Non - Contact Real - Time Measurement Of Liquid Refractive Index In Transparent Containers

Refractive index is one of the most important physical quantities of representing optical properties of transparent or translucent materials. Also, it has close relationship with the concentration, temperature, density and other physical and chemical parameters of the material in certain range. Since the refractive index is more easily to be measured than other physical and chemical parameters, to obtain other physical and chemical parameters synchronization at the same time by measuring the refractive index of the material has widely used in liquid brewing, chemical, pharmaceutical, sugar, food and other industries. This method has become one of the basic methods of measuring physical and chemical parameters of materials.In this paper, we reviewed current statues of the measurement technology of liquid. After analyzed the limiting of several common methods, a new way based on the total reflection of the light and the need of non-contact measurement technology of the liquid refractive index has been proposed in order to achieve the non-contact, real-time measurement of the liquid inside a container. In this method, a parallel laser beam which is converted into divergent light with high divergence by transmits through a scattering layer. The divergent lights generate shading effect by incident into the interface of the container and the liquid. This effect will form an image of optical dark spot on the outer surface. By using the dark spot effect and the relationship between its scale information and refractive index of liquid, we could obtain the refractive index of transparent or translucent liquid. Transparent containers used in the study of this method are measuring cylinder, beakers and glass sinks which are commonly used in laboratory.It was found that in experiments that circular dark spots formed on a plain surface and oval dark spots formed on the cylindrical surface. This paper analyzed the formation mechanism of the circular dark spots and oval dark spots in optical images, and established analytical relationship among the wall parameters, refractive of liquid and scale of dark spots in optical images. Optical dark spots images quality directly affects the accuracy of the measurement of refractive index. It was found in the experiment that incident light spot size and divergence formed by the laser beam transmitted through the trans-missive scattering layer influenced on the clarity of image boundary, thereby affecting the measurement accuracy. On the basis of the analyzing of the mechanism, we proposed an solution which can improve the clearly of the image boundary and the corresponding optimized parameters, successfully designed and built a refractive index based optical imaging system, and achieved non-contact measurement of liquid refractive index in a, container.During the extraction and processing of optical image scale information, we designed a photoelectric conversion acquisition and processing system. The optical dark spots image information which is collected by photoelectric converter has been transmitted to the computer and processed with default program in order to obtain measurements value of the refractive index of the liquid. In this paper, we choose CMOS which has advantages of low cost, low energy consumption, and have enough image quality as the automatic collection components of optical dark spots images; Matlab compile data processing program is closed to achieve a real-time access to the refractive index of liquid.We also designed two other image information collection methods:coordinates paper collection method and displacement measurement acquisition method. These two methods require visually capture to acknowledge optical dark spots image scale,which is easy to implement, but it is easily to cause an artificial bias. After compared the refractive index of transparent liquid such as ethanol acetone, and different concentrations of salt water, translucent liquid such as cola and orange soda measured with the presented experimental methods and Abbe refractometer respectively. The experimental results showed that the average relative error of refractive index during the measurements of liquid refractive index in the transparent container is 0.05%. In the measurement process, the method of this study has advantages of no loss of liquid, and the liquid refractive optical image showing as soon as the illumination, no debugging and testing, automatic acquiring and processing image information, synchronization acquiring measurement data with high measurement accuracy. It is expected to apply the physical and chemical parameters of non-contact real-time measurements to toxic, hazardous, flammable, explosive and other hazardous liquid. This method have predictable application value especially in measuring the physical and chemical parameters of transparent or translucent liquid under the state of atmospheric temperature and atmospheric pressure transparent, translucent liquid physical and chemical parameters measuring in contained in sealed container.