Compact full-color augmented reality near-eye display using freeform optics and a holographic optical combiner.

We develop a compact full-color augmented reality near-eye display system with a multicolor holographic optical combiner and a freeform relay system. The digital image is produced by a full-color micro organic light-emitting diode (Micro-OLED) display module. The freeform relay system includes four freeform optics and a holographic optical mirror, which are employed to correct both the monochromatic and chromatic aberrations caused by the holographic optical combiner. The two multicolor holographic mirrors have a three-layer laminated structure and are delicately fabricated to yield an improved diffractive efficiency and a reduced efficiency difference for red, green, and blue colors. The high degrees of freedom of freeform optics, and the thin and light nature of the holographic optical combiner yield a compact form factor near-eye display system with a diagonal field of view (FOV) of 20° and the eye-box of 5 mm × 5 mm. Two prototypes are built to demonstrate the feasibility of the proposed display system.

Quantitative Response of Gray-Level Co-Occurrence Matrix Texture Features to the Salinity of Cracked Soda Saline-Alkali Soil.

Desiccation cracking during water evaporation is a common phenomenon in soda saline-alkali soils and is mainly determined by soil salinity. Therefore, quantitative measurement of the surface cracking status of soda saline-alkali soils is highly significant in different applications. Texture features can help to determine the mechanical properties of soda saline-alkali soils, thus improving the understanding of the mechanism of desiccation cracking in saline-alkali soils. This study aims to provide a new standard describing the surface cracking conditions of soda saline-alkali soil on the basis of gray-level co-occurrence matrix (GLCM) texture analysis and to quantitatively study the responses of GLCM texture features to soil salinity. To achieve this, images of 200 field soil samples with different surface cracks were processed and calculated for GLCMs under different parameters, including directions, gray levels, and step sizes. Subsequently, correlation analysis was then conducted between texture features and electrical conductivity (EC) values. The results indicated that direction had little effect on the GLCM texture features, and that four selected texture features, contrast (CON), angular second moment (ASM), entropy (ENT), and homogeneity (HOM), were the most correlated with EC under a gray level of 2 and step size of 1 pixel. The results also showed that logarithmic models can be used to accurately describe the relationships between EC values and GLCM texture features of soda saline-alkali soils in the Songnen Plain of China, with calibration R2 ranging from 0.88 to 0.92, and RMSE from 2.12 × 10-4 to 9.68 × 10-3, respectively. This study can therefore enhance the understanding of desiccation cracking of salt-affected soil to a certain extent and can also help to improve the detection accuracy of soil salinity.

Foveated light-field display and real-time rendering for virtual reality.

Glasses-free light field displays have significantly progressed due to advances in high-resolution microdisplays and high-end graphics processing units (GPUs). However, for near-eye light-field displays requiring portability, the fundamental trade-off regarding achieved spatial resolution remains: retinal blur quality must be degraded; otherwise, computational consumption increases. This has prevented synthesizing the high-quality light field from being fast. By integrating off-the-shelf gaze tracking modules into near-eye light-field displays, we present wearable virtual reality prototypes supporting human visual system-oriented focus cues. An optimized, foveated light field is delivered to each eye subject to the gaze point, providing more natural visual experiences than state-of-the-art solutions. Importantly, the factorization runtime can be immensely reduced, since the image resolution is only high within the gaze cone. In addition, we demonstrate significant improvements in computation and retinal blur quality over counterpart near-eye displays.

Synthesis of Zirconium-Containing Polyhedral Oligometallasilsesquioxane as an Efficient Thermal Stabilizer for Silicone Rubber.

Free radicals play a negative role during the thermal degradation of silicone rubber (SR). Quenching free radicals is proposed to be an efficient way to improve the thermal-oxidative stability of SR. In this work, a novel zirconium-containing polyhedral oligometallasilsesquioxane (Zr-POSS) with free-radical quenching capability was synthesized and characterized. The incorporation of Zr-POSS effectively improved the thermal-oxidative stability of SR. The T5 (temperature at 5% weight loss) of SR/Zr-POSS significantly increased by 31.7 °C when compared to the unmodified SR. Notably, after aging 12 h at 280 °C, SR/Zr-POSS was still retaining about 65%, 60%, 75%, and 100% of the tensile strength, tear strength, elongation at break, and hardness before aging, respectively, while the mechanical properties of the unmodified SR were significantly decreased. The possible mechanism of Zr-POSS for improving the thermal-oxidative stability of SR was intensively studied and it was revealed that the POSS structure could act as a limiting point to suppress the random scission reaction of backbone. Furthermore, Zr could quench the free radicals by its empty orbital and transformation of valence states. Therefore, it effectively suppressed the thermal-oxidative degradation and crosslinking reaction of the side chains.

Adaptive image reconstruction for sparse arrays using single-cycle terahertz pulses.

We demonstrate a noniterative adaptive reconstruction technique to significantly improve the imaging performance of sparse terahertz arrays employing single-cycle pulses. Grating lobe artifacts are suppressed by an adaptive weighting factor derived from the temporal coherence of signals from neighboring array elements. Image quality is further improved by a second weighting factor based on the spatial coherence of signals across the entire array. Experiments are performed with a synthetic aperture two-dimensional sparse array of 56x56 elements. Our reconstruction technique suppresses artifacts by 30 dB.

Adaptive terahertz imaging using a virtual transceiver and coherence weighting.

We demonstrate an adaptive reconstruction technique to significantly improve the depth of focus and contrast of three-dimensional reflection-mode terahertz imaging. A laterally scanned virtual transceiver element records reflections from the object of interest. A synthetic aperture focusing technique maintains fine spatial resolution over a large image depth. Measuring the spatial coherence of the received signals across the transceiver aperture provides a non-iterative self-adaptive approach to significantly improve image contrast. Test images show a spatial resolution of 0.4 mm maintained over a 16 mm depth of field, and up to a 30 dB improvement in signal-to-noise ratio.