# Reducing the Power Consumption of VR Displays with a Field Sequential Color LCD
## 1 Introduction  
Display engines play a vital role affecting the performance of virtual reality (VR) headsets [1; 2; 3]. Recently, JDI and Inmolux independently demonstrated a 2.27\" 2016 pixels per inch (PPI) active-matrix liquid-crystal display (AMLCD) with subpixel rendering for VR displays. Such a light engine offers 32 pixels per degree (PPD) and 100\({}^{\circ}\) field of view (FoV), but its aperture ratio is lower than 10%, implying a high-power consumption. To leapfrog LCD, recently eMagin introduced a 2.1\" 4K OLED-on-silicon with 2645 PPI and 42 PPD. However, for an 8\" silicon wafer, only \(\sim\)20 such panels can be fabricated, resulting in a high cost.  
To achieve 60 PPD and 100\({}^{\circ}\) FoV with a 2.27\" subpixel rendering AMLCD, the required PPI should increase to 3780. Under such a high-resolution density, the extrapolated aperture ratio would be \(\sim\)1% if subpixel rendering is not employed [4], which implies a high-power consumption. To enlarge the aperture ratio, field sequential color (FSC) is a viable approach. By eliminating the spatial color filters, both resolution density and optical efficiency are tripled. Thus, the required resolution density can be reduced from 3780 to 1260 PPI, which will enable AMLCD to leapfrog OLED-on-silicon again while keeping a reasonably large aperture ratio (\(>\)20%) for reducing the power consumption. One critical requirement for FSC operation is that the LC response time must be fast enough to avoid color breakup. One approach is to use ferroelectric liquid crystal (FLC) because of its microsecond response time [5; 6; 7]. However, the driving circuits for realizing gray levels are complicated. For example, a 3T1C (three transistors and one capacitor) pixel circuit and DC compensation driving frame are adopted to generate 8-bit gray levels for electrically suppressed helix FLC [7]. The increased space for multiple transistors would undoubtedly lower the resolution density, while the DC compensation would reduce the frame rate; both are undesirable for VR displays. In contrast, nematic LCD is easy to drive just using 1T1C (one transistor and one capacitor), but its response time is usually slower. Other approaches include polymer-stabilized blue-phase LCs and the composites of nematicLCs with nanomaterials. Polymer-stabilized blue-phase LCs can exhibit submillisecond response times, but the operation voltage is still high and slow capacitor charging requires the pre-charging method [8]. The composites of nematic LCs with nanomaterials also help reduce response time, but the stability of such composites remains to be tested [9]. Uniform dispersion of nanomaterials such as carbon nanomaterials into the LC material is also challenging [10].  
To achieve fast response time, here we evaluate a high birefringence and low viscosity nematic LC mixture. High birefringence enables a thin cell gap to be used, which helps decrease response time and crosstalk. In a pancake-lens-based VR headset, the housing temperature can easily reach 50-60 \({}^{\circ}\)C because of the thermal effect from the light engine and imaging optics. On the positive side, such an elevated temperature helps to lower the LC viscosity dramatically. A single-domain fringe-field switching (FFS) LCD [11; 12] is selected for this study because of its wide viewing angle and weak color shift. Based on these measured parameters, our simulation results show the average gray-to-gray response time is about 1.5 ms with 1.5-\(\upmu\)m electrode width, 1.5-\(\upmu\)m electrode gap, and 0\({}^{\circ}\) rubbing angle, which can support a color subframe rate of up to 360 Hz. If overdrive and undershoot voltages are applied [13], the response time can be reduced to submillisecond.