Research On Non-contact Measurement Technology Of Liquid Refractive Index In Closed Systems

Refractive index(RI)is an important parameter of matters.Studies have shown that the properties of different liquid,such as optical properties,magnetic properties,concentration,temperature,density,and solute the change of the important parameters such as material composition,can cause the refractive index(RI,refractive index)changes.As a result,we can synchronous acquire the concentration,density,temperature etc.parameters of the liquid based on refractive index measurement,and it has been applied in the field of chemistry and chemical industry,pharmaceutical and biomedical,food and beverage,etc.in detecting technology.Based on the comprehensive analysis of the existing refractive index measurement method,we propose a simplenon-contact liquid refractive index measuring method using a glass tube after analyze the difficulty of liquid refractive index measurement in a closed glass tube.This method is based on the optical properties of the glass wall as well as the principle of total reflection of light.A transmission-scattering layer is coated on the outer face of the transparent glass wall.A parallel laser beam incident on the transmission-scattering layer,the incident light is converted into a big divergent angle of scattered light.When the laser is incident on the transmission scattering layer,a dark optical pattern is formed with a clear and high-contrast boundary.This paper analyzes the geometric parameters of optical pattern related to liquid refractive index,and achieve on-line,non-contact liquid RI measurement inside the glass tube in real-time.In order to realize the liquid refractive index measurement in industrial application,we intercept the industrial glass pipes as experiment object.A laser is used as the light source,a liquid refractive index measuring method is proposed,the liquid is in an airtight container.We theoretical and experimental analyze the eventual impact of related parameters on the result of the experiment such as the thickness of transmission-scattering layer,the incident light spot size,the laser wavelength and laser source stability.The experimental results show that when the incident light spot is smaller,the results of the final accuracy is higher;About the influence of the optical dark pattern boundary definition,there is a best value of thickness of transmission-scattering layer;In addition,this method is not affected by the stability of light source,which is one of advantages compared with other RI measuring method based on light intensity modulation.For image acquisition and processing of the oval dark pattern,in order to achieve the purpose of real-time online monitoring,we use a CMOS industrial camera for image acquisition.The image is transmitted to the computer.Using the MATLAB image processing and calculation,we can get the image key information related to the refractive index.Using MATLAB GUI graphical interface design,we can measure the liquid RI in real-time with the real-time image process.To improve the accuracy of measurement,we use a 10 million pixels CMOS camera to collect images.However,this measure affects the processing speed.To solve this problem,we use FPGA for image processing and acceleration,thus can achieve high precision liquid refractive index online monitoring in real time.We measured RIs of different concentrations of salt,sugar,water,alcohol,ethanol and other common liquid using our experimental apparatus,and the experimental results show that the measuring precision of this method can reach 10-4 RIU(refractive index).This measurement accuracy is comparable with the commonly used laboratory abbe refractometer;Meanwhile,this method can also fulfill the real-time,non-contact and online liquid RI measurement in a closed container with the help of MATLAB and the FPGA for high-speed processing image data.This method is simple and easy to use,it can be used for RI measurement without debugging,which is widely applied.