To improve the stability and measurement accuracy of the phase-shift digital holographic measurement system, a gray-level encoded phase-shift digital holographic three-dimensional measurement technology is proposed. This method uses the electronically controlled birefringence effect of the liquid crystal cell to design the gray code, converts the 0-255 gray value into an 8bit digital binary code signal and inputs it to the liquid crystal cell for electrical addressing, so that the refractive index in the liquid crystal cell is changed to achieve digital control of the modulation phase of the reference optical path. a Mach-Zehnder-type phase-shift digital holographic measurement system is designed, and a micro-lens array, with a diameter of 150μm and a sagittal height of 2.6μm is adopted as sample, through phase reconstruction and phase distortion compensation, The phase distribution of the micro-lens array is accurately obtained. The experimental results show that the relative measurement error of the micro-lens array diameter is 2.8%, and the relative measurement error of the sag height is 3.4%, which prove that the proposed method can realize the surface topography measurement of three-dimensional objects effectively.
Digital holographic microscopy is an ideal non-invasive, non-contact, and fast-response 3D measurement method. The strong coherence of the laser leads to irregularly distributed speckle noise during the digital holographic recording process, which affects the 3D reconstruction of the digital holographic microscopy system. To solve this problem, the coherence of the laser is changed by diffusion glass rotation to achieve the suppression of the speckle noise in the digital holographic measurement system. Firstly, a theoretical model of a digital holographic microscopy system based on the diffusion glass rotation is established. Then, the influence of diffusion glass rotation speed on coherence time, speckle contrast, fringe contrast, signal-to-noise ratio are simulated and analyzed, the optimal rotation speed of the digital holographic microscopy system is obtained. Finally, The speckle noise suppression system based on the Mach-Zehnder interference model is established, and the optimal parameters of the diffusion glass rotation are set, which verifies that the method is suitable for high-precision measurement.
As a new coherent imaging technology, digital holographic imaging technology has the characteristics of non-invasive, non-contact and fast response, and can quickly recover the three-dimensional shape of the object through a single frame image. However, there are some problems in the measurement of phase objects with abrupt information, such as discontinuous phase information and low precision of phase reconstruction. To solve this problem, the design and implementation of compensation phase are carried out. Firstly, according to the characteristic information of the measured phase object, the compensated phase encoding diagram is designed based on the phase modulation characteristics of the liquid crystal spatial light modulator; Then, the interference fringes with coded phase compensation are collected by the Mach-Zehnder interferometer system; Finally, the two-step phase-shifting algorithm is used to recover the compensated phase, and compared with the design value.
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