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Abstract In this paper we present a method of zero-padding temporal auto-correlation imaging (ztDCI) for imaging blood flows. Compared with temporal auto-correlation imaging (tDCI), ztDCI effectively slows down the decorrelation speed of speckle frames, so it has wider applicable speed range. We also prove that the combination of tDCI and ztDCI can compensate the non-monotonous trend of ztDCI VS speed and provides higher relative signal to noise rate (rSNR) for slow flows and short exposure times than laser speckle contrast imaging (LSCI). It captures both slow and fast flows independent of camera exposure time which will be an advantage for in vivo or clinical applications. [ABSTRACT FROM AUTHOR]

• Particle Swarm Optimization to find optimal values for image reconstruction by Phase Retrieval. • Faster convergence and better reconstruction results for 16-bit images. • Image reconstruction process is automated, eliminating the manual search of best values for reconstruction. Imaging through scattering media has become an important aspect in optical imaging. To achieve this, different approaches have been presented to recover the phase of an imaging object from its scattered speckle behind a scattering medium. However, all these algorithms require initial parameters which are obtained based on prior knowledge of the imaging object. This is impractical, as prior knowledge of the imaging object is not always available in real imaging applications. In this paper, we propose using a Particle Swarm Optimization algorithm to find these optimal parameters without requiring prior knowledge of the imaging object. These optimal values are then adopted in the Phase Retrieval algorithm based on object-modulated speckles for object image reconstruction behind a scattering medium in a single-shot imaging system. The obtention of the values for the standard deviation for Gaussian filter and the threshold control allows Phase Retrieval to converge faster and avoids the need for running the algorithm more than once in the search for the best reconstruction. [ABSTRACT FROM AUTHOR]

Our study aimed to examine the effects of COVID-19 on the physical fitness and academic performance of Chinese college students. The sample included physical fitness test data from 9,712 undergraduate students and academic performance data from 12,000 undergraduate students at a top university in China. Physical fitness was measured and evaluated according to the Chinese National Student Physical Fitness Standard. Data were analyzed using two-sample t-tests and Pearson product-moment correlations. Total physical fitness test scores of college students decreased after the outbreak, but their mean body mass index (BMI) remained at normal levels. The mean academic performance of college students unexpectedly improved during the online learning period. The positive correlation between physical test scores and academic performance was significantly higher during COVID-19 than that before the outbreak. Chinese college students with high physical fitness during COVID-19 were more likely to achieve good academic performance. [ABSTRACT FROM AUTHOR]

• Surface degradation of Ge in vacuum on different laser conditions are showed. • The influences of processing parameter and material failure behavior are analyzed. • Mechanism in damage variation with laser ablation is compared in air and vacuum. • Damage mechanism contains thermal effect, plasma shock wave and mechanical stress. Laser-induced damage of germanium (Ge) sheets plays a key role to estimate the lifetime of optical components in the vacuum and space irradiation environment. However, the dependence between the degrees of laser-induced surface damage and nanosecond pulsed irradiation remains unclear for Ge sheet. Here, we investigate damage morphology and the growth characteristic of Ge sheets under different conditions and measure the laser-induced damage threshold (LIDT) in vacuum. Our results reveal that optical reflectivity of Ge in vacuum chamber falls 92% because of the defective sites on the damaged surface. The area, the depth and the size of the damage crater increase but the hillock height and the surrounding thermal diffuse area decrease along with the increase of vacuum degree. The thermal effect, the failure effect of plasma shock wave and the mechanical stress are analyzed to be the main mechanism in the laser damage. The combined experimental and theoretical work gives an insight into the change of surface damage and material failure behavior, hence broadens the potential applications of Ge sheets in areas of laser material manufacturing and laser processing. [ABSTRACT FROM AUTHOR]

Pump-probe technique is employed to investigate the dynamic process of shock wave in water induced by femtosecond laser pulse. The time resolved shadowgraphs and holograms were obtained experimentally, and the 2-dimensional phase maps were reconstructed from holograms. From the analysis of the phase maps, the evolution process within the laser–water interaction region was described and the formation mechanism of the shock wave was discussed. [ABSTRACT FROM AUTHOR]

An asymmetric encryption method in 4f system based on interference is proposed in this paper. The original image is transformed into two white stationary noise images, and the decryption keys are designed to be different from the encryption keys to realize asymmetric encryption. The decryption process is implemented in a 4f system of image addition with grating. This method has very high security as the asymmetric cryptosystem encryption system but also can be easy implemented in optical system. Besides, holographic plate or photorefractive crystal is not necessary in the encryption process. The simulation results prove the feasibility and security of this method. [ABSTRACT FROM AUTHOR]

The recognition and classification of wire melted marks is crucial in modern fire investigation. The existing technology mainly uses physical or chemical methods to deal with wire melted marks and draws conclusions through manual observation, or manually extracts the features and train classification model, both of which consume excessive manpower and resources. The research on automatic feature extraction and recognition of wire weld marks by artificial intelligence technology is still blank. Based on the data set of wire melted mark metallographic images provided by a city fire research institute, we proposed an algorithm to recognize the type of wire melted mark metallographic images based on artificial intelligence which can help fire fighters efficiently speculate the cause of fire. In the algorithm, the TransUnet network is used to segment the melted zone by semantic segmentation to extract the melted zone containing the main features, and the mIOU reaches 92.2%. Then, the features of wire melted mark are extracted based on the melted zone image. Finally, XGBoost is used for feature modeling for classification. The F1 Score of model is 82.9%. [ABSTRACT FROM AUTHOR]

To obtain satisfactory performance in characterizing optical freeform surfaces with local features, this paper proposes a model of a radial basis function with slope-based shape factor and distribution (RBF-SSD). Compared to previous RBF-slope models with only slope-based shape factors, the RBF-SSD model relates both shape factors and distribution with the surface slope, ensuring greater fitting ability can be achieved when fitting a surface with local features. Fitting experiments for two different surfaces demonstrated the fitting ability of the RBF-SSD model. An off-axis three-mirror system with 3 ° × 3.6 ° field of view was designed as an example to show the optical design efficacy of our model. [ABSTRACT FROM AUTHOR]

A wavelength step-swept light source (WSSL) using a recirculating frequency shifter loop (RFSL) based on a single-side-band (SSB) modulator is proposed, in order to achieve a linear and fast wavelength-sweeping. The swept step can be tuned from 1.2 pm to 128 pm by adjusting a precise and stable radio frequency (RF) signal that is applied to the SSB modulator. The swept rate can be tuned up to 99 kHz in a range of over 5.12 nm. Wavelength-to-time mapping is used to measure static strain-induced or temperature-induced shifting of the reflected central wavelength of a fiber Bragg grating (FBG). Because of the high linearity of the light source, the interrogation linearity of the strain and the temperature are as high as 0.99944 and 0.99946, respectively. When a dynamic periodic strain applied to FBG sensor, the dynamic performance of the FBG sensor is successfully recorded in the time domain and its power spectral density of a fast Fourier transform (FFT) is calculated. The signal-to-noise ratio (SNR) of the power spectral density is over 40 dB for a 100 Hz dynamic strain and the calculated sensitivity is 0.048 με/Hz1/2. A sharp change in the strain frequency from 100 Hz to 500 Hz is captured in real time. [ABSTRACT FROM AUTHOR]

• Proposed a new sampling method for Fourier single pixel imaging, by which significant Fourier coefficients were effectively sampled at low sampling ratios, so the image resolution is greatly improved. • This method innovatively takes the use of the property that the Fourier domain images of most natural sceneries have light-rays pattern and the light rays corresponds to the borders in spatial domain. Therefore the discrete and sparse light rays enable effective sampling of high frequencies that are important for image resolution. • An analysis of randomly chosen 200 images from VOC image database proved most natural scenery images have the light rays pattern in Fourier domain. • The proposed method provides a potential direction to improve the sampling efficiency of Fourier single-pixel imaging. Fourier single-pixel imaging (FSI) technology has attracted wide attention for its high imaging efficiency and broad applicability. However, the existing sampling methods cannot effectively sample the key high-frequency Fourier coefficients with low sampling ratio. To address this problem, we propose an adaptive sampling method based on spectral energy continuity, especially in the directional distributed frequencies that correspond to the borders in the image. In this method, the significant high-frequency components are predicted from simply isometric sampled low frequencies and the generated sampling trajectory is close to the optimal. Because more important high-frequency information is sampled, the image quality at low sampling ratio is greatly improved. We demonstrate with both simulations and experiments that this sampling method is effective in suppressing noise and obtaining detailed information of the image at low sampling ratios. [ABSTRACT FROM AUTHOR]

The new broadband light-absorbing material is essential for effective utilization of solar energy. However, the size-controllable MoSe 2 nanospheres for the applications in broadband light-absorbing and photothermal fields, as well as the role of partially disordered features on the optical responses have not been well investigated. Here, we prepare the partially disordered MoSe 2 nanospheres by a facile solvothermal synthesis method and investigate the random impacts of the spherical diameter on optical responses. The MoSe 2 nanospheres reveal dark black color and broadband absorption from ultraviolet to near-infrared wavelength without obvious absorption edge. The photothermal properties with super-continuous laser irradiation demonstrate that MoSe 2 nanospheres have high photothermal conversion efficiency of about 44.8% in water and even also reach about 30.7% in ethanol. The superior light absorption could originate from free-carriers absorption, defective state absorption and resonant absorption. The results not only pave the way of two-dimensional material for disordered photonic researches, but also widen the potential applications of MoSe 2 in saturable absorber and photothermal agent. [Display omitted] • Size-controllable MoSe 2 nanospheres are prepared by a solvothermal synthesis method. • The randomness plays a key role in optical absorption and photothermal properties. • MoSe 2 reveals high photothermal conversion efficiency of about 44.8% in water. • Photoabsorption comes from free-carrier, defective state and resonant absorption. [ABSTRACT FROM AUTHOR]

Based on vector diffraction theory, which was described originally by Richards and Wolf, we calculate the focusing fields of the radial vector beams loading two, three and four vortex phases. The calculations and simulations show that one, two and three optical dark-holes could be generated in the focusing field by a high numerical aperture for the two, three and four vortex phases, respectively. In addition, the distances between the optical dark-holes are positively correlated with the distances between vortex phases. Based on this study, the generation of the optical dark-hole and the manipulation of the dark-hole's distance may be used in the capture and manipulation of multiple particles. [ABSTRACT FROM AUTHOR]

To measure large and heavy micro-structured workpieces in situ and improve the measurement accuracy, which strongly depends on the environment during the measurement of micro-structured surfaces, a workpiece rotating technique is proposed. This method utilizes the precise rotation of machine tools to drive the workpiece and records a set of elemental image arrays for the pickup stage to overcome the upper-resolution limit imposed by the Nyquist sampling theorem, which allows the increase in the two-dimensional spatial resolution of the computed depth images in integral imaging. By extracting the depth position, we can obtain accurate depth information and measurement results for micro-structured surfaces. We carried out simulations and experiments to demonstrate the proposed method, and the results show the feasibility of our method. [ABSTRACT FROM AUTHOR]