Switchable Lens for 3D Display, Augmented Reality and Virtual Reality Yun-Han Lee, Fenglin Peng, and Shin-Tson Wu College of Optics and Photonics, University of Central Florida, Orlando, Florida 32816, USA Abstract A switchable lens based on a twisted nematic liquid crystal cell and a polarization dependent component is demonstrated. This switchable lens has advantages in fast response time, low chromatic aberration, and low operation voltage. Its potential applications include wearable virtual reality, augmented reality, and other head mounted display devices. Keywords liquid crystal lens, 3D display, augmented reality, virtual reality. 1. Introduction Virtual reality and augmented reality [1, 2] are emerging wearable display technologies for immersive video games and interactive 3D graphics. A critical issue of these display devices is distance matching. For 3D displays based on sending different images to different eyes, e.g. Oculus Rift TM , the perceived image may locate at a distance different from the eye’s focal length, thus causing eye-brain conflict and strain. For devices such as Google Glass TM , since only one eye is receiving information, the above mentioned problem is mitigated. Instead, its major problem is the mismatch between the distance of displayed image and the surrounding image because the displayed image remains at a certain plane. In this case, the viewer cannot focus at the image from the device and the surrounding objects simultaneously. In either case mentioned above, the need for a tunable/switchable lens is apparent. Even though several types of tunable/switchable lenses have been proposed before [3], according to [4–6] to achieve a depth-fused 3D display, which forms the sense of 3D through fast switching between different focuses, it is required to switch between at least six image planes while keeping the same angular size. For a display with 60 frames per second, when switching between six focal lengths in a field sequential manner, the response time should be less than 3 ms. Among many proposed tunable/switchable lenses/ mirrors, only few candidates can achieve such a fast response time. Microelectromechanical systems (MEMS), in particular, a deformable membrane mirror device [7] is a promising candidate, while current-induced shape-changing lens [8] is another option. However, the former is relatively difficult to fabricate for large aperture and it is a reflective device, while the latter is essentially a current device, which consumes more power. In addition to flat panel displays, liquid crystals (LCs) have also been widely used in adaptive optics [9], optical coherence tomography [10], interferometry [11] and surface profilometry [12]. In this paper, we demonstrate an LC focus-switching device for 3D head-mounted displays. The LC device used here is a 90o twisted nematic (TN) cell [13]. TN is an electrically tunable achromatic half-wave (/2) plate and has been used as a polarization switch for adaptive lens [14, 15]. For a linearly polarized input light, say p- wave, the output light can be converted to s-wave if the applied voltage is off (V=0) or it remains p-wave if V>>V th (threshold voltage). For a commercial TN display, the response time can be as fast as 2 ms [16] or sub-millisecond if the cell gap is reduced to 1.6 μm [17] or a dual-frequency LC [18] is employed, while keeping the driving voltage below 10 V rms . Also, due to its achromatic nature, TN-assisted lens system has very little chromatic aberration. With these advantages, here we incorporate a TN cell as a polarization switch in our optical system. The outgoing beam would travel at different paths, depending on the p- or s-wave, when entering a polarization dependent component such as a polarizing beam splitter (PBS), a wire-grid polarizer (WGP), a dual brightness enhancement film (DBEF), or a uniaxial/biaxial plate. Upon properly controlling and recombining the output waves with retardation films, lenses, and mirrors, this path difference can be exploited to change the effective focal distance or image planes and thus effectively forms a fast switchable lens. Following this principle, different setups, such as the number of lenses/mirrors, focal length of each lens/mirror, the distance between the lens/mirror, the switching power and the direction of outgoing light, can be utilized for different applications. A special advantage of this device is that it can create depth information to a 2D image when the TN panel is pixelated. 2. Results and Discussion 2.1 Experimental setup Figure 1 shows the experimental setup to realize the focus switching. A 90° TN cell was used to switch the polarization of the input beam between s-wave and p-wave. To obtain fast response time, we infiltrated a low viscosity LC mixture DIC- LC2 ( = 32 mPa s 01 , . 21 n    and 2.0    ) into a 4.9-m TN cell. By applying 10 Vrms of an AC voltage (1 kHz frequency) to the TN cell, which was sandwiched between crossed polarizers, we measured the rise time and the decay time to be 4.3 ms and 1.0 ms, respectively. Note that here the response time was not optimized due to the limited TN cells we have. In principle, to meet Gooch—Tarry condition [19] for achromatic polarization rotation, the optimal TN cell gap should satisfy / 3 / 2, d n    where d is the cell gap and the wavelength  is usually chosen to be 550 nm. Therefore, for DIC-LC2 ( 0.121 n  ), the optimized cell gap is 3.9 m. Since the response time is proportional to d2, by reducing the cell gap from 4.9 m to 3.9 m, the rise time and the decay time can reach 2.8 ms and 0.6 ms, respectively. By using a higher birefringence LC material with a thinner cell gap, we can even obtain sub-millisecond response time [17]. As Fig. 1 depicts, our optical system consisted of a broadband PBS, two broadband /4 polymeric retarder films (450–700 nm, Edmund Optics), two lenses (L1 and L2), and two flat mirrors (M1 and M2). The first lens L1 ( 1 10 cm f  ) was located at 15 cm P  away from the object. After passing through L1, one polarization (red arrow, say s-wave) passed straight through PBS, 45°-oriented (with respect to the polarization, in the plane that was perpendicular to the propagation direction) /4 plate, and reached M1. Upon reflection, the beam passed through the /4 plate one more time and its polarization state was converted to p-wave. Thus, it was reflected by the PBS toward L2 ( 2 20 cm f  ). The total path length from L1 to L2 for the red route is 1 43 cm. d  On the other hand, the other polarization (blue arrow, say p-wave) was reflected first by the PBS toward the 45°-oriented /4 plate and M2. Upon reflection from M2, the beam passing through the /4 3-4 / Y.-H. Lee SID 2016 DIGEST • 11 ISSN 0097-966X/16/4701-0011-$1.00 © 2016 SID