2013 Vol. 6, No. 5
The image reconstruction algorithms of tomography are introduced. The characteristics of the various algorithms are compared from the points of view of the forward model simplication and reverse model mapping structure. Studies show that the various conversion methods belonging to linear algorithm have serious distortion, which can be improved by convolution filtering. The various iterative algorithms based on derivative search have strong initial value dependence and slow convergence and are easy to fall into a local optimal solution. The various Fourier transform methods have intrinsic limitation and the wavelet transform can characterize both the time and frequency domain minutiae of the image. The finite element method can simplify the forward model by smart designing pixels of the reconstruction object. With the physical background, the Monte Carlo method, simulated annealing, genetic algorithms, particle filter method and the neural network method are more suitable for complex and nonlinear image reconstruction. Moreover, intelligentization, modeling, parallelization, and integration of various algorithms are the trends for the image reconstruction algorithms of the tomography.
The bioimaging technology based on the Surface-enhanced Raman Spectroscopy(SERS) has emerged as a potential alternative due to its narrow bandwidth, weak background interference, high stability of label agent and specificity. Raman imaging technique can not only analyze a single point of the sample, but also offer an integral description of ingredient, distribution and variation for a large scale selected region. This paper introduces the principle of surface enhanced Raman scattering and its enhancement mechanism. Then, it reviews the recent development of SERS based bioimaging where both non-labeled and labelled SERS imaging techniques are included, and describes its applications to the biomedicine. Finally, it discusses some problems existed in the current technology and points out the future developing trends of the SERS-based bioimaging.
The background of Thirty Meter Telescope(TMT) and its mission that we should take in hand are introdced. The detailed specifications and the preliminary scheme for the M3CA and M3PA in the 3 mirror(M3) system are presented according to the general specification. Then, the two parts,axial support and lateral support of M3CA, are designed in detail. Due to the kinematic concept design used in both of axial support and lateral support, the mirror surface error can be reduced in a acceptable level. When the kinematic axial support scheme is taken, its surface accuracy RMS is 109.7 nm and Slope RMS is 0.95 rad, while the kinematic lateral support scheme is taken, its surface accuracy RMS is 15 nm,and the first natural frequency can reach to 17.7 Hz. The design proposal for M3PA is discussed and the control system for M3 system(M3CS) is introduced briefly.
Parallel confocal measurement technology changes traditional simple scanning into a multi-spot scanning, and it will improve the efficiency of measurement significantly. In this paper, the working principle of confocal measurement technology is introduced and some methods to produce a parallel light source in the parallel confocal measurement are surveyed. Then, it analyzes the advantages and disadvantages of these methods. At last, it presents the latest research achievements, and describes new methods to produce the parallel light source based on the micro lens array and the Digital Micro-lens Device(DMD). By analysis of the mechanism of spatial light modulation, it establishes a laser parallel astigmatism measuring system based on a parallel light source and dual beam paths.
Advances in the techniques available for natural radiative lifetime measurement of excited states of free atoms and ions are presented. The disadvantages and advantages of several often-used methods for radiative lifetime measurements of free atoms and ions of rare-earth elements are reviewed by taking La Ⅰ and Pr Ⅱ as examples. Then, the techniques of the laser ablation to produce free atoms and ions are introduced by taking Sm Ⅱ as an example, and the time-resolved laser induced fluorescence(LIF) spectroscopy for the radiative lifetime measurements is discussed in detail. The limitation of this method of LIF combined with laser ablation is summarized and a possibly useful solution is suggested, which may be helpful to further improve the precision of radiative lifetime measurements.
This paper describes the basic process of laser communications and their link types firstly. Then, it focuses on the latest developments of the laser communications in the European Space Agency(ESA), the United States, Germany, Japan, and domestic laser communication research, and also gives detailed technical parameters of some successful applications for the laser communication terminals in space, such as laser wavelengths, communication distances, communication speeds, bit error rates, and application purposes. It highlights the characteristics and advantages of each instrument in opto-mechanical structure, points out the key technical problems faced by the laser communications, as well as the future development of laser communication trends and directions through the analysis of the research status at home and abroad, which will provide a reference for the study of laser communication equipment and its related fields.
The free space optical communication technology based on a cat-eye modulating retro-reflector has great advantages as compared to the classical free space optical communication technology. In this paper, the principle of free space optical communication technology based on modulating retro-reflector is described and its main construction is introduced. Then, the research status of the technology in modulator characteristics, cat-eye optical systems and technology used in foreign countries is reviewed, and the technology development in China is introduced briefly. The key points of the technology are analyzed. Finally, the application trends and developing prospects are viewed.
The methods and characteristics of limb viewing are introduced, and the research progress of limb imaging spectrometers is discribed. Then, the technical indexes of limb imaging spectrometers in the developed countries in America and Europe are compared and analyzed. Finally, the development situation of our country about the limb imaging spectrometers is introduced. Through comparative analysis of the research progress of limb imaging spectrometers at home and abroad, it provides the theoretical background basis for our country to further improve the performance and technical index of limb imaging spectrometers.
In order to detect low clouds and heavy fogs during nighttime and twilight, and to achieve three-dimensional imaging of the clouds, the key technologies of shimmer tridimensional imaging were discussed. First, the theory of shimmer tridimensional imaging was introduced, and the wide-field coverage and multi-lens layout for imaging were analyzed. Then, the low-light imaging technology and its implementation were introduced. Finally, aiming at the larger illuminance change of an observed target, the dynamic range expanding technology is presented, and detecting technology for clouds and fogs was introduced. Emulation calculation results indicate that the EMCCD detector and push-broom imaging mode, integrated detector assembly and the multi-camera stitching program can effectively realize shimmer tridimensional imaging. The camera swath is beyond 2 800 km and elevation resolution is less than 817.7 m. By using this imaging technology, we can detect the low clouds and heavy fogs in a low-light condition, and can get high-resolution three-dimensional imaging of the clouds, which satisfies the requirements of meteorologycal and oceanic detection.
In order to research the effect of holes on the spin electron diffusion, a method of resonant spin amplication called Spin Concentration Grating(SCG) is adopt to observe the process of electron spin diffusion. Transient spin grating and spin concentration grating excited by femtosecond laser beams are used to investigate electron spin diffusion and electron spin ambipolar diffusion in intrinsic GaAs multiple quantum wells. The measured coefficient of electron spin ambipolar diffusion Das=25.4 cm2s-1 is lower than that of electron spin diffusion Ds=113.0 cm2s-1, which indicates that the influence of holes on electron spin diffusion in spin concentration grating is notable.
Light management is an important factor for high-efficiency crystalline silicon solar cells, which can enhance light absorption to increase a short-circuit current(Jsc). In this contribution, the most common light management methods including antireflection, surface scattering, and light confinement are reviewed. Researchers have developed various surface antireflection structures to lower the surface reflection loss of the cell. For example, biological structures fabricated by moth-eye type texture create a gradient refractive index profile resulting in a reflectance below 1% over a wide wavelength range. As the wafer thickness is reduced gradually, light management becomes even more critical for cell performance. Antireflection for the wide wavelength and incident angle range has attracted much attention. And light scattering and confinement may be the most effective ways to enhance light path length and absorption for thinner substrates.
Due to the presence of system noises in second harmonic signals of a laser gas analyzer based on Tunable Diode Laser Absorption Spectroscopy(TDLAS), this paper designs an Finite Impulse Response(FIR) based filtering scheme to realize the online filtering of second harmonic of the laser gas analyzer. First, based on the theory of FIR digital filting, a FIR digital filter is designed with window functions based on a MATLAB(Matrix laboratory) software platform. Then, the corresponding algorithm is transplanted to the embedded system of laser gas analyzer. Finally, on the basis of waveform analysis of the second harmonic signals before and after filterings, it proves the feasibility of the proposed filter algorithm that the FIR digital filter is embedded system to realize the online filtering of second harmonic in the laser gas analyzer. Experimental results indicate that algorithm computation time for one set of second harmonic signal is 230 ms. The FIR filter can effectively filter out the interference signals of second harmonic by comparing the waveforms of the second harmonic signals before and after filterings. The FIR digital filter can basically meet the requirements of laser gas analyzer for denoising, smoothing the second harmonic signals with noises, lowering computational complexity as well as enhancing the performance of transplanting.
To obtain thermal damage characteristics of the HgCdTe crystal irradiated by a CO2 laser, the theoretical model of Hg0.784Cd0.216Te crystal damaged by the high repetition frequency CO2 laser was developed, and then the damage thresholds of Hg0.784Cd0.216Te crystal irradiated by a single pulse laser and a high repetition frequency laser were calculated based on the finite element method. Finally, the effect of irradiation time and laser repetition frequency on the damage thresholds was analyzed. The results show that the damage threshold of the crystal radiated by a single pulse laser is 64.5 J/cm2. In the high repetition frequency(f1 kHz) CO2 laser irradiation, the damage threshold of the crystal mainly depends on the laser average power density and irradiation time, and is independent of the laser repetition frequency. With the increase of irradiation time, the damage threshold can effectively be reduced. When the laser average power density is less than 1.95 kW/cm2, the crystal damage will not occur. The conclusions have a reference value for laser processing and protection.
The template method and sol-gel technique were used for the preparation of gold nanorod-silica core-shell (GNR@SiO2) nanocomposites. The controllable synthesis, spectroscopic properties, cytotoxicity and cell imaging of these nanocomposites were discussed by using UV-Vis spectrophotometry, transmission electron microscopy, confocal microscopy and other characterized measurements. The results show that, by control of the reaction time, the thickness of silica shell can be facilely tuned in the regime of 20-30 nm; the colloidal stability of gold nanorods has been greatly improved, and their cell cytotoxicities are significantly decreased due to the presence of the silica shell. In addition, fluorescent probes can be easily attached on the silica shell by post chemical reactions. As formed fluorescent core-shell nanocomposites can be easily captured by cells through endocytosis pathway, which is useful for the optical imaging. Therefore, these nanoparticle composites have a great potential in fields of biosensors, cell imaging and photothermal therapy.
According to the feature of a wide coverage space camera, the geometrical relation of scroll imaging was analyzed.The space coordinate transform process from an object point on the earth to the image point in the focal plane was found. Furthermore, the calculation formulas of scroll image motion velocity were deduced based on a round earth model by using the transform of space coordinate system. The effect of relative image motion match error on the image quality was analyzed by the image motion mismatch modulation transfer function(MTFmismatch). By taking a wide coverage space camera for an example, image motion velocities at different scroll angles and different centrifugal angles were researched. Computation results indicate that the image motion velocities and their variation trends with the scroll angle are different in the different positions of image plane. The MTFmismatch caused by mismatch image motion velocity reaches 0.681 at the perigee if the scroll angle is 25 and the TDI stage is 48. The actual field experimental results show that it is necessary to keep the TDI stages within 16 to ensure good image quality.
This paper introduces a method that uses a Light Emitting Diode(LED) point light source as the camera calibration light. According to the characteristics of the LED point light source, a constant current source is used to provide the necessary current and a illuminometer is used to measure the luminance of the LED point light source. The constant current source is controlled by an ARM MCU and its data are exchanged with the host computer through the mode of serial communications. The PC is used as the host computer, and it adjusts the current according to the luminance of the LED point light source until the luminance achieves an anticipated value. Experimental analysis shows that the LED point light source with a higher precision can meet the requirements of the calibration light source, and can adaptively control the luminance of LED well. The system is convenient and flexible, and its performance is stable and reliable.
To solve the image degradation caused by satellite platform vibration and to improve TDI CCD image quality, the range of satellite attitude stabilization and its effect were analyzed. First, the dependence of the satellite platform vibration on the TDI CCD image quality was discussed. Then, equations which gave the relationship between image motion and attitude stabilization were derived. Finally, it was concluded that the satellite attitude stabilization was relevant to orbit altitudes and integral stages. The experiment results indicate that the higher the orbit altitude is and the bigger the integral stages are, the more rigorous the requirement of satellite attitude stabilization is. From obtained data, it is demonstrated that when the orbit altitude changes from 200 km to 1 000 km, the requirement of satellite attitude stabilization need to improve from 0.037 7 to 0.006 35 rad/s. Meanwhile, when the integral stages change from 1 to 100, the requirement of satellite attitude stabilization need to be improved from 0.014 to 0.000 14 rad/s. The work in this paper confirms the relationships of satellite attitude stabilization, orbit altitudes and integral stages, which contributes to improve TDI CCD image quality.
The focusing of a mapping camera affects its principal point, which will change the inner orientation elements and affect the mapping accuracy. Therefore, this paper analyzes in detail the effects of three usual focusing methods(lens focusing, plane mirror focusing, focal plane focusing) on the principal point position of aerospace mapping camera. First, the principles of three usual focusing methods were introduced briefly, and the effects of these focusing methods on the principal points were studied under an ideal condition. Then, through considering the assembly error, the mathematical model between the principal point offset and the focusing distance was built, and the focal plane focusing was proved to be the best focusing method through the example computation in which the positional error of principal point was less than 0.2 pixel. Finally, by taking the focusing mechanism designed for some cameras as the experiment object, the principal point changes of plane mirror focusing and focal plane focusing were measured. The results show that the computing formula proposed is accurate, and the analysis about the principal point based on different focusing methods is right. The results can be used for choosing the focusing mode of the mapping camera and can meet the needs of high precision mapping.
In order to achieve the high-precision, large-size, three-dimensional space and real-time measurement of mechanical components, a monocular vision measurement system with a light pen was established. The new technology, algorithm converting model, the optimization of camera focal length and the position calibration of the light pen probe were studied. First, a new type of light pen which was designed by using the photolithography process was proposed. Then, based on resection principle of single image space in the close- range photogrammetry, the new conversion relationship between the light pen coordinate system and the camera coordinate system was established. Optimal exterior orientation elements of single image space was solved by least squares adjustment method in the loop iteration, and basic parameters of the transformation model were determined. Finally, the impacts of focal length on the result of monocular vision measurement system were analyzed, and a method to determine the relatively accurate focus length and the position of light pen probe in the coordinate system of the light pen was proposed. Experimental results indicate that the stability errors in the x-axis, y-axis, z-axis directions of the camera coordinate system are 0.042 733 mm, 0.048 513 mm and 0.066 39 mm, respectively, and the maximum measuring error is 0.173 mm. These results meet the requirements of monocular vision measurement system with a light pen for stability and high accuracy.
To satisfy rigid performance specifications of goniometry for a telescope, this paper analyzes the impacts of the mechanical installation eccentric, roundness error and the shafting sloshing on angle measurements by an encoder with a scale tape grating, and develops a software subdivision angle measurement program based on 4 reading heads and 90 phase difference. The counterclockwise and clockwise angle measuring experiments are performed on a experimental platform with a mechanical installation eccentric of 10 m, roundness error of 2 m and shafting sloshing of 0.6. The measuring angle error RMS values from two sets of experiments are 0.393, 0.488, respectively. The experiments prove that the program can eliminate the angular measurement errors caused by the mechanical installation eccentric, machining roundness error and the shafting sloshing. It achieves arcsecond angle measurements.
An experiment platform for laser active imaging and target recognition was built combining a laser active image system and the target recognition technology, and the target recognition after laser active imaging was mainly researched. The feature vector was comprised of seven invariant Hu moments. The BP neural network algorithm comprised of 136 weight coefficients was used to study the moving target, a 43 submachine gun model at 450 m from the experiment platform at night, and excellent experiment results were obtained. It shows clear imaging effects by 68.87% of target recognition statistic probability in 2 740 frames of laser active imaging, and the probability of rotation transformation reaches 80.05%. These researches are significant to the detection and recognition of little targets at night.