In deep ocean applications, a camera’s viewing range is related to its low-light-level performance. Hence, a high-resolution ultra-low-light-level digital still camera (thereafter L3DSC), designed from the ground up for deep ocean autonomous underwater vehicle (AUV) imaging, is presented. A high-resolution (1024 × 1024) electron-multiplying CCD (EMCCD) is adopted as an image sensor to ensure low-light capability. A thermo electric cooler (TEC) is employed to lower image sensor temperature to promote low-light-level performance. Totem pole circuits that are able to generate 50-V pulse wave at 10 MHz are implemented to drive the EM pin, and power consumption of the circuit is optimized. An EMCCD digital image is buffered in a field-programmable gate array (FPGA) and then transferred through USB interface to a solid-state drive (SSD), which is installed in the image storage unit. The camera is controlled by AUV via Ethernet, and image data stored in the camera could be downloaded via the same interface after AUV retrieved from the sea. The camera module is mounted into a 6000-m depth-rate titanium alloy housing. The diagonal field of view is measured to be 58.4 deg in air. Experiments show that the minimum scene illumination of L3DSC is better than 5 × 10 − 4 Lux; underwater imaging distance is longer than 10 m, and total image data capacity is 200 gigabytes. These results demonstrate the camera’s low-light-level imaging performance and feasibility for AUV applications.
In this paper we discuss and examine approaches for detecting large objects in low-light maritime environments with a goal of improving the detection of large targets within a region of interest. More specifically, a passive ghost imaging system is proposed for using caustics illumination patterns to reconstruct a target image from correlations with intensities captured by a bucket detector.
Single-pixel imaging (SPI) represents a promising approach to multispectral imaging. We investigated the interband similarity of color images among red, green, and blue bands and found that it is highly possible for their wavelet coefficients to be at the same locations due to edges and transactions. Accordingly, we constructed a multiple measurement vectors model that includes a constraint under which the sparse coefficients of different bands have the same sparse structure, and then joint reconstruction is performed for all bands. We ran both simulated and actual experiments to validate the feasibility and effectiveness of the proposed method and found that compared with similar methods, it significantly improves the reconstruction quality of color SPI.
In order to realize accurate detection for small dim infrared maritime target, this paper proposes a target detection algorithm based on local peak detection and pipeline-filtering. This method firstly extracts some suspected targets through local peak detection and removes most of non-target peaks with self-adaptive threshold process. And then pipeline-filtering is used to eliminate residual interferences so that only real target can be retained. The experiment results prove that this method has high performance on target detection, and its missing alarm rate and false alarm rate can basically meet practical requirements.
The mobile pipeline-filtering algorithm is a real-time algorithm that performs well in detecting small dim targets, but it is particularly sensitive to interframe vibration of sequence images. When searching for small dim targets at sea based on an infrared imaging system, irregular and random vibration of the airborne imaging platform causes huge interference problems for the mobile pipeline-filtering. This paper puts forward a pipeline-filtering algorithm that has a good performance on self-adaptive anti-vibration. In the block matching method using the normalized cross-correlations coefficient (NCC), the interframe vibration of sequence images is acquired in real time and used to correct coordinates of the single-frame detection results, and then the corrected detection results are used to complete the mobile pipelinefiltering. Experimental results show that the algorithm can overcome the problem of interframe vibration of sequence images, thus realizing accurate detection of small dim maritime targets.
When searching for small targets at sea based on an infrared imaging system, irregular and random vibration of the airborne imaging platform causes intense interference for the pipeline-filtering, which is an algorithm that performs well in detecting small targets but is particularly sensitive to interframe vibrations of sequence images. This paper puts forward a pipeline-filtering algorithm that has a good performance on self-adaptive antivibration. In the block matching method that combines the normalized cross-correlations coefficient with the normalized mutual information, the interframe vibration of sequence images is acquired in real time and used to correct coordinates of the single-frame detection results, and then the corrected detection results are used to complete the pipeline-filtering. In addition, under severe sea conditions, small targets at sea may disappear transiently, leading to missing detection. This algorithm is also able to resolve this problem. Experimental results show that the algorithm can overcome the problem of interframe vibration of sequence images, thus realizing accurate detection of small maritime targets.
Using by different output transmittances of 2%, 5%, and 10%, we have investigated the continuously tunable laser performance of c-cut Tm:YAP crystal. The 5-mm long c-cut crystal with 4 at.% thulium ion was used. Without a tuning element, the slope efficiency respect to the absorbed pump power reached 44.1% with transmittance of 5%, corresponding to the conversion efficiency of 35.3%. The laser operated at TEM00 mode with a beam quality factor of M2 ∼ 1.3, which was demonstrated by a 90/10 knife-edge method. A 3-mm thick birefringent plate was used as a tuning element. At transmittance of 5%, the widest tunable range of 159 nm extended from 1899 to 2058 nm. For transmittances of 2% and 10%, the tunable ranges of 150 and 68 nm were achieved, respectively.
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