Three-dimensional measurement of objects in liquid with an unknown refractive index using fisheye stereo camera

This paper proposes a method to estimate the refractive index of liquid using a stereo camera. Sensing in liquid environment is important to research liquid structures, aquatic lives and liquid resources. Methods using cameras are often performed. However, methods using cameras are affected by the refraction of light. Therefore, refractive index is needed to know to consider the refraction of light. The use of the refractive index of the liquid is a common approach described in literature, for most of the researches of three-dimensional (3-D) measurement in liquid based on a camera. Those approaches does not take in account that the refractive index of liquid may change by dissolved material, concentration of solute, temperature. Moreover, in the real environment, a precise refractive index in liquid is needed to get an accurate measurement. In this work, we propose a stereo camera based method to measure the refractive index. An object whose scales are known is attached in front of the transparent waterproof container as reference. The refractive index is estimated measuring the distances between key feature points on the object. Experimental results show the validity of the proposed method.