| Summary: | 碩士 === 國立高雄科技大學 === 電子工程系 === 107 === Infrared spectrum sensor based on scattering spectrum according to reflecting energy from particle surfaces usually is used to distinguish different ingredients among PM2.5 for tracking polluted sources. The method, however, has a critical problem that when the particle sizes are similar in different PM2.5 pollutions, their scattering spectrums could be approach as so to fail to map out their sources. To avoid the problem, particle 3D shapes are necessary by digital holography methods.
In this paper, we develop numerical reconstruction and phase unwrapping with off-axis digital holography to observe 3D PM2.5 particle 3D shapes. Our five processes are following: 1. set the Mach-Zehnder interferometer as the experimental framework to record the hologram of the object with the CCD camera, 2. convert the hologram by fast Fourier transform to obtain real image, conjugate image and DC term in the space spectrum, 3. extract the real image by spatial filtering, 4. restore a complex image from the real image with the angular spectrum reconstruction method, and 5. gain unwrapping phase of the complex image and draw 3D particle shapes.
In addition, we also apply the least square unwrapping method to calculate the phase of the object. Based on the method, the principle problem is from the whole picture, and fit the phase unwrapping picture according to the whole wrapping phase picture, so the phase difference between adjacent pixels after expansion and before the expansion. The least square solution of the difference between adjacent phase differences, the original phase of the object to be tested is restored. The phase is no long to limit folding between -π and π rad.
The test objects in our experiment including USAF 1951 resolution test chart、sand powder、incense ash、cigarette ash and chalk dust. First, we use USAF 1951 resolution test chart as a sample to measure the height. According to our method, the height map revealed 300 nm, which is consistent with other the results of other research, confirming the feasibility of phase reconstruction. After the experiment of the PM2.5 particle, the results of PM2.5 particles are firstly the image of the particles after the angular spectrum reconstruction, and second the phase unwrapping method is used to sample the particles. The three-dimensional shape is reduced, and the difference in the particle sample is separately obtained.
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