Light Measurement Method Research Articles

Research on 3D dimensional measurement of structures based on a dual monocular structured light system

In recent years, machine vision has been increasingly applied in automated monitoring of civil engineering. However, the rapid and accurate acquisition of displacement and shape changes of structures remains a challenge. In the use of structured light to measure medium and large-sized objects, there generally exist shadow and insufficient field of view problems, especially for the traditional monocular structured light measurement method. To solve these, this article discusses a dual monocular structured light measurement method, which projects two-way stripe structured light and performs three-step phase shift, also proposes a new phase-unwrapping method of structured light, and discusses a point filtering algorithm of fusion and stitching of the point cloud data. The phase-unwrapping method and the point filtering algorithm proposed, respectively, improves the measuring speed, especially for large-sized objects, and accuracy, compared with gray-coding method and traditional ICP (iterative closest point) method. This article performs three-dimensional measurements of ball work pieces, and the experimental results show that the fused and stitched data is significantly better than the single-field data, and the diameter error is <0.15mm for 35mm ball workpieces, and 0.32mm for 45mm ball workpieces. In addition, this article compares the experimental results with gray-coding method and traditional ICP method.

Accurate 3D reconstruction of highly reflective mechanical part surfaces using a structured light method with combined phase shift and gray code

The structured light measurement method is applied in quality control across various sectors including manufacturing, medicine, fashion, and technology. This approach facilitates the generation of rapid and consistent evaluation reports by analyzing errors, geometric dimensions, and deviations in measured parts, aligning them with design specifications. Such capabilities significantly enhance the speed and quality of production processes, particularly in the mechanical industry. This paper proposes an effective alternative solution for the inaccurate measurement of shiny mechanical surfaces. It explores ambient light to achieve optimal calibration and enhance measurement accuracy. The appropriate exposure time for surface measurement was determined by analyzing the image intensity histogram. This step helps avoid saturation in the detection device and ensures the generation of high-quality point clouds by merging them at suitable exposure times. To validate the effectiveness of the proposed system, two comparisons were conducted: one between the 3D profile of the part measured by commercial software versus the proposed method, and another between the 3D profile of the plastic part as measured by the proposed method versus its designed profile. The results demonstrate the efficiency of the proposed method in accurately testing full-field reflective surfaces, without requiring additional equipment or hardware.

Research on the rapid 3D measurement of satellite antenna reflectors using stereo tracking technique

Measurement and analysis of surface accuracy are crucial steps in the manufacturing process of satellite antenna reflectors. The conventional close-range photogrammetry requires attaching target points on the surface, which is time-consuming and may damage the surface quality. To overcome these drawbacks, a rapid measurement technique of non-contact three-dimensional (3D) shapes is proposed, which is based on the integration of a stereo tracking method and a structured light measurement method. This paper focuses on addressing the issue of obtaining pose transformation matrices for measurement devices in motion within this method. Initially, a plane tracking scheme and tracking plate pattern is designed. Furthermore, the feature points of the tracking plate are extracted from the target area of image processing. Moreover, the pose transformation matrix of the tracking plate relative to the fixed world coordinate system at different positions is obtained using the triangulation method. Finally, a measurement experiment is conducted, resulting in a stereo tracking scheme error within 0.5%.

Simple and precise calibration of the line-structured light vision system using a planar target.

When calibrating a line-structured light vision system using a planar target, noise easily affects the solution of the coordinates of light stripe points at the camera coordinate frame. Therefore, the planar target must be placed in the measurement space many times to capture more target images for improving calibration stability and achieving relatively high calibration accuracy. This complicates the calibration process. This paper proposes a calibration method considering the measurement baselines of a planar target. The planar target is placed only two times, and two target images are captured correspondingly. A three-point subset is made up of the two calibration points that form the measurement baseline with the longest 2D projection and any other calibration point. In this way, it is less affected by noise when using the three-point subsets to establish the equations. Then, we use the lengths of the measurement baselines provided by all three-point subsets and their 2D projections to solve the coordinates of light stripe points at the camera coordinate frame more accurately to calibrate the line-structured light vision system. Both the simulation and actual experiment results demonstrate the feasibility of our method. Based on our calibration method, the RMS error is 0.035mm for length measurement and 0.054mm for height measurement. Compared with other existing methods, our method needs only two target images. It can also achieve more accurate calibration results than the other methods. In addition, our calibration method increases the applicability of the line-structured light measurement method by reducing the number of target swings.

Line-encoded structured light measurement method in measuring shiny and transparent objects

Structured light measurement (SLM) has been widely used in acquiring the three-dimensional (3D) shape of objects because of its high precision and high speed. However, the intensity based structured light coding strategies like sinusoidal patterns is difficult to measure transparent and shiny objects with high dynamic range surface reflectance due to inaccurate light intensity information. Divergent multi-line structured light projection and traditional gray-code method avoids this problem by extracting lines. However, traditional gray-code method requires additional many patterns to distinguish the order of lines, which reduces the measurement efficiency. To overcome defects, this paper proposes a line-encoded few-pattern SLM method in measuring shiny and transparent objects. In this method, we simultaneously extract the edge line, the center line and obtain their orders by projecting only five patterns. Many experiments were implemented to prove its accuracy and robustness. Because only five patterns are required, the proposed method greatly accelerate the process of reconstruction, which is indispensable for 3D real-time reconstruction. We conducted a dental model dynamic scanning experiment to prove it.

Fast Multi-Line Structured Light Measurement Method Integrated with Region Matching

Fast Multi-Line Structured Light Measurement Method Integrated with Region Matching

A Three-Dimensional Structured Light Vision System by Using a Combination of Single-Line and Three-Line Lasers

A multi-line structured light measurement method that combines a single-line and a three-line laser, in which precision sliding rails and displacement measurement equipment are not required, is proposed in this paper. During the measurement, the single-line structured light projects onto the surface of an object and the three-line structured light remains fixed. The single-line laser is moved and intersects with the three-line laser to form three intersection points. The single-line light plane can be solved using the camera coordinates of three intersection points, thus completing the real-time calibration of the scanned light plane. The single-line laser can be scanned at any angle to determine the overall complete three-dimensional (3D) shape of the object during the process. Experimental results show that this method overcomes the difficulty of obtaining information about certain angles and locations and can effectively recover the 3D shape of the object. The measurement system's repetition error is under 0.16 mm, which is sufficient to measure the 3D shapes of complicated workpieces.

Study on microscopic fringe structured light measurement method for tiny and complex structural components

Study on microscopic fringe structured light measurement method for tiny and complex structural components

A virtual binocular line-structured light measurement method based on a plane mirror

A virtual binocular line-structured light measurement method based on a plane mirror is proposed. A checkerboard target and its virtual image in a front coating plane mirror are captured by a camera in a single picture. The target is imaged in several positions to determine the spatial relationship between the camera and plane mirror. The centers of light stripes are matched with its mirror images by the vanishing point constraint between the checkerboard coplanar point and its corresponding mirrored point. By scanning the object on a moving stage, 3D point cloud data can be obtained. Compared with the traditional binocular line-structured light measurement system, the proposed method just needs a single camera, and two images can be obtained in one procedure. It improves the measurement speed, reduces the cost of measurement, and avoids measurement errors caused by non-strict synchronization between the left and right camera in binocular vision. Experimental results demonstrate good performance with the ability to reconstruction surfaces, and measurement accuracy (RMSE = 0.032 mm) can meet the industrial requirement.

A Multi-View Structured Light Measurement Method Based on Pose Estimation Using Deep Learning

To enhance the effectiveness and flexibility of the data alignment in the multi-view measurement system, a measurement strategy based on pose estimation using deep learning is proposed. The object pose is estimated and established through a single-shot pose estimation network. Then, the coarse alignment of the data acquired from different views is performed using the estimated 6D pose. The ICP algorithm is utilized for global refinement. Different shapes are used to verify the effectiveness, robustness, and flexibility of the deep learning-based multi-view measurement strategy. Furthermore, error comparisons of data fusion using markers and deep learning are implemented. The translation error of pose estimation is 1.8-5 mm, and the angle error can reach 0.5-1 degree. The difference between the marker-based and proposed data alignment method is only 0.02 mm. The proposed method can achieve comparable data alignment accuracy with the marker-based method. Moreover, it increases the flexibility and convenience of the data alignment and provides an improved way for existing marker- and shape-based multi-view measurement systems.

A Light Measurement Method for 9–150 kHz Disturbances in Power Grids Comparable to CISPR Quasi-Peak

This study proposes a novel measurement method to assess the disturbances in the electrical grid for the CISPR Band A (9 to 150 kHz), as no normative grid measurement method for these frequencies exists yet. Compatibility levels in IEC 61000-2-2 in this frequency range are defined based on the CISPR 16-1-1 method using the quasi-peak (QP) detector. However, this method is not directly applicable for grid measurements as it was originally designed for laboratory conditions and for measuring radio disturbances and immunity. The method proposed in this paper (Light-QP method) overcomes these limitations, along with lower complexity, computational burden and memory requirements than CISPR 16-1-1. The Light-QP method uses a digital quasi-peak detector that processes Root Mean Square (RMS) values of spectral components, calculated by adapting the IEC 61000-4-7 standard to the CISPR Band A. The proposed Light-QP method is applied to real measurements from a Low Voltage (LV) distribution grid and compared to the quasi-peak outputs from a digital implementation of the CISPR 16-1-1 method. The results are comparable and can be used for assessment against compatibility levels. The Light-QP method has been presented to IEC SC77A/WG9 for its potential inclusion as a normative measurement method in a new version of the IEC 61000-4-30 standard.

Adaptive Fringe Projection for 3D Shape Measurement with Large Reflectivity Variations by Using Image Fusion and Predicted Search

There is always a great challenge for the structured light technique that it is difficult to deal with the surface with large reflectivity variations or specular reflection. This paper proposes a flexible and adaptive digital fringe projection method based on image fusion and interpolated prediction search algorithm. The multiple mask images are fused to obtain the required saturation threshold, and the interpolated prediction search algorithm is used to calculate the optimal projection gray-level intensity. Then, the projection intensity is reduced to achieve coordinate matching in the unsaturated condition, and the adaptive digital fringes with the optimal projection intensity are subsequently projected for phase calculation by using the heterodyne multifrequency phase-shifted method. The experiments demonstrate that the proposed method is effective for measuring the high-reflective surface and unwrapping the phase in the local overexposure region completely. Compared with the traditional structured light measurement methods, our method can decrease the number of projected and captured images with higher modulation and better contrast. In addition, the measurement process only needs two prior steps and avoids hardware complexity, which is more convenient to apply to the industry.

Research Progress of Automated Visual Surface Defect Detection for Industrial Metal Planar Materials.

The computer-vision-based surface defect detection of metal planar materials is a research hotspot in the field of metallurgical industry. The high standard of planar surface quality in the metal manufacturing industry requires that the performance of an automated visual inspection system and its algorithms are constantly improved. This paper attempts to present a comprehensive survey on both two-dimensional and three-dimensional surface defect detection technologies based on reviewing over 160 publications for some typical metal planar material products of steel, aluminum, copper plates and strips. According to the algorithm properties as well as the image features, the existing two-dimensional methodologies are categorized into four groups: statistical, spectral, model, and machine learning-based methods. On the basis of three-dimensional data acquisition, the three-dimensional technologies are divided into stereoscopic vision, photometric stereo, laser scanner, and structured light measurement methods. These classical algorithms and emerging methods are introduced, analyzed, and compared in this review. Finally, the remaining challenges and future research trends of visual defect detection are discussed and forecasted at an abstract level.

Development of an In-Situ Laser Machining System Using a Three-Dimensional Galvanometer Scanner

Abstract In this study, a three-dimensional (3D) in-situ laser machining system integrating laser measurement and machining was built using a 3D galvanometer scanner equipped with a side-axis industrial camera. A line structured light measurement model based on a galvanometer scanner was proposed to obtain the 3D information of the workpiece. A height calibration method was proposed to further ensure measurement accuracy, so as to achieve accurate laser focusing. In-situ machining software was developed to realize time-saving and labor-saving 3D laser processing. The feasibility and practicability of this in-situ laser machining system were verified using specific cases. In comparison with the conventional line structured light measurement method, the proposed methods do not require light plane calibration, and do not need additional motion axes for 3D reconstruction; thus they provide technical and cost advantages. The in-situ laser machining system realizes a simple operation process by integrating measurement and machining, which greatly reduces labor and time costs.

Measuring Daylight: A Review of Dosimetry in Daylight Photodynamic Therapy.

Successful daylight photodynamic therapy (DPDT) relies on the interaction of light, photosensitisers and oxygen. Therefore, the ‘dose’ of light that a patient receives during treatment is a clinically relevant quantity, with a minimum dose for effective treatment recommended in the literature. However, there are many different light measurement methods used in the published literature, which may lead to confusion surrounding reliable and traceable dose measurement in DPDT, and what the most appropriate method of light measurement in DPDT might be. Furthermore, for the majority of practitioners who do not carry out any formal dosimetry and for the patients receiving DPDT, building confidence in the evidence supporting this important treatment option is of key importance. This review seeks to clarify the methodology of DPDT and discusses the literature relating to DPDT dosimetry.

Observing the Impact of WWF Earth Hour on Urban Light Pollution: A Case Study in Berlin 2018 Using Differential Photometry

Earth Hour is one of the most successful coordinated mass efforts worldwide to raise awareness of environmental issues, with excessive energy consumption being one driver of climate change. The campaign, first organized by the World Wildlife Fund in Australia in 2007, has grown across borders and cultures and was celebrated in 188 countries in 2018. It calls for voluntarily reduction of electricity consumption for a single hour of one day each year. Switching off non-essential electric lights is a central theme and resulted in 17,900 landmarks going dark in 2018. This switch-off of lights during Earth Hour also leads to reduction of light pollution for this specific period. In principle, Earth Hour allows the study of light pollution and the linkage to electricity consumption of lighting. However, quantitative analysis of the impact of Earth Hour on light pollution (and electricity consumption) are sparse, with only a few studies published showing no clear impact or the reverse, suffering from residual twilight and unstable weather conditions. In this work, light pollution measurements during Earth Hour 2018 in an urban park (Tiergarten) in Berlin, Germany, are reported. A novel light measurement method using differential photometry with calibrated digital cameras enables tracking of the switching off and switching back on of the lights of Berlin’s iconic Brandenburg Gate and the buildings of Potsdamer Platz adjacent to the park. Light pollution reduction during the event was measurable, despite the presence of moonlight. Strategies for future work on light pollution using such events are discussed.

Prediction of weld bead geometry of MAG welding based on XGBoost algorithm

Evaluating the welding joint quality in real time is difficult for chassis parts robotic gas-shielded welding. Series of metal active gas (MAG) joints were conducted in this paper to investigate the relationship between welding current, welding speed, energy input, and weld bead geometry. Bead width and bead reinforcement are obtained using a line-structured light measurement method, and the penetration depth of the bead is measured with the macroscopic metallurgical microscope. The ratio of penetration depth to the plate thickness and reinforcement is chosen as the evaluation criterion of the joint quality. Based on XGBoost algorithm, two data-driven models are proposed to recognize penetration status and predict the bead reinforcement. In the prediction results, the absolute error of the penetration coefficient is 0.079 at the maximum, and the average relative error is 11.06%. For the test result of reinforcement prediction model, the relative error is 20.5% on average. The test results show that the XGBoost-based models can be used for real-time prediction of welding quality.

The dark art of light measurement: accurate radiometry for low-level light therapy.

Lasers and light-emitting diodes are used for a range of biomedical applications with many studies reporting their beneficial effects. However, three main concerns exist regarding much of the low-level light therapy (LLLT) or photobiomodulation literature; (1) incomplete, inaccurate and unverified irradiation parameters, (2) miscalculation of ‘dose,’ and (3) the misuse of appropriate light property terminology. The aim of this systematic review was to assess where, and to what extent, these inadequacies exist and to provide an overview of ‘best practice’ in light measurement methods and importance of correct light measurement. A review of recent relevant literature was performed in PubMed using the terms LLLT and photobiomodulation (March 2014–March 2015) to investigate the contemporary information available in LLLT and photobiomodulation literature in terms of reporting light properties and irradiation parameters. A total of 74 articles formed the basis of this systematic review. Although most articles reported beneficial effects following LLLT, the majority contained no information in terms of how light was measured (73 %) and relied on manufacturer-stated values. For all papers reviewed, missing information for specific light parameters included wavelength (3 %), light source type (8 %), power (41 %), pulse frequency (52 %), beam area (40 %), irradiance (43 %), exposure time (16 %), radiant energy (74 %) and fluence (16 %). Frequent use of incorrect terminology was also observed within the reviewed literature. A poor understanding of photophysics is evident as a significant number of papers neglected to report or misreported important radiometric data. These errors affect repeatability and reliability of studies shared between scientists, manufacturers and clinicians and could degrade efficacy of patient treatments. Researchers need a physicist or appropriately skilled engineer on the team, and manuscript reviewers should reject papers that do not report beam measurement methods and all ten key parameters: wavelength, power, irradiation time, beam area (at the skin or culture surface; this is not necessarily the same size as the aperture), radiant energy, radiant exposure, pulse parameters, number of treatments, interval between treatments and anatomical location. Inclusion of these parameters will improve the information available to compare and contrast study outcomes and improve repeatability, reliability of studies.

球面反射式日冕仪实验装置的杂光分析和实验验证

This paper focuses on the imaging and stray light distribution levels of a Lyman alpha reflection coronagraph, and attempts to get a reliable stray light measurement method. A new sphere reflection coronagraph experimental device is designed and manufactured based on the design principles of coronagraphes to observe the corona radiation in Lyman alpha region when the sun is with a solar diameter (Rinfo/inf) of 1.3~2.5Rinfo/inf. The imaging resolution of the experimental device is measured. The results show that the spatial resolution of the system is 17.96 lp/mm, and in agreement with the design results. A coronagraph simulation model is established by using the Trace pro software, and the stray light distributions in the meridian direction are compared with the simulation model and the experimental device at a 0 field. By comparison, a Point Source Transmittance (PST) curve of the coronagraph experimental device is obtained, and the corresponding stray light of the device is 0.278. Moreover, the main source of stray light is also analyzed. , 2015, Chinese Academy of Sciences. All right reserved.

High Resolution Measurement of Light in Terrestrial Ecosystems Using Photodegrading Dyes

Incoming solar radiation is the main determinant of terrestrial ecosystem processes, such as primary production, litter decomposition, or soil mineralization rates. Light in terrestrial ecosystems is spatially and temporally heterogeneous due to the interaction among sunlight angle, cloud cover and tree-canopy structure. To integrate this variability and to know light distribution over time and space, a high number of measurements are needed, but tools to do this are usually expensive and limited. An easy-to-use and inexpensive method that can be used to measure light over time and space is needed. We used two photodegrading fluorescent organic dyes, rhodamine WT (RWT) and fluorescein, for the quantification of light. We measured dye photodegradation as the decrease in fluorescence across an irradiance gradient from full sunlight to deep shade. Then, we correlated it to accumulated light measured with PAR quantum sensors and obtained a model for this behavior. Rhodamine WT and fluorescein photodegradation followed an exponential decay curve with respect to accumulated light. Rhodamine WT degraded slower than fluorescein and remained unaltered after exposure to temperature changes. Under controlled conditions, fluorescence of both dyes decreased when temperatures increased, but returned to its initial values after cooling to the pre-heating temperature, indicating no degradation. RWT and fluorescein can be used to measure light under a varying range of light conditions in terrestrial ecosystems. This method is particularly useful to integrate solar radiation over time and to measure light simultaneously at different locations, and might be a better alternative to the expensive and time consuming traditional light measurement methods. The accuracy, low price and ease of this method make it a powerful tool for intensive sampling of large areas and for developing high resolution maps of light in an ecosystem.