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Angle measurement is an essential component of precision measurement and serves as a crucial prerequisite for high-end manufacturing. It guides the implementation of precision manufacturing and assembly. The current angle measurement methods mainly focus on multiple axes, high precision, and large measurement ranges. This article introduces the technology of angle measurement from the perspectives of single-axis and multi-axis measurement schemes. Firstly, the single-axis measurement scheme is primarily achieved through optical methods, such as encoder discs that measure energy changes and interferometric phase changes, as well as mechanical, electromagnetic, and inertial angle measurement methods, among which interferometric methods offer the highest accuracy, with high cost, and encoder discs provide the largest measurement range with an ordinary price. Secondly, in the multi-axis measurement scheme, autocollimation instruments, including plane mirrors, gratings, and self-designed targets, are the main options. Although grating encoders can achieve three degrees of freedom in angle measurement with an ordinary price, they are limited in terms of measurement range and sensitivity compared to self-designed targets. Lastly, artificial intelligence assistance precision measurement is increasingly being embraced due to significant advancements in computer performance, making it more convenient to identify the relationship between measured values and detection values. In conclusion, angle measurement plays a crucial role in precision manufacturing, and the evolving and improving technologies provide the manufacturing industry with greater choices. The purpose of this review is to help readers quickly find more suitable technical solutions according to current application requirements, such as single/multiple axes, accuracy level, measuring range, budget, etc.

In advanced manufacturing, nanotechnology, and aerospace fields, the demand for precision is increasing. Driven by this demand, multi-degree-of-freedom grating encoders have become particularly crucial in high-precision displacement and angle measurement. Over the years, these encoders have evolved from one-dimensional systems to complex multi-degree-of-freedom measurement solutions that can achieve real-time synchronization. There can also be high-resolution feedback. Its structure is relatively compact, the signal output is also very stable, and the integration degree is high. This gives it a significant advantage in complex measurement tasks. Recently, there have been new developments. The functions of grating encoders in terms of principle, system architecture, error modeling, and signal processing strategies have all been expanded. For instance, accuracy can be improved by integrating multiple reading-heads, while innovative strategies such as error decoupling and robustness enhancement have further advanced system performance. This article will focus on the development of two-dimensional, three-dimensional and multi-degree-of-freedom grating encoders, exploring how the measurement degrees of freedom have evolved, and emphasizing key developments in spatial decoupling, error compensation and system integration. At the same time, it will also discuss some challenges, such as error coupling, system stability and intelligent algorithms for integrating real-time error correction. The future of grating encoders holds great potential. Their applications in precision control, semiconductor calibration, calibration systems, and next-generation intelligent manufacturing technologies can bring promising progress to both industrial and scientific fields.

The unique diffractive properties of gratings have made them essential in a wide range of applications, including spectral analysis, precision measurement, optical data storage, laser technology, and biomedical imaging. With advancements in micro- and nanotechnologies, the demand for more precise and efficient grating fabrication has increased. This review discusses the latest advancements in grating manufacturing techniques, particularly highlighting laser interference lithography, which excels in sub-beam generation through wavefront and amplitude division. Techniques such as Lloyd’s mirror configurations produce stable interference fringe fields for grating patterning in a single exposure. Orthogonal and non-orthogonal, two-axis Lloyd’s mirror interferometers have advanced the fabrication of two-dimensional gratings and large-area gratings, respectively, while laser interference combined with concave lenses enables the creation of concave gratings. Grating interferometry, utilizing optical interference principles, allows for highly precise measurements of minute displacements at the nanometer to sub-nanometer scale. This review also examines the application of grating interferometry in high-precision, absolute, and multi-degree-of-freedom measurement systems. Progress in grating fabrication has significantly advanced spectrometer technology, with integrated structures such as concave gratings, Fresnel gratings, and grating–microlens arrays driving the miniaturization of spectrometers and expanding their use in compact analytical instruments.

Optical interferometry provides high-precision displacement and angle measurement solutions for a wide range of cutting-edge industrial applications. One of the key factors to achieve such precision lies in highly accurate optical encoder signal processing, as well as the calibration and compensation techniques customized for specific measurement principles. Optical interferometric techniques, including laser interferometry and grating interferometry, are usually classified into homodyne and heterodyne systems according to their working principles. In homodyne interferometry, the displacement is determined by analyzing the phase variation of amplitude-modulated signals, and common demodulation methods include error calibration methods and ellipse parameter estimation methods. Heterodyne interferometry obtains displacement information through the phase variation of beat-frequency signals generated by the interference of two light beams with shifted frequencies, and its demodulation techniques include pulse-counting methods, quadrature phase-locked methods, and Kalman filtering. This paper comprehensively reviews the widely used signal processing techniques in optical interferometric measurements over the past two decades and conducts a comparative analysis based on the characteristics of different methods to highlight their respective advantages and limitations. Finally, the hardware platforms commonly used for optical interference signal processing are introduced.

An absolute-type four-degree-of-freedom (four-DOF) grating encoder that can simultaneously measure the three-axis pose (θx, θy, θz) and one-axis out-of-plane position (Z) of an object with high accuracy is demonstrated for the first time in this research. This grating encoder is composed of a stationary reading head and a movable grating reflector. A light beam from the reading head is projected onto the grating, and three diffracted beams (0th-, +1st-, and −1st-order) are generated, collimated, and received by three separate quadrant photodetectors (QPDs). The information of θx, θy, θz, and Z is coded into spot positions of these three diffracted beams on the QPDs. Thus, the modeling and decoupling algorithms were investigated, and an independent calculation of these four-DOF absolute positions was theoretically guaranteed. A prototype was then designed, constructed, and evaluated. Experimental results verified that the proposed grating encoder could achieve the absolute measurement of four-DOF θx, θy, θz, and Z with an accuracy of sub-arcseconds and sub-micrometers. To the best of our knowledge, the proposed encoder in this research is the first one to achieve absolute simultaneous measurements of four-DOF position and pose with a large measurement range. The success of this new grating encoder can benefit various multi-DOF positioning applications, especially for large-scale synthetic aperture optics (SAO), including stitching off-axis parabolic mirrors and pulse compression grating.

Absolute linear encoders have emerged as a core technical enabler in the fields of high-end manufacturing and precision displacement measurement, owing to their inherent advantages such as the elimination of the need for homing operations and the retention of position data even upon power failure. However, there remains a notable scarcity of comprehensive review materials that can provide systematic guidance for practitioners engaged in the field of absolute linear encoder measurement technology. The present study aims to address this gap by offering a practical reference to professionals in this domain. In this research, we first systematically delineate the three fundamental categories of measurement principles underlying absolute linear encoders. Subsequently, we analyze the evolutionary trajectory of coding technologies, encompassing the design logics and application characteristics of quasi-absolute coding (including non-embedded and embedded variants) as well as absolute coding (covering multi-track and single-track configurations). Furthermore, we summarize the primary error sources that influence measurement accuracy and explore the operational mechanisms of various types of errors. This study clarifies the key technical pathways and existing challenges associated with absolute linear encoders, thereby providing practitioners in relevant fields with a decision-making guide for technology selection and insights into future development directions. Moving forward, efforts should focus on achieving breakthroughs in critical technologies such as high fault-tolerant coding design, integrated manufacturing, and error compensation, so as to advance the development of absolute linear encoders toward higher precision, miniaturization, cost reduction, and enhanced reliability.

Optical metrology and perception technologies employ light as an information carrier to enable non-contact, high-precision measurement of geometry, dynamics, and material properties. They are widely deployed in industrial and consumer domains, from nanoscale defect inspection in semiconductor manufacturing to environmental perception in autonomous driving and spatial tracking in AR/VR. However, existing reviews often treat individual modalities—such as interferometry, imaging, or spectroscopy—in isolation, overlooking the increasing cross-domain integration in emerging systems. This review proposes a hierarchical taxonomy encompassing four core systems: interferometry, imaging, spectroscopy, and hybrid/advanced methods. It introduces a “theory–application–innovation” framework to unify fundamental principles, application scenarios, and evolutionary trends, revealing synergies across modalities. By mapping technological progress to industrial and societal needs, including AI-driven optimization and quantum-enhanced sensing, this work provides a structured, evolving knowledge base. The framework supports both cross-disciplinary understanding and strategic decision-making, offering researchers and engineers a consolidated reference for navigating the rapidly expanding frontiers of optical metrology and perception.

Background The rapid advancement of Internet technologies has become deeply integrated into multiple aspects of social life, providing an intelligent and diversified platform for information acquisition, social interaction and civic participation. Within the context of accelerated digitalisation and urbanisation, the association between Internet use and health among rural-to-urban migrants, as well as the potential pathways that link them, remains insufficiently explored. Methods This study utilized data from the China Family Panel Studies conducted between 2014 and 2022, focusing on a sample of 18,302 rural-to-urban migrants. Ordered logit and probit models were primarily used to examine the relationship between Internet use and self-rated health. Chronic disease status and mental health were treated as alternative health outcomes, analyzed using binary logit and OLS regressions, respectively. The instrumental variable (IV) method and propensity score matching (PSM) were adopted to address endogeneity and selection bias. Additionally, the Karlson–Holm–Breen (KHB) and an IV-based mediation method were applied to evaluate the mediating role of social integration across relational, structural, and institutional dimensions. Finally, stratified analyses were performed to explore heterogeneity across demographic subgroups and Internet use patterns, and to assess potential non-linear relationships of usage duration on health. Results The findings indicated that Internet use was positively associated with better self-rated health, lower chronic disease risk, and reduced psychological distress among rural-to-urban migrants. Social integration mediated this relationship by enhancing interpersonal interactions, fostering a sense of community belonging, and increasing institutional trust. IV-adjusted decomposition results show that, after instrumenting Internet use, the indirect associations via relational, structural, and institutional integration were 0.051 ( p  < 0.01), 0.022 ( p  < 0.01), and 0.036 ( p  < 0.05), respectively. The study also identified demographic heterogeneity, with stronger associations observed among men, middle-aged individuals, those with higher educational attainment and intraprovincial migrants. Additionally, compared to non-users, multi-type and low-frequency Internet users showed stronger positive associations with self-rated health. Finally, a nonlinear, inverted U-shaped relationship was found between Internet use duration and self-rated health, with an optimal threshold of approximately 2.69 h per day. Conclusions Improving digital accessibility and narrowing the digital divide are critical for maximizing the health benefits of Internet use among rural-to-urban migrants. Policy efforts should prioritize digital inclusivity by integrating migrants into local “Digital China” initiatives. First, digital literacy should be strengthened through accessible formats like community-based education and short videos, encouraging migrants to leverage the Internet for learning, social interaction, and healthcare. Second, inclusive digital design is essential; policies should promote age-friendly and simplified adaptations for vulnerable subgroups, such as older adults and those with lower educational attainment. Finally, promoting healthy digital habits is crucial through community-based programs that balance online engagement with offline interaction to mitigate risks of information overload and social isolation. Together, these measures will foster a health-oriented digital environment, enabling migrants to better navigate an increasingly urbanized and digitized society.

Background Among the social determinants of health, social interaction is considered a modifiable factor and an essential component of the global active ageing strategy. This study examined the associations of various types and frequencies of social interaction with health outcomes among older adults in China, while accounting for potential simultaneity and heterogeneity biases. Methods This study used data from the Chinese Health and Retirement Longitudinal Study (CHARLS), a five-wave Panel survey conducted in 2011, 2013, 2015, 2018, and 2020, comprising 38,420 observations from 7,864 individuals aged 60 years and above. Social interaction was categorised into three types: leisure-based individual interaction, community-based organisational interaction, and responsibility-driven caregiving interaction, to capture its diversity. Descriptive statistics were used to summarise health status and social interaction. Generalised estimating equation regression models were used to examine the associations between one- or two-wave-lagged social interaction and health outcomes (self-rated health, mental health, cognitive function, and diagnosed diseases). Random-effects estimation addressed individual-level heterogeneity. The 2SLS model was applied to assess potential bidirectional associations between interaction frequency and health, followed by endogeneity test. Results Social interaction was associated with more favourable health outcomes among older adults, particularly in the medium to long term. One-wave-lagged interaction was linked to better self-rated health (β = 0.014, P  < 0.05), lower mental distress (β=-0.232, P  < 0.05), and better cognitive function (β = 0.233, P  < 0.001), with no significant association with diagnosed disease. Leisure-based and community-based interactions corresponded to more favourable physical and mental health, whereas responsibility-driven interactions were associated with better cognition but also greater mental distress. Interaction frequency was positively associated with health, and was higher among those with better access to facilities and public transport. Living with children or a spouse, being employed and having a higher income tended to report more favourable health outcomes. Conclusion Active social interaction, regular participation in leisure activities, organized social activities, and informal social interactions are associated with more favourable health outcomes among older adults. Policies should prioritize supportive environments and age-friendly community renovations, while families and society should strengthen internal and external support systems to help foster active and healthy aging.

Background and AimsEndoscopic ultrasound-guided fine-needle aspiration (EUS-FNA) is increasingly being used for diagnosing lymphadenopathy. We aim to systematically review the accuracy of EUS-FNA in differentiating benign and malignant mediastinal and abdominal lymph nodes (LNs).MethodsA comprehensive literature search was performed on multiple electronic databases through February 2020. A random or fixed effect model generated the pooled sensitivity, specificity, likelihood ratio (LR), and diagnostic odds ratio (DOR) of EUS-FNA. Subgroup analyses and meta-regression were used to explore sources of heterogeneity.ResultsTwenty-six studies involving 2753 patients with 2833 LNs were included. In the differential diagnosis of benign and malignant LNs, EUS-FNA had a pooled sensitivity, specificity, positive LR, and negative LR of 87% (95% confidence interval [CI] 86–90%), 100% (95% CI 99–100%), 68.98 (95% CI 42.10–113.02), and 0.14 (95% CI 0.11–0.17), respectively. The pooled rate of adverse events associated with EUS-FNA was 1.57% (95% CI 1.06–2.24%). The summary receiver operating characteristic (SROC) yielded an area under the curve (AUC) of 0.9912. EUS-FNA performed in mediastinal LNs gained a sensitivity of 85% (95% CI 81–88%), while in abdominal LNs, it reached 87% (95% CI 82–91%). The sensitivity of the subgroup with rapid on-site evaluation (ROSE) was 91% (95% CI 89–93%), while non-ROSE was 85% (95% CI 82–87%).ConclusionsEUS-FNA is a sensitive, highly specific, and safe method for distinguishing benign and malignant mediastinal or abdominal LNs. However, the sensitivity of EUS-FNA still varies significantly among different centers.