Research On Digital Speckle Correlation Method Based On Frequency Domain

In recent years, digital speckle correlation technology is developed as a high-precision method for the measurement in object plane or strain. It has the advantages of easier light path,non-contact measurement in full-field and no processing stage of interference when compared with other optical measurement techniques. Therefore it has a wide range of applications in theoretical research and practical projects.Digital speckle correlation method in frequency domain is an important branch of digital speckle correlation technology. It makes up the low real-time property of traditional digital speckle correlation method, but for the low measurement accuracy, it couldn't meet the demand for precision in the actual applications. According to these problems, the digital speckle correlation technology in frequency domain is systematically researched. In order to improve the measurement accuracy, a digital speckle correlation measurement method based on fractal interpolation is proposed. By inheriting traditional digital speckle correlation in frequency domain, the Hanning window function is adopted to filter the images and fractal interpolation is used to get the peak position more accurately in this method. Moreover, a comparison between the output of Fandel-Lugt correlator and joint transform correlator is made to determine the fact that the former correlator is chosen to take the match filtering experiment of the speckle correlation method in frequency domain. Finally, the measurement accuracy is assessed by taking an experiment in rigid translations, and the results show the reliability of the method. The experimental results show that the method proposed above has the advantages of low environmental requirements, high accuracy and strong real-time when the measured object has a continuous deformation in full-field.For the measurement of discontinuous deformation, a displacement measurement method of digital speckle correlation which combining the frequency domain with the spatial domain is proposed. The whole pixel displacement information is calculated by frequency phase correlation method, and this information is regarded as an initial estimate of reverse combination Gauss Newton method for the measurement of sub-pixel displacement. By using frequency domain method to calculate the sub-image interest point displacement anddeformation information, the actuality of solving initial value per column line by line search when using the traditional digital speckle correlation can be avoided. The verified results show that this method has a higher accuracy and faster calculation speed compared with the Newton iteration method.