolution density could be tripled, but the trade-off is a tripled system refresh rate. For example, a 60-Hz display frame rate with FSC design needs 180 Hz color field rate. Recently, the image shifting and overlaying design were also demonstrated in AR displays [105; 111]. By generating two light fields in each frame, the perceived resolution is enhanced. Similar to an FSC display, the required field rate is doubled.  
In real-time holographic displays with phase-only LCoS, the required resolution is different from 6K6K as in intensity modulation. Taking phase grating as an example, the FoV correlates to the maximum diffraction angle, which is inversely proportional to the pixel pitch. On the other hand, the angular resolution is inversely proportional to the panel size according to the theory of Fourier transform. In the path light projected to human eyes, the applied optical system adds on factors to the inversely proportional relationships, so that the FoV and angular resolution could be increased or reduced while keeping the same ratio. To obtain a larger FoV with finer angular resolution, AR displays call for higher spatial resolution LCoS panels. Fitting in the same panel size for compact form factor, the pixel pitch decreases as the resolution increases. The state-of-the-art pixel pitch has reached 2.5-3 \(\upmu\)m. Ideally, AR displays call for a 1-\(\upmu\)m pitch. Challenges of fabricating such a small pitch come from both the electrical and optical sides. The limited space in the pixel area could accommodate very few transistors and storage capacitors, resulting in a low applicable voltage on the LC layer. The fringing  
Figure 13: An illustration of the slow LC response time-impaired gray level accuracy.  
field effect and low fill factor impair the throughput optical power. Presently, the relatively large pixel size and limited resolution provided by commercial LCoS panels cannot provide a satisfactory 3D image quality. An upgrade in hardware or new optical system design is highly desirable.