Study On Digital Holographic Imaging For Strong Diffuse-Reflection Metal Surface

Digital holography is a novel imaging technique appeared with holographic technology and computer technology developing. Its advantage is not only simplifying the experimental process of traditional holography, abandoning the darkroom experimental treatment procedure with higher operating requirements in the traditional holography, but also we can simulate the regeneration process, and obtain the reconstructed 3D topography by numerical reconstruction algorithm. And in this process, we can deal with the image results by some algorithms. The difference between digital holography and traditional optical holography is that the digital holography employs the CCD(Charge Coupled Device), CMOS(Complementary Metal Oxide Semiconductor) or other components as the recording medium to collect object information, and then the hologram is stored into the computer. Compared with the optical holography, digital holography's advantage is that it has the characteristics of fast, real-time and non-contact measurement.Diffuse reflection is the most common reflection phenomena in nature. The measurement of the surface structure of the diffuse object is important, and but it is difficult for imaging the micro structure of a diffuse object because of its diffuse-reflection light noise. The measurement of the stress and deformation of the metal surface has been an urgent problem to be solved in the scientific research, which may be realized by topography imaging of the mental object. The purpose of this paper is for imaging the topographic surface of diffuse-reflection object by digital holography, and in the future, the digital holography will further provide the measurement of stress and deformation for a strong diffuse object with a kind of non-contact, high sensitivity imaging.In this paper, the development of digital holography is introduced, some applications of digital holography in object shape measurement are described, and the advantage of digital holography is explained. The digital holographic imaging method and system used for strong diffuse-reflection metal surface are investigated. Digital holographic imaging for diffuse objects is performed in experiment by using the Fresnel holographic optical path, the image plane holographic optical path and the lensless Fourier transform optical path, respectively. Through the analysis of the imaging results, it is proved that due to the influence of the dc term, the traditional Fresnel holographic method can't get the strong diffuse reflection type object surface structure information. Due to the limitation of the lens the image plane holography can't get all the information about this object. And the digital holographic microscopy can't get the effective information of the object clearly because of the problem of the micro objective. On the basis of this, the holographic subtraction method based on Fresnel digital holography is proposed. This method uses the hologram to subtract the object light and reference light. And the concept of the traditional filter is improved in this paper. The method of half extraction is used to remove the direct current, and then the information of the topographic surface of the strong diffuse-reflection alloy is obtained. The quantitative evaluation of the experimental results is carried out. The digital holographic map of the steel plate surface is compared with the topographic map obtained by the laser ranging method, so the advantages and feasibility of this method are proved.