| Objects with specular reflection characteristics are widely used in people's daily life and industrial production.How to measure the three-dimensional shape of mirror without damage and with high precision is a hot issue in optical three-dimensional measurement.Phase measuring deflectometry(PMD)has become the main optical measurement method for mirror objects due to its advantages of large dynamic range,high measurement accuracy,fast measurement speed and non-contact.Direct PMD(DPMD)method can be used to measure mirror objects with discontinuous and large gradient surfaces by directly establishing the relationship between phase and depth.In the PMD measurement system,liquid crystal display(LCD)has become the necessary display equipment to build the measurement system.However,LCD is generally considered as an ideal light-emitting plane,ignoring the nonideal characteristics such as transparent layer refraction,nonlinear distortion,surface flatness and so on.Therefore,further improving the measurement accuracy by analyzing and compensating the non-ideal characteristics of LCD becomes a research direction worthy of further exploration.In order to improve the measurement accuracy of the system,this dissertation analyzes the refraction effect of the transparent layer of the display screen in the DPMD system and proposes an error compensation method to compensate the measurement results to improve the measurement accuracy.The main research contents of this dissertation include:First of all,theoretical analysis and modeling of transparent layer projection effect and non-ideal characteristics are carried out.Several transparent layers of LCD are modeled as a single transparent layer,and the formula of pixel offset is deduced according to the refraction principle.Secondly,the refractive model of DPMD is established.There are three unknown parameters in the refractive model: the thickness of display screen,the refractive index of display screen and the refractive angle.The optimal solution of screen thickness and refractive index in the model is calculated by light equation and trust region refraction algorithm,and the refraction angle in the model is calculated by phase data and system parameters.Then,by combining the refraction model of DPMD with the offset formula,the depth error compensation formula of DPMD system is obtained,which is used to compensate the calculation accuracy of the system.Based on the proposed method,the effectiveness and accuracy of the proposed method are verified by simulation and actual experiments.In the simulation system,DPMD measurement model and DPMD refraction model are established,then the simulated object is measured to restore the three-dimensional shape and analyze the error effect caused by refraction effect.Through the simulation experiment,it is found that the refraction effect mainly affects the measurement accuracy of the object edge range.A practical system is built to measure a mirror reflection step and an optical concave mirror.After compensation,the accuracy is improved about 30% compared with that before compensation.The experimental results show that the proposed method can improve the measurement accuracy of the system effectively. |
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