Super-second-generation and third-generation image intensifiers are two types of image intensifiers that use different technologies. Super-second-generation image intensifiers employ a Na2KSb(Cs) photocathode, whereas third-generation image intensifiers employ a GaAs photocathode. Third-generation image intensifiers employ higher cathode voltages than those employed by super-second-generation image intensifiers. In addition, third-generation image intensifiers employ an antireflection coating between the glass input window and GaAs photocathode; however, this is not employed in super second-generation image intensifiers. Furthermore, third-generation image intensifiers employ ion barriers on their MCP(microchannel plate), whereas super-second-generation image intensifiers do not. In terms of limiting resolution, despite the small initial electron velocity, narrow exit angle distribution, and high cathode voltage of the third-generation image intensifiers, the limiting resolutions of the two types of image intensifiers are the same; the advantages of the GaAs photocathode of the third-generation image intensifiers have not been introduced under the existing limiting resolution level. In terms of signal-to-noise ratio, the GaAs photocathode has a higher cathode sensitivity, normally more than twice that of the super-second-generation image intensifier. Thus, theoretically, the third-generation image intensifiers have signal-to-noise ratios that are 1.4 times those of the super-second-generation image intensifiers. However, the two types of image intensifiers are basically the same owing to the influence of higher cathode voltage and ion barrier transmittance and the advantage of not introducing the high sensitivity of the GaAs photocathode of the third-generation image intensifiers. In terms of gain, although the third-generation image intensifiers have higher cathode sensitivity and cathode voltage, the super-second-generation image intensifiers compensate for the shortcomings of cathode sensitivity and cathode voltage by increasing the working voltage of the microchannel plate. Therefore, in terms of the existing image intensifier gain, the gains of the two types of image intensifiers are identical. In terms of equivalent background illumination(EBI), owing to the higher sensitivity of the GaAs photocathode, the third-generation image intensifiers can obtain lower equivalent background illumination under the same photocathode dark current. Therefore, the third-generation image intensifiers have higher initial contrast than that of the super-second-generation image intensifiers. The higher the initial contrast of the input image, the higher the contrast of the output image. In terms of halo, because the photocathode of the third-generation image intensifiers has high sensitivity and an ion barrier film, theoretically, the third-generation image intensifiers have higher halo brightness than that of the super-second-generation image intensifiers. However, in actual situation, the halo brightness levels of the two types of image intensifiers are basically the same. In terms of stray light, the GaAs photocathode has an antireflection coating; thus, the stray light is lower than that of the super-second-generation image intensifier, so the imaging of the third-generation image intensifier is clearer and the sense of gradation is better. In terms of spectral response beyond the long-wavelength threshold, because the spectral responses beyond the long-wavelength threshold of the super-second-generation image intensifiers are higher than those of the third-generation image intensifiers, the super-second-generation image intensifiers have better imaging performance than that of the third-generation image intensifier under supplementary illumination using the near-infrared waveband. For example, without the presence of any light, the super-second-generation image intensifiers can obtain better images at a supplementary illumination of 980 nm wavelength, whereas the third-generation image intensifiers cannot. In terms of the resolution of low illumination, the super-second- and third-generation image intensifiers with similar performance parameters have the same low luminance resolution. It should be noted that this conclusion was obtained under the test conditions of a standard A light source. When the actual environmental emission spectrum distribution is different from that of a standard illuminant A, the low illumination resolutions of the two types of image intensifiers are different. Photocathode sensitivity is a parameter of the photocathode and not of the image intensifier. Thus, the performances of the two types of image intensifiers cannot be compared in terms of photocathode sensitivity. The difference between the super-second and third-generations cannot be understood using the meaning of “generation;” their differences do not lie in the meaning of “generation.”
Semiconductor photocathodes with high quantum efficiency and low dark current are widely used in various vacuum photoelectric detection and imaging devices, such as photomultiplier tubes and image intensifiers, promoting the development of ultrafast detection and imaging technology for extremely weak light. Vacuum electron sources capable of producing high-quality electron beams are used in accelerator photoinjectors, electron microscopes, and other scientific equipment. First, this review introduces the classification of semiconductor photocathodes and their applications in the fields of vacuum photoelectric detection and imaging and vacuum electron sources. Then, preparation techniques for three types of typical semiconductor photocathodes, namely, alkali telluride, alkali antimonide, and GaAs photocathodes, are summarized. Subsequently, applications of new technologies, such as micro-nano structures, low-dimensional materials, and single-crystal epitaxy, in the development of semiconductor photocathodes are introduced. Finally, the technical development of the semiconductor photocathodes is discussed.
A new type of charge-sensitive three-stage amplifier was developed. The circuit design mainly adopted an ADA4817 high-speed and low-noise integrated operational amplifier chip. The three-stage amplifier had low noise, good stability, a simple circuit structure, high cost performance, and ease of maintenance. It could amplify signals with rise time in nanoseconds without distortion, and the output signal quality of the amplifier was excellent. It was used with a multi-channel analyzer MCA8000D to read a single photoelectron spectrum of a microchannel plate (MCP) assembly or a single channel electron multiplier (CEM). The test results show that the custom-made amplifier fully satisfied the pulse performance test requirements of MCP or CEM detectors, and its technical indicators were close to those of imported A250 amplifiers.
In this study, we epitaxially grew a multilayer structure of gallium nitride (GaN) photocathode film material on a sapphire substrate and conducted a surface photovoltage test. The effects of doping type, thickness, and doping method on the surface photovoltage of the gallium nitride material were compared and analyzed, and the mechanism of surface photovoltage generation of the multi-layered gallium nitride material was determined. A surface photovoltage test was performed on uniformly doped and delta-doped gallium nitride photocathode thin film materials using sub-band-gap laser. Experimental data shows that better growth quality was achieved using δ-doping than that achieved using uniform doping; however, δ-doping increased the density of defect states in the (Ev+0.65)–(Ev+1.07) eV energy levels.才项目”。
Aiming at the problem that the brightness gains of super Gen. Ⅱ image intensifiers, equipped with high-voltage DC power supplies, substantially decrease at high temperatures, according to the theoretical analysis, this study developed high and low temperature test platforms to study the temperature characteristics of an image intensifier with a high-voltage DC power supply, an image intensifier tube, and a single high-voltage DC power supply. The experimental results show that the brightness gain of this type of image intensifier at a high temperature (55℃) decreased by approximately 65% than that at a low temperature (-55℃). However, on supplying constant cathode, MCP, and anode voltages to the image intensifier tube, the brightness gain decreased by 20%, which was mainly due to the decrease in the cathode sensitivity and luminous efficiency of the fluorescent screen with the increase in temperature. Compared with low temperature (-55℃), the cathode voltage of the high-voltage DC power supply was reduced by approximately 40 V at high temperature (55℃), whereas the MCP and anode voltages were reduced by approximately 18 and 100 V, respectively. These three factors accelerated the attenuation of the brightness gain of the image intensifier at high temperatures. Therefore, compensating the cathode, MCP, and anode voltages using software and/or hardware to the power supply can be an effective method to improve the brightness gain consistency of super Gen. Ⅱ image intensifiers with a high-voltage DC power supply at various temperatures.
Image intensifiers, which are core imaging devices for low-light-level night-vision techniques, are used worldwide as advanced military night-vision devices. A method of directly comparing and analyzing the final output image characteristics of different image intensifiers is the most intuitive routine for assessing the image quality of image intensifiers. However, because image intensifiers are direct-view imaging devices, they face many challenges, such as field experimental set-up, synchronous image capturing, and adaption of image intensifiers with different types. Therefore, an image quality assessment method and image quality comparison system for image intensifiers were proposed. Dual-channel image intensifiers, a low-light-level CMOS and a laser rangefinder were integrated into the system; the main optical axis of each device was parallel. The images of the image intensifiers and CMOS were synchronously observed and captured. The system is compatible with different types of image intensifiers and power supply modes. The distance between the target and the system was also measured. Moreover, several no-reference image-quality-assessment metrics were integrated into the software to assist the observer in assessing the image quality of different image intensifiers. The actual field test results show that the proposed system is convenient and practical for comparing the quality of different imaging devices, and can facilitate optimization of the manufacture of image intensifiers, thereby promoting the development of low-light-level night-vision technology.
The open area ratio is an important performance index for microchannel plates(MCPs). The detection efficiency and noise factor of MCPs can be significantly improved by expanding the entrance of the pores, which has potential application in the military and civil fields, such as low-light-level night-vision devices and particle detectors. MCPs with funneled pores prepared through wet etching are difficult to apply on a mass scale because of problems such as poor consistency of the process and difficulty in conforming funnel size caused by selective corrosion. A dry-etching method for MCP flaring was proposed for the first time. The principle and feasibility of dry etching for MCP flaring are described in this paper. A theoretical model was established to investigate the effects of dry etching process parameters, such as etching angle and etching time, on the MCP opening area ratio, etching depth in the channel, etching taper on the channel wall, and other performance parameters. The appropriate range of the process parameters was calculated theoretically, which provides a foundation for experimental research.
GaAs photocathodes are widely used in low-light night vision owing to their high quantum efficiency and adjustable spectra. In particular, they are distinguished from multi-alkali photocathodes based on their high integration sensitivity. The negative electron affinity of GaAs photocathodes is determined through Cs, and O activation is achieved. However, after activation, the maintenance of negative electron affinity is affected by many factors, such as the activation source, activation method, and gas atmosphere. To explore the factors that affect the stability of GaAs photocathodes in ultra-high vacuum systems, an activation and stability experiment was performed with a GaAs photocathode. The activation photocurrent curve and gas composition in a chamber were monitored. The experimental results show that in a high-vacuum system with vacuum degree less than 1×10.6 Pa, the stability of the GaAs photocathode was not directly affected by the degree of vacuum but by the gas composition in the chamber. Among these, H2O had the greatest impact on stability. The increase in the H2O partial pressure in the vacuum system rapidly destroyed the Cs and O activation layers of the GaAs photocathode and dramatically reduced the photoemission.
HgCdTe has dominated the high-performance IR detector market for decades. Owing to its numerous merits, including precise energy band structure control and device structure growth, the MBE(molecular beam epitaxy) growth of HgCdTe has become the main tool for fabricating third-generation IR focal plane arrays. CdZnTe is widely considered to be an ideal substrate for HgCdTe epitaxy because of the matched lattice through Zn fraction adjustment. Therefore, HgCdTe/CdZnTe has a high crystal quality with a typical etch pit density in the range of 1.104–1.105 cm-2. However, several limitations, such as high cost, small wafer size, and low yield, still exist in the (211) CdZnTe substrate, which results in high cost and limits the array format size in infrared detectors based on HgCdTe/CdZnTe. Compared with CdZnTe substrates, alternative substrates (e.g., Si, Ge, GaAs, and GaSb) have large wafer size, low cost, and convenience in standard semiconductor equipment, which have the potential to fabricate low-cost high-performance focal plane arrays. The main issue in HgCdTe on alternative substrates is the large lattice mismatch between the substrate and epi-layer (19.3%, 14.3%, 14.4%, and 6.1% for Si, Ge, GaAs, and GaSb, respectively), which is responsible for the high dislocation density of 106–107 cm-2 in HgCdTe films. The high dislocation density hampers the application of this material to long-wavelength and very long-wavelength infrared detectors. The variation in dislocation density with film thickness in the as-grown HgCdTe film grown on an alternative substrate was modeled, and the results from the ρ~1/h law agreed well with the experimental data. This indicates that the dislocation annihilation radius is the leading cause of impeding the dislocation density below 5.106 cm-2 in HgCdTe; thus, dislocation reduction is urgently needed. Moreover, the theory and research progress on three dislocation reduction methods, namely thermal cycle annealing (TCA), dislocation blocking, and mesa dislocation gettering (MDG), are summarized in this paper. Prospects and priorities for future development are also discussed. Overall, TCA and dislocation blocking techniques are likely to be harder in technical breakthroughs and have less development potential in dislocation reduction to below 5. 105 cm-2. By contrast, the MDG technique has shown tremendous development potential and high value in low-cost long-wavelength infrared detectors; however, process integration between the MDG technique and standard focal plane array fabrication is needed.
According to the structural characteristics and working conditions of a low light level multispectral imager, an integrated device of focusing and image motion compensation is designed to be smaller, better imaging quality and low illumination imaging. The focusing function is realized by the screw nut and the wedge slider. The realization of the motion compensation function depends on the voice coil motor, and with the dynamic and static two-stage locking device. The reliability and impact resistance of the mechanism are significantly improved. The overall dimension of the structure is 349 mm×192 mm×174 mm, the focusing range is ±2 mm, the image motion compensation is 3 mm, the focusing resolution is 0.05 .m, and the actual positioning accuracy is ±5.7 .m. The first order mode is 225 Hz, which is consistent with the result of finite element simulation. The results of the sine vibration test and random vibration test meet the requirements of the technical indicators. It shows that it has good dynamic stiffness and can effectively avoid the resonance phenomenon. The focusing and image motion compensation mechanism has small size and high structural strength, which meet the working conditions of low light level cameras.
Short-wave infrared (SWIR) cameras have several advantages over medium-wave infrared (MWIR) and long-wave infrared (LWIR) cameras. Hence, a method for evaluating their performance is crucial for the application and development of electro-optical systems. We suggest a method that can be used to evaluate the performance of an SWIR camera based on the minimum resolvable vontrast(MRC) test. An integrating sphere and five targets with different contrasts were used. The intensity of the radiation source, aliasing, and observation patterns were controlled to evaluate the SWIR camera performance. We applied this method to test the MRC of the SWIR system. Furthermore, a series of field experiments was conducted, and the results were in agreement with the MRC testing data. The uncertainty of the method reached 2.11%, which supports the conclusion that the MRC method can be applied to evaluate and predict the performance of SWIR cameras.
Infrared and visible image fusion is widely used in target tracking, detection, and recognition. To preserve image details and enhance contrast, this study proposed an infrared and visible image fusion method based on latent low-rank representation. The latent low-rank representation was used to decompose the source images into base and significant layers, in which the base layers contained the main content and structure information, and the salient layers contained the local area with relatively concentrated energy. The ratio of low-pass pyramid was also adopted to decompose the base layer into low-frequency and high-frequency layers. The corresponding fusion rules were designed according to the characteristics of the different layers. A sparse representation was used to express the relatively dispersed energy of the low-frequency base, and the rules of the maximum L1 norm and maximum sparse coefficient were weighted averages to retain different significant features. The absolute value of the high-frequency part of the base layer was used to enhance the contrast. Local variance was used for the salient layer to measure significance, and the weighted average was used to highlight the target area with enhanced contrast. Experimental results on the TNO datasets show that the proposed method performed well in both qualitative and quantitative evaluations. The method based on low-rank decomposition can enhance the contrast of the targets and retain rich details in infrared and visible fusion images.
This paper presents a fuzzy clustering method based on similarity thresholding to detect an overheating fault region from an infrared image of a transmission line. In this method, the original iteration mechanism of fuzzy clustering was improved and a thresholding fuzzy clustering model was built. Thus, a fuzzy member was utilized to measure the neighboring pixels t by conducting cluster analysis on the object region with local neighboring pixels. This ensured similarity during the clustering of the local neighboring pixels into the cluster center. In addition, the maximum similarity thresholding rule was applied to determine the final thresholding using the strategy of thresholding from top to bottom, thus improving the efficiency of the method in obtaining the final region of interest in the infrared image using fuzzy clustering. Finally, experimental results on infrared images of transmission lines show that the good performance of the proposed method and that the proposed method is suitable for fault detection in transmission lines.
This work introduces a thermal fault diagnosis method that integrates superpixel segmentation and low-rank representation for diagnosis. The proposed method comprises two main steps. First, an input infrared image is transformed using a principal component analysis (PCA) algorithm, and a superpixel segmentation method is employed for the first principal component (PC). The first PC is divided into non-overlapping homogeneous superpixels. Then, the thermal fault region is detected by employing low-rank representation in a superpixel-by-superpixel manner. Experimental results show that the proposed diagnosis method has a better detection performance than that of current state-of-the-art detectors.
To improve the effectiveness of infrared image dehazing, an improved dark channel was proposed. First, because the penetration ability of near-infrared light in the sky region is different from that in the non-sky region, and the infrared energy in the sky region is smaller than that in the non-sky region, the region was divided into sky and non-sky regions using the energy difference. Second, the atmospheric light value of the sky region was calculated using the average pixel brightness of the sliding window, the near-infrared wave attenuation was considered for the transmittance, and the atmospheric light value and transmittance of the non-sky region were calculated using the improved dark channel algorithm. Finally, the dehazed image was recovered from the atmospheric light value and transmittance of each region. The experimental results show that the dehazing output of the infrared image was clearer, the image detail information was better, and the evaluation index was better than those of other algorithms.
The inspection and identification of writing ink are important in the field of forensic science. A Fourier transform infrared spectrometer was used to test 20 types of direct liquid ball pen ink samples, and chemometrics was used to rapidly test the direct liquid ball pen ink. The Fourier transform infrared spectrum data were standardized, and the spectrum was pre-processed using three methods: automatic baseline correction, peak area normalization, and Savitzky-Golay 5-point smoothing. The optimal value of classification K was determined using the sum of squares error(SSE). The samples were analyzed using K-means clustering, and the clustering results were explained. The principal component analysis method was used to verify the results of K-means clustering. The group mean equality test was used to investigate the contribution of principal component variables to the Fisher discriminant analysis (FDA) model, and the FDA discriminant model of straight liquid ball pen ink was constructed. The results show that all the ink samples were clustered into three categories using K-means clustering. The principal components analysis–Fisher discriminant analysis (PCA–FDA) model achieved 100% prediction and classification of different categories of straight-liquid ball pen inks with an accuracy of 100% after cross-validation. Infrared spectroscopy combined with the PCA–FDA model can be used for rapid and accurate inspection and identification of direct liquid ball pen inks.