Structural Design and Verification of an Effective-Area Measurement Device Detection System.

The effective-area method is a new way to measure aperture area. It defines aperture area by directly using the beam-limiting effect of the aperture in radiometric measurement. Due to the special structure of the measurement device, it is necessary to find a suitable method to design the detection system. In this paper, the measurement system model is constructed in the TracePro program. The real circumstances of light propagation for the measurement beam are simulated, and the responses of the detector are given. It is proved that the relative change in the detector response is the lowest when the detector is at the position of 132°. And this is the best structure design of the detection system. The experimental results are designed to verify the feasibility of the structure design of the detection system.

Analysis of Stray Light and Enhancement of SNR in DMD-Based Spectrometers.

Due to advantages such as the high efficiency of light utilization, small volume, and vibration resistance, digital micro-mirror device (DMD)-based spectrometers are widely used in ocean investigations, mountain surveys, and other field science research. In order to eliminate the stray light caused by DMDs, the stray light in DMD-based spectrometers was first measured and analyzed. Then, the stray light was classified into wavelength-related components and wavelength-unrelated components. Moreover, the noise caused by the stray light was analyzed from the perspective of encoding equation, and the de-noising decoding equation was deduced. The results showed that the accuracy range of absorbance was enhanced from [0, 1.9] to [0, 3.1] in single-stripe mode and the accuracy range of absorbance was enhanced from [0, 3.8] to [0, 6.3] in Hadamard transform (HT) multiple-stripe mode. A conclusion can be drawn that the de-noising strategy is feasible and effective for enhancing the SNR in DMD-based spectrometers.

The Color Improvement of Underwater Images Based on Light Source and Detector.

As one of the most direct approaches to perceive the world, optical images can provide plenty of useful information for underwater applications. However, underwater images often present color deviation due to the light attenuation in the water, which reduces the efficiency and accuracy in underwater applications. To improve the color reproduction of underwater images, we proposed a method with adjusting the spectral component of the light source and the spectral response of the detector. Then, we built the experimental setup to study the color deviation of underwater images with different lamps and different cameras. The experimental results showed that, a) in terms of light source, the color deviation of an underwater image with warm light LED (Light Emitting Diode) (with the value of Δa*2+Δb*2 being 26.58) was the smallest compared with other lamps, b) in terms of detectors, the color deviation of images with the 3×CMOS RGB camera (a novel underwater camera with three CMOS sensors developed for suppressing the color deviation in our team) (with the value of Δa*2+Δb*2 being 25.25) was the smallest compared with other cameras. The experimental result (i.e., the result of color improvement between different lamps or between different cameras) verified our assumption that the underwater image color could be improved by adjusting the spectral component of the light source and the spectral response of the detector. Differing from the color improvement method with image processing, this color-improvement method was based on hardware, which had advantages, including more image information being retained and less-time being consumed.

An Underwater Image Enhancement Method for Different Illumination Conditions Based on Color Tone Correction and Fusion-Based Descattering.

In the shallow-water environment, underwater images often present problems like color deviation and low contrast due to light absorption and scattering in the water body, but for deep-sea images, additional problems like uneven brightness and regional color shift can also exist, due to the use of chromatic and inhomogeneous artificial lighting devices. Since the latter situation is rarely studied in the field of underwater image enhancement, we propose a new model to include it in the analysis of underwater image degradation. Based on the theoretical study of the new model, a comprehensive method for enhancing underwater images under different illumination conditions is proposed in this paper. The proposed method is composed of two modules: color-tone correction and fusion-based descattering. In the first module, the regional or full-extent color deviation caused by different types of incident light is corrected via frequency-based color-tone estimation. And in the second module, the residual low contrast and pixel-wise color shift problems are handled by combining the descattering results under the assumption of different states of the image. The proposed method is experimented on laboratory and open-water images of different depths and illumination states. Qualitative and quantitative evaluation results demonstrate that the proposed method outperforms many other methods in enhancing the quality of different types of underwater images, and is especially effective in improving the color accuracy and information content in badly-illuminated regions of underwater images with non-uniform illumination, such as deep-sea images.

[Analysis and Correction of Spectral Curvature in Hadamard Transform Spectrometer with DMD].

Due to the advantages of its low cost and high utilization rate of light energy and no moving parts, Hadamard transform spectrometer with DMD has become a focus in the research of spectrometer. In order to solve the reduction of spectral resolution caused by the spectral curvature of Hadamard transform spectrometer with DMD (Digital Micro-mirror Device), the spectral aliasing in the spectrometer was investigated. Firstly, the mathematical relationship of spectral aliasing to radius of spectral curvature was deduced. Then, two procedures were proposed to solve the spectral aliasing. One is making the DMD encoded spectral band accordant with the standard spectral band as far as possible by adjusting the DMD-encoded stripe, and another is correcting remaining spectral aliasing by means of data processing. Finally, by analyzing and correcting spectral curvature in six situations of the curvature radius of 15.8 x 104, 7.8 x 104, 9.7 x 104 µm and etc, we fit out the relationship of spectral aliasing and spectrum correction effect of spectral-curvature to the curvature radius. The simulation indicates that the spectral resolution increases to the resolution of optical system. It shows that the proposed methods are universal, simple and effective in the improvement of spectral resolution.