Study On Digital Holography And Its Applications In The Field Of Measurement

Digital holography in measurement technology has become one of the most interested research fields all over the world. Mang progresses have been made at it's applications in temperature field measurement, roughness measurement, granularity analysis, profilometry and shape microvariation measurement, cell culture observations, micro-circuit examination. Digital holography is a unique holographic imaging technology, which uses the charge-coupled device (CCD) camera instead of the traditional holographic material, eliminating the wet process at holographic imaging and fixing. The measurement of physical parameters and physical processes observation can be achieved by simulating the optical diffraction process and reconstructing the wavefront of the original object field.This paper addresses some issues of digital holography, including the resolution and the phase correction method, the application in temperature field and roughness measurement based on the theoretical analysis of digital holographic recording and reconstruction, and the feasibility and accurate results are acquired by some experiments.The main contents and the significant results are as follows:1. Progresses in holographic technology are described, including the research results in optical holography and digital holography. These prospects in digital holography field are discussed: recording optical path of digital hologram, separation of the zero-order diffraction image and first-order diffraction image, reconstruction algorithm, reconstruction resolution, compensation for phase distortion and the application in measurement technology.2. Principles of recording and reconstruction in optical and digital holography are studied. Several common recording methods are discussed, and the advantages of digital and optical holography are compared respectively. The constraints of off-axis Fourier transform digital holographic are derived, with point source and plane wave as reference beam: point source bias limits, plane wave tilt limits and recording distance limits. The construction algorithms of digital holography are addressed, and the advantages of various algorithms are compared.3. Resolution of the reconstructed image in digital holography has been studied, and point out that the spatial resolution and lateral resolution of reconstructed image are different. The impact factors on lateral resolution of the reconstructed image are discussed, and several ways are provided to improve the resolution of the constructed image. With point source as reference beam, principles of recording and reconstruction in off-axis lensless Fourier transform digital hologram are derived theoretically. The resolution of the reconstructed image is obtained by Fourier transform and the angular spectrum algorithms respectively. Furthermore, the reconstructed image resolution derived by angular spectrum algorithm is proved by experimental results. The reasons of phase distortion in reconstructed image are analyzed based on the recording and reconstruction of off-axis digital holography with pre-magnification. The automatic compensation is proposed to correct the phase distortion, and the result of phase compensation is simulated. The result shows that the effect of compensation is better in low noise situation, and it's not better when compensate more times in high noise situation.4. We discuss the current measurement methods about the temperature field, especially focusing on several optical measurement methods, and point out the advantages and disadvantages of various measurement methods. Application of digital holography method is proposed to measure the temperature field based on the off-axis lensless Fourier digital holography, and the feasibility of this method is analyzed theory. The phase distribution is obtained by digital holography, and then we get the relation between temperature and the unwrapped phase acquired through the appropriate phase unwrapping algorithm. To verify the feasibility and accuracy of this method, we design the transmission and reflection optical setups in which our own device that can provide and monitor the temperature is used. Compared with the result measured by thermocouple in real time, the error range is [-1.01K, 0.19K] by the transmission optical setup, and the error range is [-0.22K, 0.50K] by the reflection optical setup. The confidence probability is 99.75%. Thus, the feasibility and accuracy of this method can be proved.5. Based on the national standards we expound the definition about the main parameters of surface roughness and the related basic test theory. The current detection methods of roughness are discussed, and the digital holography method is proposed to measure the surface roughness of object. We study the feasibility of this method theoretically, and then the relation between the surface roughness and the unwrapped phase distribution is obtained based on the theory of the off-axis lensless digital holography. It is found that the real phase distribution by digital holography is not necessary, and the relative unwrapped phase distribution is enough in this method. The experimental device is designed, and the standard roughness samples are applied as the test objects. Comparing the arithmetical mean deviation of profile Ra detected by our method with the parameters of the standard roughness samples, the measurement error of the standard roughness sample with Ra 3.2μm is -0.45μm and the fluctuation range is±0.03μm; the measurement error of the standard roughness sample with Ra 1.6μm is 0.23μm and the fluctuation range is±0.03μm. The results demonstrate that it is feasible to measure the surface roughness of object by digital holography and the variability is low.