Laser Line Scan System Research Articles

Assessment of out‐of‐plane structural defects using parallel laser line scanning system

AbstractA precise parallel laser line scanning system has been developed to assess the depth of out‐of‐plane structural defects on concrete surfaces. This system comprises a digital camera, dual line laser diodes, and positioning rigid arms that create a triangulation‐based setup. Laser lines are distorted when projected onto an out‐of‐plane defect. A new image processing algorithm has been devised to extract depth information from the distorted laser strips. Parallel laser lines are used to ensure that imaged laser strips do not intersect, thereby simplifying the depth assessment of defects at different distances and enabling the generation of defect profiles from a single image. The system has been validated through laboratory and field tests, demonstrating its effectiveness and accuracy. Compared to other noncontact measurement techniques, this system stands out due to its simplicity, cost‐effectiveness, efficiency, and superior accuracy for long‐range measurements, rendering it suitable for on‐site scanning of textureless uneven engineering objects.

Kinematic Modeling of an Automated Laser Line Point Cloud Scanning System

This paper describes the geometric transformation of coordinates in the elements of an automated laser line scanning system caused by movements required for scanning a component surface. The automated scanning system elements are a laser line scanner mounted on a robot arm or a probe tip with coordinate measuring machine. A major challenge is determining the kinematic relationship between the elements of an automated laser scanning system that functions as a mechanical linkage in order to obtain a trajectory for scanning a surface. This paper solves the forward kinematic equations to get the validation pose for the elements of the system, derives the equation of component surface considering a spherical component, and uses inverse kinematics to characterize the movement of the entire automated scanning system on the point trajectory. The result is the position and orientation matrix for the laser line scanner beam and its movement on the trajectory in relation to a component surface. This kinematic chain of linkages and the obtained poses of point trajectory on the component surface is a starting point for future work that aims to develop optimal scan paths that can collect “best” point cloud datasets. The outcomes of this work contribute to enhancing the use of laser line scanners for inspection of component surfaces in manufacturing, remanufacturing, and reverse engineering applications.

3D color reconstruction based on underwater RGB laser line scanning system

Laser line scanning (LLS) as an optical 3D survey method is recently used for underwater 3D imaging. This paper presents an approach for both 3D reconstruction and color restoration based on the LLS system. The system is set up with RGB laser scanning components for acquiring 3D data and color data at the same time. Direct calibration algorithms and RGB additive color model are involved for processing 3D color data. The experiment result shows that underwater targets can be feasibly 3D reconstructed with color texture simultaneously, at millimeter scales.

An experimental evaluation method for the performance of a laser line scanning system with multiple sensors

Laser line scanning 3D digitising systems have a wide range of applications. Their working performance is mainly determined by the system calibration procedure and is also affected by the working conditions, CCD camera imperfections, and object surface optical characteristics. Therefore, a comprehensive evaluation of working performance is necessary before and during use. This study proposes an experimental method for the performance evaluation of a laser line scanner (LLS) with 8 scanning sensors developed in our laboratory. This method first obtains the dense point clouds of standard parts composed of disks, cylinders, and squares. Next, the single-layer point clouds located in horizontal planes of different heights are fitted using the least squares method to obtain the enclosed contours S. Three parameters, namely, the standard deviation of the distance distribution between points and S, the mean distance of the distance distribution, and the shape feature sizes, are used to evaluate the performance. The proposed method evaluates both the scanner as a whole and each scanning sensor. Using this method, more comprehensive information can be acquired to evaluate the scanner performance. The experimental results show that the absolute dimension size error and relative error are less than 5mm and 3%, respectively, and the relative shape error is less than 2%; therefore, the evaluated LLS system can meet the requirements for human anthropometry applications. Although each scanning sensor has different random and systematic error, these errors are the function of measurement depth. These conclusions are helpful for the further use of this scanner system and can be utilised to optimise this LLS system further.

Visualization and Image Enhancement for Multistatic Underwater Laser Line Scan System Using Image-Based Rendering

Over the last several decades, developments in underwater laser line scan (LLS) serial imaging sensors have resulted in significant improvements in turbid water imaging performance. In the last few years, there has been renewed interest in distributed, truly multistatic time-varying intensity (TVI) (i.e., multiple transmitter nonsynchronous LLS) sensor configurations. In addition to being capable of high-quality image acquisition through tens of beam attenuation lengths, while simultaneously establishing a non-line-of-sight free-space communications link, these system architectures also have the potential to provide a more synoptic image coverage of larger regions of seabed and the flexibility to simultaneously examine a target from different perspectives. A related issue worth investigation is how to utilize these capabilities to improve rendering of the underwater scenes. In this regard, light field rendering (LFR)-a type of image-based rendering (IBR) technique-offers several advantages. Compared to other IBR techniques, LFR can provide signal-to-noise ratio (SNR) improvements and the ability to image through obscuring objects in front of the target. On the other hand, multistatic nonsynchronous LLS can be readily configured to acquire image sequences needed to generate LFR. This paper investigates the application of LFR to images taken from a distributed bistatic nonsynchronous LLS imager using both line-ofsight and non-line-of-sight imaging geometries to create multiperspective rendering of an unknown underwater scene. The issues related to effectively applying this technique to underwater LLS imagery are analyzed and an image postprocessing flow to address these issues is proposed. The results from a series of experiments at the Harbor Branch Oceanographic Institute at the Florida Atlantic University (HBOI-FAU, Fort Pierce, FL, USA) optical imaging test tank demonstrated the capability of using bistatic/multistatic nonsynchronous LLS system to generated LFR and, therefore, verify the proposed image processing flow. The benefits of LFR to underwater imaging in challenging environments were further demonstrated via imaging against a variety of obstacles such as mesh screens, bubbles, and water at different turbidity. Image quality metrics based on mutual information and texture features were used in the analysis of the experimental results.

The Investigation of Arch Model Acting in Mass-Flow Hoppers

This paper presents the experimental results of mass-flow hopper arch geometry investigation, which was conducted using a variable geometry plane-flow bin. The cohesive arches formed under different critical outlet openings and hopper half-angles were measured using a 360° two-dimensional laser line scan system. This system was employed to obtain the complete surface profile of each arch across the width of the outlet by moving the rotating laser along the total length of the outlet. The test results were analyzed using Matlab, adopting stationary wavelet transformation de-noising to decrease the signal noise generated during the testing process. The geometric data for each single line scan was smoothed and combined to present a three-dimensional arch surface profile shown to be in good agreement with the observed experimental arch profiles. The angle η at the intersection of the arch with the hopper walls was then calculated by running a Matlab program and a new angle η' is introduced to the arch shape study. The detailed results are discussed in the paper. Arch geometry models, such as the parabolic arc and circular arc arch models developed, respectively, by Walker [ and Enstad [ are reviewed and their relevance is discussed based on the experimental results presented in this paper.

Multicolor Super-Resolution Imaging using a Laser Line Scanning Hyperspectral Microscope

Single-molecule super-resolution imaging techniques have proven to be successful in observing cellular structures with a resolution below the diffraction limit. However, these experiments often restrict observations to a region of shallow z-depth near the cover slip due to TIRF imaging or else suffer from high backgrounds due to wide field excitation. Confocal microscopy, which offers good background rejection, is prohibitively slow and cannot be used effectively for single-molecule localization based super-resolution. Here we introduce a new method that uses a fast, laser line scanning system that can be used to image a z-section anywhere within a cell and is not restricted to regions near the cover slip. The line scanning system allows a trade-off between acquisition speed and background rejection. Emission light is passed through a prism spectrometer, and a complete spectrum is collected for each spatial pixel. We exploit this hyperspectral emission path to make simultaneous measurements of multiple fluorophore species in the same sample.

Enhanced extended range underwater imaging via structured illumination

In this article, the utility of structured illumination in order to enhance the contrast and subsequent range capability of an underwater imaging system is explored. The proposed method consists of transmitting a short pulse of light in a grid like pattern that consists of multiple, narrow, delta-function like beams. The grid pattern can be arranged in either a one-dimensional line or an area as a two-dimensional pattern. Scanning the pattern in time results in the sequential illumination of the entire scene. The receiving system architecture imposes the exact same, grid-like pattern sensitivity on the reflected light with a simple subsequent superposition of the time-sequenced images. The system can be viewed as a parallel implementation of a Laser Line Scan System where multiple beams are projected and received instead of a single one. The performance enhancement over more conventional systems that project either a sheet or an area of light is compared for a challenging underwater environment via computer simulations. The resulting images are analyzed as a function of the spacing between the projected light beams to characterize contrast and resolution. The results indicate that reasonable gains are obtainable for close spacing between the beams while quite significant gains are predicted for larger ones. Structured illumination systems can therefore collect images more rapidly than systems that scan a single beam; however with concomitant trade-offs in contrast and resolution.

Performance considerations for continuous-wave and pulsed laser line scan (LLS) imaging systems

Extended range underwater optical imaging techniques can be classified into one of two broad categories; those which use synchronously scanned narrow source and receiver paths to restrict both back-scattered and forward-scattered light reaching the receiver (continuouswave laser line scan); and those which use pulsed sources and time-gating to remove back-scatter noise (pulsed laser line scan and pulse-gated laser line scan). Laser line scan systems are observed to perform at up to 5 to 6 optical attenuation lengths, but greater standoff distances are desirable for seabed imaging using the growing fleet of autonomous underwater vehicles (AUVs). Currently, a moderate physical separation between laser and receiver is necessary to reject near-field multiple back-scatter, which imposes restrictions on AUV miniaturization. Recent experiments and theoretical modeling reveal that significant imaging improvements are possible over the existing continuous-wave laser line scan systems (LLS), by using a pulsed-gated version of the LLS (PG-LLS). However, the use of such a technique has a greater advantage in reducing the overall form factor over conventional LLS imaging system, as well as providing greater depth-of-field. In this paper, we present experimental results comparing both LLS and PG-LLS systems for several source-receiver separations and standoff distances. These results compare favorably with images obtained from validated LLS image simulation tools, and indicate the potential for reducing the source-receiver separation and therefore the system size.

The role of porosity fluctuations in scattering from sand sediments and in propagation losses within the sediment.

Acoustic backscattering from a diver-smoothed sand sediment was measured at frequencies from 200 to 500 kHz as a function of grazing angle. The residual roughness of this smoothed surface was measured using a laser line scanning system capable of measuring sub-millimeter heights over a 4-meter track. Using the measured sediment roughness, perturbation theory underestimates the scattering strength at all frequencies for angles greater than the critical grazing angle. The absence of large, discrete scatterers in the sediment suggests that scattering from fluctuations in the sediment properties may be the dominant scattering mechanism for these angles. The sound speed and attenuation were also measured in this sediment and the attenuation was found to exhibit a linear frequency dependence similar to that observed for other sand sediments. To account for this linear attenuation, a theory that incorporated scattering losses due to porosity fluctuations into the effective density fluid model has been developed. This theory suggests that the propagation losses may be connected by the same physical mechanism to the scattering of sound from the sediment. This connection is explored in the context of these scattering and propagation measurements.

Innovative Laser Materials Processing for Photovoltaics

AbstractThere are several laser‐based processes which are currently used in solar cell manufacturing or show a high potential for being implemented in series production in the future. Among these are laser edge isolation, laser fired contacts, metal/emitter wrap through, laser doping as well as P1, P2 and P3 structuring of thin film solar cells. Currently, all these direct‐patterning processes are mostly performed by single‐mode lasers with Gaussian intensity distributions. In contrast, top hat profiles offer larger process stability as well as cost savings. Another process that is still in the research and development stage is the crystallization of amorphous Si into polycrystalline Si. Here an increase of the electron mobility results in a better separation of the charge carriers in the cell and finally increased cell efficiency. A novel diode laser line scanning system and first processing results are presented in this article.

The effects of roughness on the frequency dependence of specular scattering.

Specular scattering data were collected at grazing angles from 7 to 77 deg grazing and frequencies from 5 to 50 kHz at the experimental validation of acoustic modeling techniques (EVA) sea test near Isola in October 2006. High resolution microbathymetry data taken using a laser line scan system were also collected. The amplitude and phase of the specular scattered data exhibit a distinctive frequency dependence. This frequency dependence may be due to Bragg scattering from small interface ripples on the ocean bottom indicated by peaks in the measured interface roughness power spectrum. This study simulates the scattering using realizations of interface roughness in a finite element model to determine if the frequency dependence is due solely to scattering or if other mechanisms such as layering are important factors. Results will be compared to analytic approximations such as the Kirchhoff approximation as well as exact integral equations. [Work sponsored by the ONR, Ocean Acoustics.]

Comparison of finite element model of scattering with experimental results from the Experimental Validation of Acoustic modeling techniques (EVA) sea test

Reflection coefficient measurements from 5 50 kHz at grazing angles from 10–80 degrees were taken as part of the Experiments for Validation of Acoustic modeling techniques (EVA) sea test on the north shore of Isola dElba, Italy. The microtopography of the seafloor was also measured using an ROV mounted laser line scan system. The seafloor roughness measurements were used as inputs in a finite element model to calculate the effects of scattering on the reflection coefficient measurement. The variance of the calculated reflection coefficient is compared with the measured variance to determine the contribution from interface scattering at different frequencies and angles. [Work supported by ONR, Ocean Acoustics.]

Performance bounds on synchronous laser line scan systems

In this article a performance bound is derived for feasible image resolution among a class of imaging systems that can be referred to as synchronous laser line scan systems. Most often, these systems use a narrow beam projected source (typically a laser) in conjunction with a very small field of view receiver that is synchronously scanned. Here, a bound on the maximum system resolution is derived when both source and receiver are "delta function like". The bound demonstrates that the best achievable overall system point spread function is the square of the one way point spread function for the medium.

Using Laser Line Scan Imaging Technology to Assess Deepwater Seafloor Habitats in the Monterey Bay National Marine Sanctuary

We conducted a 9-day field test of laser line scan (LLS) imaging technology to characterize and explore seafloor habitats in and around the Big Creek Marine Ecological Reserve (BCER) within the Monterey Bay National Marine Sanctuary off the central California coast. Our goal was to evaluate the utility of LLS technology for assessing the distribution and abundance offish, megafaunal invertebrates, and seafloor habitats, and to compare LLS images with those acquired from side-scan sonar and a remotely operated vehicle (ROV). We surveyed an area approximately 2.6 km long and 0.4 km wide, extending inside and directly outside BCER. Using LLS technology, we imaged isolated rock outcrops with patches of large sea anemones and dense groups of fishes, drift kelp, sea pens, salp chains, and individual, sedentary benthic fishes (including California halibut, Pacific electric ray, ratfish, and juvenile lingcod). As can be achieved with side-scan sonar, the LLS system also did an excellent job of imaging structural details of low-relief features such as sand waves and ripples. The LLS system offered the advantage of imaging some of the biogenic components of habitat and of describing their spatial relationships with detail that currently is not possible using acoustic techniques such as side-scan and multibeam sonar. However, the ROV far exceeded the capability of a LLS system to discern organisms at the highest possible taxonomic level, which ultimately will affect measures of diversity. The ability to visualize the seafloor in a broad context from a mosaic of LLS images could help us understand physical and anthropogenic (e.g., bottom trawling) processes that influence dynamic benthic habitats. Further development and use of LLS technology have the potential to improve our understanding, quantification, and protection of fish habitats.

Use of laser line scan for assessment of response of benthic habitats and demersal fish to seafloor disturbance

High resolution imaging of the seafloor has been limited to fixed sensors and/or limited field-of-view instruments. As a result, our ability to characterize large areas of benthic habitat has relied on acoustic maps and photomosaics combined with point sampling. In addition, the response of motile epibenthos and demersal fish to habitat patches, structure and disturbance has been largely inferred or based on disruptive sampling and manipulative experiments. Direct quantitative observation of large areal variation in these factors is possible through a combination of traditional video census techniques with high resolution optical imaging of the seafloor. Laser Line Scan Systems (LLSS) are capable of imaging continuous swaths of the seafloor from 3 to 15 m wide at speeds up to 5 km/h. Image quality is dependent on system performance and turbidity but can exceed limits required to distinguish benthic habitats and census undisturbed fish and epibenthos. This areal coverage rate (1 km/day) is several orders of magnitude higher than video sleds and can complement high resolution acoustic surveys (multibeam or side scan). We present results from several applications of these systems for assessment of benthic resources and disturbance in turbid estuarine environments.

Time-evolution of high-resolution topographic measurements of the sea floor using a 3-D laser line scan mapping system

A laser line scan system was used to characterize the high-resolution spatial variability and temporal evolution of the sea floor as a function of environmental conditions for a small section of the sea floor in the Gulf of Mexico during a period of nine days. High-resolution bathymetric profiles (<1 mm) covering about two sand ripple periods on the sea floor were acquired over a one-dimensional transect 1.35 m in length. The system was also used to measure reflectance and to produce three-dimensional bottom maps of the test area. Over the nine-day period, the sand ripple peak-to-trough height was observed to decrease slowly from about 2.5 cm to about 2.0 cm. Similar gradual changes we also observed in the time-evolution of bottom profile correlations and of the bottom roughness spectra. However, when smaller sections were examined individually, the time-evolution of the profile correlation was observed to vary acutely and in a transitory manner in some regions but not in others, and without preference for crests or troughs. In general, bottom roughness for spatial frequencies greater than 0.044 cycles/cm completely decorrelated within 20-30 h. However, this gradual trend was also marked by acute and transitory changes in bottom topography believed to be primarily from fish feeding on epibenthic prey.

Intercalibration method for underwater three-dimensional mapping laser line scan systems

I present a method to calibrate absolutely a new family of underwater bathymetric laser scanner systems that use a single target range. Depth is calculated by triangulation among the source, the receiver, and the instantaneous position of a single spot produced by a highly collimated laser beam scanned across the target. For calibration, the refractive index of seawater is used to modulate the triangulation parameters rather than use of different target ranges to calibrate for extended range operation. Exploitation of a unique triangulation configuration allows the receiver characteristics to be ignored initially so that the scanner parameters can be determined. This allows the accurate positioning of the laser spot so that the receiver, in turn, can be calibrated. This intercalibration method provides high-resolution parameterization of the source-receiver characteristics and their geometric layout within the system so that an absolute bathymetric calibration can be realized in situ. In addition, a theoretical model derived to develop the methodology can be used to characterize and optimize future designs.