High reflectivity of mirrors is very important for many applications in the vacuum ultraviolet, such as for space observation, synchrotron radiation. This paper focuses on the substrate temperature’s effect on the performance of Al mirrors when depositing the upper MgF2 layer. Al films are deposited on the substrates at room temperature by thermal evaporation, and a 5 nm MgF2 film is deposited on Al coating at room temperature immediately. Heating the substrate to various temperatures ranging from room temperature to 350°, then a 20 nm MgF2 film is deposited on the surface of Al/MgF2. The thickness of each layer is characterized using grazing incidence x-ray reflectivity. The reflectivity of sample is measured at the incident angle of 5° in the wavelength range of 105~130 nm. The reflectivity of all samples fabricated at above room temperature is higher than the sample at room temperature below 115nm. The reflectivity of mirror at 350° temperature is lower than other mirrors, and the reflectivity of the samples at 300° and 200° is similar. There are more black dots on the surface of mirror at 350° than 300°, and no black dot on the surface of mirror at 200°. The measured results using surface profiler show that the black dots are small holes that increase the roughness of mirror and reduce the reflectivity. So the best temperature for depositing the upper MgF2 layer is in 200~300° to obtain high reflectivity of Al mirrors in vacuum ultraviolet.
An image Mosaic algorithm utilizing image overlap rate prior is proposed for bionic compound eye imaging system based on micro-surface fiber faceplate in this paper. Firstly, the two images to be spliced together are both divided into overlapping regions and non-overlapping regions using the prior of relative position and overlap rate of the sub-eye images. Then, Feature points in overlapping regions are extracted using Speeded Up Robust Features (SURF) detector and described by Binary Robust Independent Elementary Features (BRIEF) descriptor. The initial matching of the feature points is made with the hamming distance matching. Random Sample Consensus (RANSAC) and angular consistency of the pairing feature points are used to further purify the feature point pairs. Finally, the weighted mean method is used on the images after registration to get the blended image. The sub-eye images are spliced in each layer and then the spliced images of each layer are stitched together successively to get the final panoramic image. Experimental results showed that the splicing speed of the proposed algorithm is 2 to 3 times higher than that of Scale Invariant Feature Transform (SIFT) algorithm. Compared with SURF algorithm, the splicing speed also increased by about 50%. In addition, more correct matching can be remained, so that the results of image registration and splicing can be more reliable. Thus, the proposed algorithm can promote the images real-time processing of the compound eye imaging system.
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