KEYWORDS: Target detection, Signal to noise ratio, Digital filtering, Image segmentation, Image filtering, Target recognition, Detection and tracking algorithms, Image processing, Image processing algorithms and systems, Signal detection
For target detection within a large-field cluttered background from a long distance, several difficulties, involving low
contrast between target and background, little occupancy, illumination ununiformity caused by vignetting of lens, and
system noise, make it a challenging problem. The existing approaches to dim target detection can be roughly divided into
two categories: detection before tracking (DBT) and tracking before detection (TBD). The DBT-based scheme has been
widely used in practical applications due to its simplicity, but it often requires working in the situation with a higher
signal-to-noise ratio (SNR). In contrast, the TBD-based methods can provide impressive detection results even in the
cases of very low SNR; unfortunately, the large memory requirement and high computational load prevents these
methods from real-time tasks. In this paper, we propose a new method for dim target detection. We address this problem
by combining the advantages of the DBT-based scheme in computational efficiency and of the TBD-based in detection
capability. Our method first predicts the local background, and then employs the energy accumulation and median filter
to remove background clutter. The dim target is finally located by double window filtering together with an improved
high order correlation which speeds up the convergence. The proposed method is implemented on a hardware platform
and performs suitably in outside experiments.
Defect in thin film is one of the most important factors influencing laser-induced damage in infrared laser systems, and
always a major concern. The infrared thin film is required for reflecting infrared and visible wavelengths, and the
materials should be low-absorbing in the spectral wavelengths as well. In general, this multilayer coating consists of a
number of high and low reflective index materials with alternate layers. Zinc Sulfide is used as high index material
because it has good visible and IR optical properties. Low index material can be Thorium Fluoride, but ThF4 is
radioactive and toxic, several fluoride coatings are studied in order to select an appropriate material instead of ThF4 in
this paper. Single layer of YbF3, LaF3,YBC(a new combined fluoride) and PrF2 coating are produced by thermal
evaporation, and classifications and derivation of defects in these thin films are introduced firstly. Then, the influence of
depositing method on surface defect density of laser thin film is analyzed. Finally, result shows that YbF3 is an
appropriate fluoride in place of ThF4, and the multilayer thin film deposited by YbF3 and ZnS material can be applied in
infrared laser systems.
Image-stabilization systems are widely used during astronomical image integration because of their large gain of image
quality and relatively simple control system. Probably the simplest system is the one that tilts a mirror to correct angular
variations caused by atmospheric fluctuations, vibration tilt of the telescope, or angular errors of internal components.
Lightweight (structured) mirrors based on piezoelectric actuators are of important part of these systems. The coating of
lightweight (structured) mirrors and the associated support systems involves the prediction of the magnitude and nature
of the elastic deformation of mirror surface due to mechanical and thermal residual stress during coating. The clamping
ways are firstly analyzed in this paper, and the finite element method of structural analysis makes it possible to include
conveniently the deformation and stress of a lightweight mirror in consideration of effects of thermal gradients and
mechanical loads during coating. Two models with different clamping ways are set up by using FEA soft. The
distribution of deformation and stress of lightweight mirror caused by thermal residual stress is analyzed. The coated
lightweight mirror is measured by interferometer and the result shows the calculated and measured results have good
compatibility. Based on the results of these studies, we select a better clamping way to coating the lightweight mirror.
The lightweight mirror for astronomical telescope intensified with silver has good optical properties, but the silver mirror
has the faults of weak adhesion to glass substrate and the mirror is easily corroded by atmospheric pollution. In order to
solve this problem, several silver adhesion layers are studied. Due to mutual action of Al2O3 and silver film, the
developed Al2O3-based silver intensified mirror has overcome the above faults and achieved good effect. Finally, the
coating is measured by making use of spectrometer. The average reflectance of coating is greater than 98% in 400-1500nm range, and the lightweight mirror can satisfy practical requirement.
Defects in thin film are the most important factors resulting in laser-induced damage of far-infrared laser thin film components, and always a major concern. The defect is a primary problem for infrared thin film of 3.8um in some optical systems. In this paper, single layer of ZnS YbF3 and multilayer coating is produced by thermal evaporation, and species, properties and derivation of defects in these thin films are introduced firstly, Then, the influence of material and evaporation rate on the surface defect density of laser thin film is analyzed. Finally, this paper put forward the appropriate deposition rates of thin film. The result shows that the nodule and concave hole defects are the mainly in the infrared thin film of 3.8um, and YbF3 has a great effect on the defect density in thin film. Also, the deposition rate of YbF3 has a large effect on the number and area of particles deposited on the substrate, as the evaporation rate increases, the number of particles increases markedly. It is possible that the spitting change of fused deposition material increases when the rate increases, since the centre temperature of fused deposition material increases. The defect density from 7.3X10-3 reduces to 6.8X10-4 through reducing the rate. Finally, the authors found it was appropriate when deposition rates were 4Å/s for ZnS and 2Å/s for YbF3, respectively.
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