Study On Holographic Characteristics Of Off-axis Particle Field Based On Filter Size

Laser holography is a three-dimensional imaging technique,it not only has the advantages of general optical technology,but also has the unique dominance that not subject to the shape of the object limit,therefore,it is widely used in particle field measurement.However,When the laser holography is used to measure the high density particle field,the contrast of the interference fringes are obviously decreased,and the noise in the reconstructed particle image is obviously increased,and the particle can not be accurately and effectively identified.The use of filtering holography can effectively reduce the background noise of the reproduced image and improve the quality of the reconstructed particle image,so as to realize the measurement of the higher density particle field.High-pass filter size,4F optical transmission system front focal length,particle size and other parameters is an important factor affecting filtering holographic application.In this paper,the characteristics and laws of the off-axis filtering holographic image under different parameters are studied in the theoretical and experimental aspects systematically.It provides a reference for filter design and subsequent image data processing to obtain particle information.The main research contents and achievements are summarized as follows:(1)The parameters of off-axis filtered holographic system were determined.The effect of absorption rate of high pass filter,amplification ratio of the 4F optical transmission system and the angle of light and reference light on the highest frequency of off-axis filtered hologram fringes,fringe contrast and contrast of reconstructed image were studied,combined with actual measurement conditions,to determine the numerical simulation and the experiment using the absorption rate of 1high pass filter and 2 times the amplification 4F optical transmission system,the angle was set to ?/6;(2)The intensity distribution expressions of the reconstructed circular particle image by off-axis filtered holography was verified and improved,he influence of the parameters such as the size of the high pass filter on the filtered holographic image is studied according to the formula.The results show that the off-axis filtered holographic reconstructed images are bright particles and dark background.As the high pass filter size,particle size increases or the 4F optical transmission system front focal length decreases,the reconstructed image exhibits more high-frequencyinformation,Particle images gradually change from uniform bright particles to concentric rings,while the boundary contours are more pronounced.Combined with the influence of each parameter on the filtered holographic image,the empirical formula of particle image feature is obtained,which provides the basis for the design of high pass filter.(3)Numerical simulation of particle off-axis filtering holography was carried out,and circular and rectangular particle reconstruction images were obtained.The numerical simulation results of circular particle filter holography are consistent with the calculated results;The results of rectangular particle filter holography show that the size of the high pass filter increases or the 4F optical transmission system front focal length decreases,the intensity of the particle center decreases and the boundary gradient increases,and the particle image can be effectively identified by the boundary contour.(4)Direct imaging experiments and off-axis filtering holography experiments were carried out.The effects of different high pass filter sizes on the particle image frequency were studied by direct imaging experiment.The reconstructed images of different shape particles and particles with different particle sizes under different high pass filter size were obtained.The experimental results are the same as those of the theoretical analysis,and the influence of the parameters such as the size of the high pass filter on the holographic image of the off-axis filter is verified.