Research Of Optical Rotation Measurement System Based On Centroid Algorithm And Error Analysis

Transmitting through some transparent materials or solutions,the vibration plane consisting by electric vector and propagation direction of linearly polarized light is rotated a certain angle along the propagation direction,which is called optically active phenomenon.Polarimeter is the instrument which is capable of measuring such rotation angle.With the development of polarization measurement technique,polarimeter is widely applied to all kinds of identification and analysis fields.Due to the feature of nondestructivity,polarimeter is increasingly combined with other analytical tools,which plays a crucial role in chiral materials and molecule researches.Therefore,researches on polarimeter are paid great attention.In this paper,an optical rotation measurement system based on digital signal processor(DSP),modulated laser,and step motor rotating stage is proposed.Based on optical null method,a curve of optical intensity versus rotating angles is obtained by DSP sampling.Moreover,due to the advantage of multiply-accumulate calculation(MAC)of DSP,data processing method with centroid algorithm is proposed.Light intensity signals with or without testing sample is processed with centroid algorithm to obtain the optical rotating angle through the step difference between two centroids.Compared with fitting algorithm on DSP,the real-time capability of centroid algorithm is about 30 times better than fitting algorithm.Hence,featured with fast and simple calculation,centroid algorithm can be introduced to effectively improve the real-time ability of whole measurement system.The system performance is proved experimentally with standard quartz tubes and glucose solutions.After various measurements,the relative error and precision of the system are determined to 0.4% and 0.004?,which demonstrates the reliable repeatability and high accuracy of whole measurement system.Moreover,an error analysis using a rotating coordinate system with three parameters of linearly polarized light—incidence angle,azimuth angle on the front surface,and angle between the incidence and vibration planes—on a rotating birefringent prism system is proposed.The Glan-Taylor prism is taken as example to analyze.Birefringence ellipsoid model is introduced to calculate the refractive index of ordinary and extraordinary light.The transmittance and angles are discussed plane-by-plane and the final transmitted intensity equation is deduced.The effects of oblique incidence,light interference,beam convergence,and misalignment of the rotation and prism axes are discussed in detail and corresponding equations are derived.We simulate the entire error model using MATLAB and conduct experiments based on built polarimeter.The simulation and experimental results under the same parameters are consistent and demonstrate the rationality and validity of our proposed error analysis model.