Advances and challenges in microdisplays and imaging optics for virtual reality and mixed reality

In this review, we discuss emerging approaches to advance virtual-reality (VR) displays in the following criteria: field of view (FoV), resolution density, formfactor, color gamut, and optical efficiency. To improve the resolution density and optical efficiency, two display technologies, field-sequential-color liquid-crystal displays (LCDs) with suppressed color breakup and tandem micro-organic light-emitting diodes (OLEDs), are under active development. While white micro-OLEDs can provide a high resolution density, the use of color filters dramatically decreases the optical efficiency. RGB micro-OLEDs can achieve a higher efficiency but face their own set of challenges in manufacturing. The amalgamation of these technical demands also creates additional challenges. For example, to achieve a wide FoV with a small display size requires a small f-number imaging lens; thus, a delicate optical system design is needed to mitigate the aberrations. We also discuss four main architectures of the viewing optics; i.e., Fresnel optics and three types of pancake lens: traditional, double-path, and Faraday rotator-based lens.