For decades it has been expected that near-eye displays such as augmented reality (AR) and virtual reality (VR) glasses and headsets will eventually take over conventional displays. Nevertheless, these technologies currently have barely penetrated everyday life. This hinderance can be explained by a lack of true next-generation near-eye display architectures that overcome the critical issues of stereoscopic wearables – notably vergence-accommodation conflict (VAC). The lack of such display architectures is directly related to the slow evolution and reorientation of image source industry. A major issue is the light transmission efficiency from an image-source towards the eyes of a viewer, directly impacted by the emission angle of light sources versus the need for collimated light. Collimation is a wasteful process, therefore, there is a limit to image brightness achievable with the currently available solid-state light sources. Inevitably, designers turn to more collimated light sources – lasers. While this approach yields improvements in size, it comes at the cost of image fidelity by introducing speckle patterns. Other alternatives (such as OLED microdisplays) are possible but are also not without issues. Thus, there needs to be a breakthrough in available image-sources for AR displays to reach at least a comparable image to what the 2D display counterparts can currently offer. Be it a full-color solid state uLED microdisplay, superluminescent LEDs, or developments in photonics by integration of RGB light sources into compact packages, the key-challenge is to leverage these advancements enabling a next-generation near-eye display architecture.
In this work we present reactive sputtered SiOxNy films with a variable refractive index as a convienent solution for contrast improvement of liquid crystal diffuser multi stacks in near-to-eye AR/VR displays. The focus concerns minimization of light reflections between internal structures, in particular ITO, by optimizing internal layers through tailored properties of thin film coatings, as well as subsequent laser patterning of thin film stack. Inorganic thin films have been deposited on glass by physical vapor deposition. Corresponding refractive index, thickness, uniformity and dielectric characteristics and other electro-optical properties have been measured and their impact on the resulting optical performance of the final integrated element stack has been compared against counterparts utilizing traditional polyimide and SiOx films.
Volumetric display implementations come in different forms. One of the most robust ways is an entirely solid-state volumetric architecture based on cholesteric liquid crystal optical diffuser elements. Stacking these elements enables formation of a volumetric screen – a projection volume, which is scanned time-sequentially. In this way two crucial components are working in conjunction – an image projector and an electronically switchable diffuser-element stack. LightSpace Technologies have been researching this concept of solid-state volumetric display technology since 2014. Improvements to the key enabling component – optical diffuser element – have been achieved over this period and include improved responsiveness, as well as enhanced optical characteristics – haze and transparency over visible spectrum. This work overviews and discusses key aspects of diffuser elements as well as a large-scale volumetric screen as a whole. Key characteristics of diffuser elements have been discussed and studied in regards to application in high refreshrate 3D display systems. Methods of improving optical performance of diffuser stacks have been analyzed and supported by experimental results. The base of a volumetric screen element within this work was a polymer-free chiral nematic liquid crystal filled in a homeotropic cell. A typical switching time of a full image cycle was around 1.5 ms. Influence of cell gap and driving voltage on the switching characteristics have been analyzed. In terms of volumetric screen improvements, viability and gains of lamination approach has been investigated. It has been found that even a non-ideal refractive index matching improves an overall light transmittance through a stack of diffuser elements significantly.
It is foreseen that the most convenient hardware for depiction of augmented reality (AR) will be optical seethrough head-mounted displays. Currently such systems are utilizing single focal plane and are inflicting vergenceaccommodation conflict to the human visual system – limiting wide acceptance. In this work, we analyze an optical seethrough AR head-mounted display prototype which has four focal planes operating in time-sequential mode thus mitigating limitation of single focal plane devices. Nevertheless, optical see-through nature implies requirement of very short motion-to-photon latency not to cause noticeable misalignment between the digital content and real-world scene. The utilized prototype display relies on commercial visual-SLAM spatial tracking module (Intel realsense T265) and within this work we analyzed factors improving motion-to-photon latency with the provided hardware setup. The performance analysis of the T265 module revealed slight translational and angular jitter – on the order of <1 mm and <15 arcseconds, and velocity readout of few cm/s from a completely still IMU. The experimentally determined motion-tophoton latency and render-to-photon latency was 46±6 ms and 38 ms respectively. To overcome IMU positional jitter, pose averaging with variable width of the averaging window was implemented. Based on immediate acceleration and velocity data the size of the averaging window was adjusted. To perform pose prediction a basic rotational-axis offset model was verified. Based on prerecorded head movements, a training model reduced the error between the predicted and actual recorded pose. The optimization parameters were corresponding offset values of the IMU’s rotational axis, translational and angular velocity as well as angular acceleration. As expected, the highest weight for the most accurate predictions was observed for velocities following angular acceleration. The role of offset values wasn’t significant. For improved perceived experience and motion-to-photon latency reduction we consider further investigation of simple trained neural networks for more accurate real-time pose prediction as well as investigation of content-driven adaptive image output overriding default order of image plane output in a time-sequential sequence.
We describe the sample preparation and experimental setup for second harmonic generation measurement of electro-optical (EO) chromophore/polymer system at the time of contact-poling. Different types of spacers for avoiding electric breakdown due to avalanche multiplication are compared. Electric field threshold values for second harmonic generation are observed in all samples.
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