Laser Speckle Noise Research Articles

Research on Student's T-Distribution Point Cloud Registration Algorithm Based on Local Features.

LiDAR offers a wide range of uses in autonomous driving, remote sensing, urban planning, and other areas. The laser 3D point cloud acquired by LiDAR typically encounters issues during registration, including laser speckle noise, Gaussian noise, data loss, and data disorder. This work suggests a novel Student's t-distribution point cloud registration algorithm based on the local features of point clouds to address these issues. The approach uses Student's t-distribution mixture model (SMM) to generate the probability distribution of point cloud registration, which can accurately describe the data distribution, in order to tackle the problem of the missing laser 3D point cloud data and data disorder. Owing to the disparity in the point cloud registration task, a full-rank covariance matrix is built based on the local features of the point cloud during the objective function design process. The combined penalty of point-to-point and point-to-plane distance is then added to the objective function adaptively. Simultaneously, by analyzing the imaging characteristics of LiDAR, according to the influence of the laser waveform and detector on the LiDAR imaging, the composite weight coefficient is added to improve the pertinence of the algorithm. Based on the public dataset and the laser 3D point cloud dataset acquired in the laboratory, the experimental findings demonstrate that the proposed algorithm has high practicability and dependability and outperforms the five comparison algorithms in terms of accuracy and robustness.

Quantitative real-time phase microscopy for extended depth-of-field imaging based on the 3D single-shot differential phase contrast (ssDPC) imaging method.

Optical diffraction tomography (ODT) is a promising label-free imaging method capable of quantitatively measuring the three-dimensional (3D) refractive index distribution of transparent samples. In recent years, partially coherent ODT (PC-ODT) has attracted increasing attention due to its system simplicity and absence of laser speckle noise. Quantitative phase imaging (QPI) technologies represented by Fourier ptychographic microscopy (FPM), differential phase contrast (DPC) imaging and intensity diffraction tomography (IDT) need to collect several or hundreds of intensity images, which usually introduce motion artifacts when shooting fast-moving targets, leading to a decrease in image quality. Hence, a quantitative real-time phase microscopy (qRPM) for extended depth of field (DOF) imaging based on 3D single-shot differential phase contrast (ssDPC) imaging method is proposed in this research study. qRPM incorporates a microlens array (MLA) to simultaneously collect spatial information and angular information. In subsequent optical information processing, a deconvolution method is used to obtain intensity stacks under different illumination angles in a raw light field image. Importing the obtained intensity stack into the 3D DPC imaging model is able to finally obtain the 3D refractive index distribution. The captured four-dimensional light field information enables the reconstruction of 3D information in a single snapshot and extending the DOF of qRPM. The imaging capability of the proposed qRPM system is experimental verified on different samples, achieve single-exposure 3D label-free imaging with an extended DOF for 160 µm which is nearly 30 times higher than the traditional microscope system.

Laser speckle noise suppression method based on empirical mode decomposition

Laser speckle noise suppression method based on empirical mode decomposition

Improved Weighted Non-Local Mean Filtering Algorithm for Laser Image Speckle Suppression.

Laser speckle noise caused by coherence between lasers greatly influences the produced image. In order to suppress the effect of laser speckles on images, in this paper we set up a combination of a laser-structured light module and an infrared camera to acquire laser images, and propose an improved weighted non-local mean (IW-NLM) filtering method that adopts an SSI-based adaptive h-solving method to select the optimal h in the weight function. The analysis shows that the algorithm not only denoises the laser image but also smooths pixel jumps in the image, while preserving the image details. The experimental results show that compared with the original laser image, the equivalent number of looks (ENL) index of the IW-NLM filtered image improved by 0.80%. The speckle suppression index (SSI) of local images dropped from 4.69 to 2.55%. Compared with non-local mean filtering algorithms, the algorithm proposed in this paper is an improvement and provides more accurate data support for subsequent image processing analysis.

Speckle Noise Suppression Based on Empirical Mode Decomposition and Improved Anisotropic Diffusion Equation

Existing methods to eliminate the laser speckle noise in quantitative phase imaging always suffer from the loss of detailed phase information and the resolution reduction in the reproduced image. To overcome these problems, this paper proposes a speckle noise suppression method based on empirical mode decomposition. Our proposed method requires only one image without additional equipment and avoids the complicated process of searching the optimal processing parameters. In this method, we use empirical mode decomposition to highlight the high frequency information of the interference image and use the Canny operator to perform edge detection, so the diffusion denoising process is guided by high-precision detection results to achieve better results. To validate the performance of our proposed method, the phase maps processed by our proposed method are compared with the phase maps processed by the improved anisotropic diffusion equation method with edge detection, the mean filter method and the median filter method. The experimental results show that the method proposed in this paper not only has a better denoising effect but also preserves more details and achieves higher phase reconstruction accuracy.

In-situ 3D shape measurement system for a high-temperature object in a plasma wind tunnel via DMD-projection laser structured light and adaptive speckle filtering

The evaluation of an object’s 3D shape change during ablation in wind tunnel testing is critical for the optimization of the thermal protection system. However, strong thermal radiation and a harsh environment cause challenges to the traditional measurement methods. A single camera digital micro-mirror device (DMD) projector-structured laser light system is proposed for in-situ 3D shape measurement in a wind tunnel test. A customized DMD combined with a laser source is devised for projecting concentrated full-field high-power structured light stripes. To filter laser speckle noise in projected chessboard images for model calibration, an auto-window Kuwahara adaptive filter is put forward. The 3D shape of a blunt-head cylinder was in-situ measured in a plasma wind tunnel which reached a maximum temperature of 1950 °C. The diameter reconstruction error was within 0.85% (0.17/20.03 mm). With projected laser light and multi-step filters, the ablation process of the front face of a dome-top cylinder at 1950 °C was clearly observed at a long working distance over 1.5 m.

High spatial and temporal resolution synthetic aperture phase microscopy

A new optical microscopy technique, termed high spatial and temporal resolution synthetic aperture phase microscopy (HISTR-SAPM), is proposed to improve the lateral resolution of wide-field coherent imaging. Under plane wave illumination, the resolution is increased by twofold to around 260 nm, while achieving millisecond-level temporal resolution. In HISTR-SAPM, digital micromirror devices are used to actively change the sample illumination beam angle at high speed with high stability. An off-axis interferometer is used to measure the sample scattered complex fields, which are then processed to reconstruct high-resolution phase images. Using HISTR-SAPM, we are able to map the height profiles of subwavelength photonic structures and resolve the period structures that have 198 nm linewidth and 132 nm gap (i.e., a full pitch of 330 nm). As the reconstruction averages out laser speckle noise while maintaining high temporal resolution, HISTR-SAPM further enables imaging and quantification of nanoscale dynamics of live cells, such as red blood cell membrane fluctuations and subcellular structure dynamics within nucleated cells. We envision that HISTR-SAPM will broadly benefit research in material science and biology.

Laser Stripe Center Detection Under the Condition of Uneven Scattering Metal Surface for Geometric Measurement

The uneven reflective metal surface has great influence on the high-precision measurement of the structured light 3-D shape. By analyzing the laser scattering and reflection models, a set of light stripe center’s subpixel extraction methods which has strong robustness is proposed. The method first cuts off the influence of the uneven fixed background by using the difference image method to process the bright field and the dark field. Then, the regional growth statistics method is used to eliminate the influence of random laser speckle noise, and then the gray-gravity method is used to obtain the coarse center of the laser stripe. The Sobel operator is used to obtain the gradient vector of the stripe pixel point, and then the normal direction field of the light stripe is obtained. The normal direction field vector is taken as the direction to find the $5\times5$ neighborhood of the coarse light stripe’s center, and then the gray-gravity method is reused to determine the center position of the laser stripe in the normal line direction; the pixel coordinates of the subpixel level are obtained by using bilinear interpolation. The experimental results show that the method can effectively eliminate the interference of flash point noise and has strong robustness. The detection error of the stripe is less than 0.1 pixels, which ensures the system measurement accuracy. Compared with other methods, the system’s resolution can reach 0.02 mm.

Speckle noise attenuation in orbital laser vibrometer seismology

The interior structures of small planetary bodies such as asteroids and comets are an enigma. Determining the internal structure is important for understanding the formation and evolution of the solar system and also has implications for in-situ resource exploitation and planetary defense. Despite the immense value, no detailed investigation of the interior properties of an asteroid has yet been made. Although orbital radar and lander based seismological approaches have been proposed to make direct interior observations, it has not yet been demonstrated that radar could transmit through a rocky asteroid, nor that landing multiple seismometers is feasible or affordable. An elegant alternative to landed seismometers is orbital laser Doppler vibrometry, which could record seismic shaking of a small body without contact with the surface. Laser Doppler vibrometers (LDVs) are mature instruments for terrestrial applications and could be adapted for a space environment. However, when incident on a rough surface like an asteroid regolith, an LDV is subject to laser speckle noise which may be misinterpreted as seismic shaking. We address the challenge of making LDV measurements on naturally rough surfaces by quantifying the laser speckle noise that an orbital LDV would encounter during a hypothetical orbital measurement. Specifically, we simulate an LDV measurement of a seismic signal generated by an impact source on the asteroid 101955 Bennu. We demonstrate that speckle noise can be attenuated by combining multiple signals recorded by an orbital seismometer equipped with multiple LDV sensors. By mitigating laser speckle, we demonstrate that an orbital LDV can record seismic signals on a natural asteroid surface, which would enable an orbital seismometer to achieve the dense global coverage necessary for high resolution interior imaging.

Flexible and lightweight speckle noise suppression module based on generation of dynamic speckles with multimode fiber and macro fiber composite

We propose a flexible and lightweight laser speckle noise suppression module based on the combination of a multimode fiber (MMF) and a macro fiber composite (MFC). The mechanism underlying speckle suppression involves the time-averaging of dynamic speckles during the exposure time of a charge-coupled device, wherein the MFC causes MMF vibration via its fast and periodic stretching due to the piezoelectric effect. The most efficient speckle reduction condition is realized when the MMF is vibrating in a resonant mode under the application of the highest MFC driving voltage and a large MMF effective numerical aperture. We obtain a speckle contrast of 0.08 when this “most efficient” speckle reduction condition is used to illuminate a diffuser. Further, the image quality is improved when our module is applied in a projector, which makes the device a flexible and lightweight module for speckle reduction in laser displays.

SNR analysis with speckle noise in interferometry using monochromatic expanded source and fringe localization

As a strongly coherent light source, laser is widely used in various high-precision interferometry techniques. However, the coherence of laser also produces coherence noise. Laser coherence noise degrades the signal-to-noise (SNR) ratio and the interferogram resolution, leading to difficulties in data classification and information restoration. Here, we aim to derive the expressions of laser speckle contrast and SNR of interferograms through the theory of laser speckle noise. The factors affecting the speckle contrast and SNR of images are analyzed theoretically. The problem of fringe localization caused by dynamic scatter is discussed. We verify the relevant expression by using an experimental system and find that it can serve as a basis of laser interferometry to analyze measurement accuracy and suppress speckle noise.

DeepLSR: a deep learning approach for laser speckle reduction.

Speckle artifacts degrade image quality in virtually all modalities that utilize coherent energy, including optical coherence tomography, reflectance confocal microscopy, ultrasound, and widefield imaging with laser illumination. We present an adversarial deep learning framework for laser speckle reduction, called DeepLSR (https://durr.jhu.edu/DeepLSR), that transforms images from a source domain of coherent illumination to a target domain of speckle-free, incoherent illumination. We apply this method to widefield images of objects and tissues illuminated with a multi-wavelength laser, using light emitting diode-illuminated images as ground truth. In images of gastrointestinal tissues, DeepLSR reduces laser speckle noise by 6.4 dB, compared to a 2.9 dB reduction from optimized non-local means processing, a 3.0 dB reduction from BM3D, and a 3.7 dB reduction from an optical speckle reducer utilizing an oscillating diffuser. Further, DeepLSR can be combined with optical speckle reduction to reduce speckle noise by 9.4 dB. This dramatic reduction in speckle noise may enable the use of coherent light sources in applications that require small illumination sources and high-quality imaging, including medical endoscopy.

Compound denoising method of laser speckle noise in laser inherent field imaging

Noise is an important factor affecting the image quality of laser coherent field high resolution imaging system. And there exists not only background light additive noise but also laser multiplicative speckle noise in a laser coherent field imaging system. Both of the above noise affect the imaging quality of laser coherent field system. In order to improve the imaging quality from the perspective of noise suppression and settle the imaging quality degradation problem of laser multiplicative speckle noise and background additive noise in the laser coherent field imaging system, the model for the influence of multiplicative speckle noise and background additive noise on laser echo field demodulated signal is established in atmospheric downlink. Then, based on the model, a novel homomorphic filter and sparse matrix trace cascade compound de-noising algorithm is put forward. Firstly, based on the homomorphic filtering theory, the laser multiplicative speckle noise in the laser echo demodulated signal is converted into the additive noise by logarithmic transformation. Then the low-frequency laser multiplicative speckle noise is filtered by the high-pass filter, and the high-frequency demodulated signal is retained. The logarithmic inverse transform is used to obtain the laser echo demodulation signal after the multiplicative speckle noise has been filtered out. Next, the phase random disturbance of atmosphere in laser echo demodulated signal is suppressed by phase closure technology and the imaging spectrum component is reconstructed by the spectrum iterative reconstruction method. Then the high resolution image is obtained by spectrum component inverse Fourier transform. Finally, the effect of background additive noise on the image quality is suppressed by the sparse base tracking theory. The simulated and outdoor experiment result are used to verify the denoising effect and image quality enhancement effect of the composite de-noising method. Compared with the existing single denoising method, the composite denoising method is shown to be able to effectively eliminate laser multiplicative speckle noise and background additive noise at one time. The proposed method can improve image contrast and promote the Strehl ratio of imaging quality in a coherent imaging system. It provides a theoretical basis for improving imaging quality and denosing laser multiplicative speckle noise and background additive noise in coherent field imaging system.

Sub-Diffraction Visible Imaging Using Macroscopic Fourier Ptychography and Regularization by Denoising.

Imaging past the diffraction limit is of significance to an optical system. Fourier ptychography (FP) is a novel coherent imaging technique that can achieve this goal and it is widely used in microscopic imaging. Most phase retrieval algorithms for FP reconstruction are based on Gaussian measurements which cannot extend straightforwardly to long range, sub-diffraction imaging setup because of laser speckle noise corruption. In this work, a new FP reconstruction framework is proposed for macroscopic visible imaging. When compared with existing research, the reweighted amplitude flow algorithm is adopted for better signal modeling, and the Regularization by Denoising (RED) scheme is introduced to reduce the effects of speckle. Experiments demonstrate that the proposed method can obtain state-of-the-art recovered results on both visual and quantitative metrics without increasing computation cost, and it is flexible for real imaging applications.

Reduction of laser speckle noise by using particle-dispersed liquid crystals

ABSTRACTLaser speckle noise occurs owing to the strong interference that originates in the high coherency of laser light and the surface topography of screen. The suppression of the speckle noise is necessary for laser displays such as an image projection system. To reduce the speckle noise, we have tried to control the speckle pattern by using liquid crystals (LCs). In this paper, we offer an idea for suppressing the speckle noise using particle-dispersed LC cells in which the speckle pattern can be varied in addition to the optical diffusive effect.

Investigation of laser speckle noise suppression by using polymer-stabilized liquid crystals within twisted nematic cell

We propose a robust method to suppress laser speckle using a polymer-stabilized liquid crystal (PSLC) device with high initial transmittance. With applied voltage, a large modulation depth has been produced through light scattering because of the refractive index mismatch between the rotated nematic liquid crystals and the polymer networks. By using PSLCs with 5 wt % monomer, a speckle noise reduction rate of approximately 54.7% can be achieved with an applied voltage of 6 V. The lowest speckle contrast of approximately 0.025 with relatively high discrimination from the projected AF image has also been demonstrated through a wedge PSLC cell.

Control of Laser Speckle Noise by Using Polymer-Dispersed LC

Laser speckle noise occurs owing to the strong interference that originates in the high coherency of laser light and the surface topography of screen. The suppression of the speckle noise is necessary for laser displays such as an image projection system. To reduce the speckle noise, we have tried to control the speckle pattern by using liquid crystals (LCs). In this paper, we offer an idea for suppressing the speckle noise using a polymer-dispersed LC (PDLC) cell in which the speckle pattern can be varied in addition to the optical diffusive effect.

Control of Laser Speckle Noise Using Liquid Crystals

Laser speckle noise occurs owing to the strong interference that originates in the high coherency of laser light and the surface topography of the screen. The suppression of the speckle noise is necessary for laser displays such as in an image projection system. In order to reduce speckle noise, we try to control the speckle pattern by using liquid crystals (LCs) that have electrically controllable optical characteristics. In this paper, we formulate an idea for suppressing speckle noise using a voltage-gradient LC cell and a polymer-dispersed LC cell.

Control of Laser Speckle Noise Using Liquid Crystals

Laser speckle noise occurs owing to the strong interference that originates in the high coherency of laser light and the surface topography of the screen. The suppression of the speckle noise is necessary for laser displays such as in an image projection system. In order to reduce speckle noise, we try to control the speckle pattern by using liquid crystals that have electrically controllable optical characteristics. The modulation of polarized light using the birefringence of liquid crystals is very effective. However, the ratio of noise reduction does not exceed 30%. This is because the polarized light transmitted through the liquid crystal medium is formed by only two optical eigenmodes. The wedge-type liquid crystal cell can control the position of the laser spot on the screen. The incident-angle modulation of laser light is also effective for noise reduction.

Control of Laser Speckle Noise Using Liquid Crystals

Laser speckle noise occurs owing to the strong interference that originates in the high coherency of laser light and the surface topography of the screen. The suppression of the speckle noise is necessary for laser displays such as in an image projection system. In order to reduce speckle noise, we try to control the speckle pattern by using liquid crystals that have electrically controllable optical characteristics. The modulation of polarized light using the birefringence of liquid crystals is very effective. However, the ratio of noise reduction does not exceed 30%. This is because the polarized light transmitted through the liquid crystal medium is formed by only two optical eigenmodes. The wedge-type liquid crystal cell can control the position of the laser spot on the screen. The incident-angle modulation of laser light is also effective for noise reduction.