Effective method for low-light image enhancement based on the JND and OCTM models.

Low-light images always suffer from dim overall brightness, low contrast, and low dynamic ranges, thus result in image degradation. In this paper, we propose an effective method for low-light image enhancement based on the just-noticeable-difference (JND) and the optimal contrast-tone mapping (OCTM) models. First, the guided filter decomposes the original images into base and detail images. After this filtering, detail images are processed based on the visual masking model to enhance details effectively. At the same time, the brightness of base images is adjusted based on the JND and OCTM models. Finally, we propose a new method to generate a sequence of artificial images to adjust the brightness of the output, which has a better performance in image detail preservation compared with other single-input algorithms. Experiments have demonstrated that the proposed method not only achieves low-light image enhancement, but also outperforms state-of-the-art methods qualitatively and quantitatively.

Effective enhancement method of low-light-level images based on the guided filter and multi-scale fusion.

Aiming to solve the problem of low-light-level (LLL) images with dim overall brightness, uneven gray distribution, and low contrast, in this paper, we propose an effective LLL image enhancement method based on the guided filter and multi-scale fusion for contrast enhancement and detail preservation. First, a base image and detail image(s) are obtained by using the guided filter. After this procedure, the base image is processed by a maximum entropy-based Gamma correction to stretch the gray level distribution. Unlike the existing methods, we enhance the detail image(s) based on the guided filter kernel, which reflects the image area information. Finally, a new method is proposed to generate a sequence of artificial images to adjust the brightness of the output, which has a better performance in image detail preservation compared with other single-input algorithms. Experiments show that the proposed method can provide a more significant performance in enhancing contrast, preserving details, and maintaining the natural feeling of the image than the state of the art.

Adaptive method for image dynamic range adjustment and detail enhancement.

Tone mapping operators (TMOs) aim to adjust high dynamic range (HDR) images to low dynamic range (LDR) ones so that they can be displayed on conventional devices with visual information retained. Nonetheless, existing TMOs can successfully tone-map only limited types of HDR images, and the parameters need to be manually adjusted to yield the best subjective-quality tone-mapped outputs. To cope with the aforementioned issues, an adaptive parameter-free and scene-adaptive TMO for dynamic range adjusting and detail enhancing is proposed to yield a high-resolution and high-subjective-quality tone-mapped output. This method is based on detail/base layer decomposition to decompose the input HDR image into coarse detail, fine detail, and base images. After that, we adopt different strategies to process each layer to adjust the overall brightness and contrast and to retain as much scene information. Finally, a new method, to the best of our knowledge, is proposed for visualization to generate a sequence of artificial images to adjust the brightness. Experiments with numerous HDR images and state-of-the-art TMOs are conducted; the results demonstrate that the proposed method consistently produces better quality tone-mapped images than the state-of-the-art methods.

Dynamic photoelectron transport in stepwise-doped GaAs photocathodes.

We present a theoretical model describing photoelectron transport dynamics in stepwise-doped GaAs photocathodes. Built-in electric field caused by the doping structure is analyzed, and the time-evolution of electron concentration in the active layer induced by a femtosecond laser pulse is solved. The predictions of the model show excellent agreement with the experimental data measured with pump-probe transient reflectometry, demonstrating the capability of the theoretical model in predicting photoelectron behaviors in real devices. Comparisons are also made between this stepwise doping model and the conventional gradient doping model with a continuous doping profile, thereby providing the first quantitative evaluation of the effectiveness and the limitation of the gradient doping model in describing actual stepwise-doped devices.

Defect detection system for silicon solar panels under all-day irradiation.

In terms of the shortcomings of defect detection based on the electroluminescence of conventional silicon solar panels, which can only be performed under darkroom conditions, a defect detection system that can work under the Sun with any irradiance in all weather is designed. The system electrifies solar panels through a modulated current source, uses high frame rate InGaAs area array detectors for image data acquisition, and transmits images via CameraLink. Using these image data as data sources, a defect display algorithm model is designed. Through experiments, it can effectively collect the defect information of solar panels within the range of 0.2 to 1300W/m2 of sunlight irradiance. Based on this, according to the relationship of the modulated phase difference between the defective points and the nondefective points found in the experiment, an enhancing algorithm for image saliency is proposed. The results show that this algorithm can reduce background interference in an effective way and improve the contrast of defects displayed under high irradiance.

Automatic evaluation of vertex structural defects on the anode surface of a low-light-level image intensifier based on proposed individual image processing strategies.

In the process of microchannel plate (MCP) making and physicochemical treatment of a low-light-level (LLL) image intensifier, multifilament fixed pattern noise, also known as structural defects, is one of the most common defects in the anode surface. The appearance of this defect will seriously affect the imaging quality of an image intensifier, so it should be found in time before delivery. The traditional evaluation method of this defect relies on subjective judgment, and the disadvantage is that the division of the dense defect area and the measurement of defect gray difference (GD) are not standardized. To address this problem, an automatic evaluation method of vertex structural defects of an LLL image intensifier based on proposed individual image processing strategies is presented, which provides a digital evaluation scheme for such defects. This method is composed of two parts: quasi-circular defect detection and defect GD calculation. The first part is composed of coarse detection and fine detection. Coarse detection is to scan the anode surface and take the two ends of a pair of adjacent line segments with a large gradient sum and opposite gray change direction as the defect boundaries; fine detection is to establish the image patch from defect boundaries, extract the edge segment from the image patch, and judge whether it conforms to the shape of a circle. In order to substantiate the performance of the quasi-circular defect detection strategy, two relevant techniques are used as comparison. One is based on a Gaussian filter, and the other is based on a fixed-size window template. The comparison results show that our method, to the best of our knowledge, has the best detection performance for vertex structural defects. The second part consists of region of interest (ROI) cropping, secondary defect detection, shortest distance sequence establishment, effective distance extraction, triplet set construction, and triplet GD calculation. First, the location histogram of defects is established to cut ROI; then, the secondary defect detection is performed to extract more vertex structural defects from ROI; after that, the shortest distance sequence of defects is constructed, and the effective distances are extracted by using the structural features of multifilament. Finally, the triplet set is generated according to the effective distance, and the triplet GD is calculated based on the gray information near the triplet baseline. The GD of vertex structural defects corresponds to the maximum GD of triplets. So as to verify the effectiveness of vertex defect GD calculation strategy, several image tubes with different degrees of such defects are used for experiments, and the subjective evaluation method is used as comparison. The experimental results substantiate that this method is superior to the subjective method in locating ROI accurately and calculating defect GD quantitatively. In general, the automatic evaluation method can be regarded as an effective evaluation scheme for vertex structural defects of an LLL image intensifier.

Improved preparation techniques for preparing high-performance GaAs photocathodes.

Efficiency and lifetime are always problems raised with photocathodes during operation. With the purpose of obtaining high-performance GaAs photocathodes with high sensitivity and long operational lifetime, it is necessary to investigate the preparation techniques during both the cleaning and the activation procedure. By comparison with the classical preparation techniques, the improved preparation techniques with an optimized chemical etching method and activation procedure are proposed. The experimental results show that the optimized chemical etching solution is more effective in removing oxide and carbon contamination, which can help photocathodes obtain higher sensitivity. On this basis, better long wavelength response and longer operational lifetime can be obtained with the help of the more competitive activation procedure. The proposed preparation techniques will be useful for applications as a source of spin-polarized electrons.

Comparison of photoemission performance of a GaAs photocathode between white light and monochromatic light illumination during activation.

To obtain higher quantum efficiency and longer operational lifetime of negative-electron-affinity GaAs-based photocathodes, illumination conditions using different light sources during the activation process are explored. GaAs photocathodes were activated under white light and 633 nm monochromatic light with different intensities, and experimental quantum efficiencies and photocurrent degradations were compared after activation and recaesiation. The results show that GaAs photocathode can obtain higher quantum efficiency and longer lifetime by using illumination of 633 nm monochromatic light, and this advantage can hardly be achieved by adjusting the intensity of white light. This work verifies an improved solution for preparing GaAs-based photocathodes with satisfactory capability.

Improved quantum efficiency and stability of GaAs photocathode using favorable illumination during activation.

Considering that illumination using the light source is required to monitor the activation progress of negative-electron-affinity GaAs-based photocathodes, it is important to understand if and how the illumination affects the activation. To improve the photoemission performance of GaAs photocathode, epitaxial GaAs samples are Cs/O2 activated and recesiated under illumination of monochromatic light of blue (460 nm), green (532 nm), and red (633 nm) with approximately the same incident photons, and halogen tungsten lamp as white light source, respectively, to induce photoemission. The performance characteristics including quantum efficiency and photocurrent degradation among the samples treated by different illumination conditions are compared to investigate their photoemission capability and stability. The results show that GaAs photocathodes activated and recesiated under illumination of monochromatic red light, can obtain higher quantum efficiency, and better stability. This work provides a favorable illumination condition during activation for obtaining satisfactory capability of GaAs-based photocathodes, which will be propitious for applications as spin-polarized electron sources.

Effect of graded bandgap structure on photoelectric performance of transmission-mode AlxGa1-xAs/GaAs photocathode modules.

The graded bandgap AlxGa1-xAs/GaAs photocathode with graded composition and exponential doping structure has shown great potential for improving photoemission capability. In order to better study the performance of transmission-mode AlxGa1-xAs/GaAs photocathode with the complex graded bandgap structure, the experimental optical properties and quantum efficiency are measured by comparison with uniform composition and exponential doping Al0.7Ga0.3As/GaAs photocathode. The theoretical optical properties of the multilayer AlxGa1-xAs/GaAs photocathode modules are calculated by matrix formula on the basis of thin-film optical principles. The effect of cathode thickness and aluminum proportion on optical properties are analyzed by simulation. The results show that these parameters have complicated effects on the optical properties. Different parameters are presented as the changes of peak and valley of the optical property curves. Meanwhile, the emission layer has a significant effect on the absorptivity values of the photocathode modules, which will obviously influence photoemission performance. By using the optical properties via calculation, a better fit of the experimental data with the theoretical model can be achieved, which would make reasonable guidance for further investigation of these complex graded bandgap photoemitters.

Comparison of degradation and recaesiation between GaAs and AlGaAs photocathodes in an unbaked vacuum system.

The lifetime and reliability of a photocathode during operation are always raised problems and the photocathode performance depends on the vacuum condition. With the purpose of investigating the stability and reliability of a GaAs-based photocathode in a harsher vacuum environment, reflection-mode exponential-doped GaAs and AlGaAs photocathodes are metalorganic vapor-phase epitaxial grown and then (Cs, O) activated inside an unbaked vacuum chamber. The degraded photocurrents are compared after activation and recaesiations between GaAs and AlGaAs photocathdoes under illumination with an equal initial photocurrent and an equal optical flux, respectively. It is found that the performance on degradation and recaesiations between GaAs and AlGaAs photocathodes are different. In the unbaked vacuum system, the stability of an AlGaAs photocathode after (Cs, O) activation is always better than that of a GaAs photocathode. After multiple recaesiations, the photocurrent decay curves of the AlGaAs photocathode are nearly coincident, which means a nearly constant operational lifetime. Moreover, operational lifetime of an AlGaAs photocathode is longer than that of a GaAs photocathode, which further illuminates that AlGaAs photocathodes are superior to GaAs photocathodes in stability and repeatability under markedly harsher vacuum conditions.

Comparative study of Al(x)Ga(1-x)As/GaAs photocathodes with different aluminum concentrations by surface photovoltage spectroscopy.

The influence of aluminum concentration in an Al(x)Ga(1-x)As window layer on the performance of Al(x)Ga(1-x)As/GaAs photocathodes was investigated. Three types of transmission-mode photocathode materials with different aluminum concentrations were designed for the comparative research. The surface photovoltage technique was applied to prepare samples. After the Cs-O activation process, spectral response curves of Al(x)Ga(1-x)As/GaAs photocathodes were obtained. Comparative studies show that a higher aluminum composition in the window layer is beneficial to improve the response of Al(x)Ga(1-x)As/GaAs photocathodes in the shortwave region. The surface photovoltage calculation formula of photocathode materials was put forward and used to obtain key performance parameters of Al(x)Ga(1-x)As/GaAs photocathodes by fitting calculations. Through calculations, the Al(x)Ga(1-x)As/GaAs interface recombination velocity, the minority carrier diffusion length of the window layer, and the emission layer were deduced, and there is a positive correlation between the aluminum composition in the window layer and the Al(x)Ga(1-x)As/GaAs interface recombination velocity, which is negative with the performance of photocathodes.

Photoemission from advanced heterostructured Al(x)Ga(1-x)As/GaAs photocathodes under multilevel built-in electric field.

A heterostructured Al(x)Ga(1-x)As/GaAs photocathode consisting of a composition-graded buffer layer and an exponential-doped emission layer is developed to improve the photoemission performance over the wavelength region of interest. The theoretical quantum efficiency models for reflection-mode and transmission-mode Al(x)Ga(1-x)As/GaAs photocathodes are deduced based on one-dimensional continuity equations, respectively. By comparison of simulated results with conventional quantum efficiency models, it is found that the multilevel built-in electric field can effectively improve the quantum efficiency, which is related to the buffer layer parameters and cathode thicknesses. This special graded bandgap structure arising from the compositional grade in the buffer layer and doping grade in the emission layer would bring about the reduction of back interface recombination losses and the efficient collection of photons generating photoelectrons. Moreover, a best fit of the experimental quantum efficiency data can be achieved with the aid of the deduced models, which would provide an effective approach to evaluate internal parameters for the special graded bandgap photoemitters.

Resolution properties of transmission-mode exponential-doping Ga0.37Al0.63As photocathodes.

Using the modulation transfer function obtained by establishing and solving the two-dimensional continuity equation, we have calculated and comparatively analyzed the resolution characteristics of transmission-mode exponential-doping and uniform-doping Ga0.37Al0.63As photocathodes. The calculations show that compared with a uniform-doping Ga0.37Al0.63As photocathode, the exponential-doping structure can significantly improve not only the resolution, but also the quantum efficiency of the photocathode. This improvement is different from the approach for high resolution by reducing the emission layer thickness Te and electron diffusion length LD, or by increasing the recombination velocity of the back-interface, SV, which results in low quantum efficiency. Furthermore, the improvement in resolution and quantum efficiency for the transmission-mode exponential-doping Ga0.37Al0.63As photocathodes is the result of the effect of the built-in electric field on electron transport and lateral diffusion.

Resolution characteristics for reflection-mode exponential-doping GaN photocathode.

According to the expression for modulation transfer function obtained by solving the established 2D continuity equation, the resolution characteristics for reflection-mode exponential-doping and uniform-doping GaN photocathodes have been calculated and comparatively analyzed. These calculated results show that the exponential-doping structure can upgrade not only the resolution capability but also the quantum efficiency for a GaN photocathode. The improvement mechanism is different from the approach for high resolution applied by reducing Te and L(D) or increasing S(V), which leads to low quantum efficiency. The main contribution factor of this improvement is that the mechanism that transports electrons toward the NEA surface is facilitated by the built-in electric field formed by this exponential-doping structure, and the corresponding lateral diffusion is reduced.

Comparative research on GaAs photocathodes before and after activation.

Obtaining higher quantum efficiency and more stability has been an important developing direction in the investigation of GaAs photocathodes. By solving the one-dimensional diffusion equation for no-equilibrium minority carriers of reflection-mode GaAs photocathode materials, we can get the equations of the surface photovoltage curve before activation and the spectral response curve after activation for uniform and exponential doping GaAs materials. Through experiments and fitting calculations for two doping structural materials designed by us, the parameters of the body materials are exactly measured by the surface photovoltage curves, and the curves for surface escape probability are also accurately fitted by the comparative research before and after activation. The differences for the fitting results of two doping structures are also well analyzed. This comparative research can form a closed-loop research for GaAs photocathodes and will help us to deeply study the varied doping structures and optimize Cs-O activation technology for GaAs photocathodes in the future.