Three-dimensional Shape Absolute Measurement Based On Longitudinal Correlation Properties Of Laser Speckles Field

With the development of modern science and technology, the development of optical 3-D shape measurement is huge. With the advantage of high precision and efficiency, optical 3-D shape measurement has become the most widely used in various fields of non-contact measurement, which has a huge business and application value. The 3-D shape measurement technology based on the structured light illumination has been considered as the most potential measurement method. Especially,3-D shape measurement based on digital fringe projection has been studied deeply and applied widely with the advantage of high precision and quick measurement speed. But, because of the periodicity of digital fringes, the phase map must be calculated by phase unwrapping algorithms in this technique. So some scholars proposed a 3-D shape measurement based on digital speckle pattern projection instead of digital fringes projection. The phase unwrapping process is released in data processing. It can reconstruct 3-D shape, even though the surface of the object is abrupt or complex. But there will have an error when the object is out of the depth of field of a projection-lens system.The longitudinal coherence properties of laser speckles are investigated. The properties are then utilized for three-dimensional(3-D) shape absolute measurement, bringing a feature that phase unwrapping is released. A sequence of speckle images is captured by a CCD camera at different longitudinal positions. The lateral displacement of the speckle can be obtained by calculating the correlation among the speckle images. According to the relationship between the longitudinal length and the lateral displacement, absolute measurement of height from the object surface to reference plane can be realized. Additionally, the phase unwrapping process is released in data processing. A projection-lens system in the technique of projected digital speckle patterns is also released, because the laser speckle field is generated by a laser which illuminates on a frosted glass. The structure of the system is simple and convenient for implement. This technique is potential to be used in practice.