Research On Liquid Refractive Index Measurement System Based On Critical Angle Measurement

Refractive index is an important parameter to reflect the physical properties of materials,and with the help of the value of refractive index,the purity,concentration and dispersion of liquid can be understood,which plays a very important role in many fields such as material identification,food detection and purity detection.There are many principles and methods for measuring refractive index,which are mainly divided into geometric optics,wave optics and optical fiber sensing.At present,most refractive index measuring instruments in the market can only measure the refractive index of liquid less than 2.00.However,with the emergence of some new materials with high refractive index,the research on measuring liquid with high refractive index is gradually increasing by improving and optimizing the existing measurement methods.In recent years,the instruments which can measure the liquid materials with high refractive index above 2.00 are mainly imported,although they have high measurement range,they also have some shortcomings of high price and demanding test environment.It is necessary to study a liquid refractive index measuring instruments which can measure high refractive index,have high measurement accuracy and low cost.In this thesis,by comparing the measurement principles and methods of refractive index of various liquids,a three-edged vessel was designed as a carrying container of liquid based on the critical angle measurement method,and the liquid itself forms a prism.Then,the mathematical model of the measured refractive index was analyzed,which showed that the refractive index of the liquid to be measured could be calculated from the top angle of the ’liquid prism’,the incident angle of light and the exit angle(critical angle).In addition,the refractive index range that can be measured will also change when the top angle of the ’liquid prism’ is different.The smaller the angle is,the larger the refractive index value can be measured.For example,the refractive that can be measured theoretically at the top angle of 30° is(1.8715,3.0288).The corresponding optical path structure and the overall scheme of the measurement system were designed,and the required devices were selected according to the measurement principle and optical path,including light source,camera,lens,etc.Three trigonous utensils with top angles of 30 °,45 ° and 72 ° were prepared,and the experimental system was built on the optical platform to verify the feasibility of the proposed scheme.After many experiments,the hardware selection scheme was optimized,the mechanical structure of the instrument was designed with threedimensional drawing software Solid Works,and then it is processed and assembled into a prototype.The application software of the system was compiled with C++ language under VS2019 environment,and the software interface was written with MFC.The software includes three parts: image acquisition,image processing and function module.Image acquisition realizes the communication function from hardware to software.The image processing includes the process of average smoothing,filtering and binarization,to achieve the analysis of the collected image and the calculation of refractive index.The functional module was designed directly for software users.In addition to interface design and real-time data display,it also had the functions of data saving,query and report generation.The prototype was combined with the application software to form a complete measurement system,which was used to measure the test samples.The measurement steps and calibration peocess were introduced,and the measured data were analyzed.The measurement data show that the measurement uncertainty of the refractive index measurement system is less than 5×10^(-4),and the repeatability index RSD is less than 1%,which basically achieved the expected results.