Real-time measuring system for liquid refractive index based on Fourier transform spectrum analysis

A digital, high-precision and portable real-time interferometric measurement system for liquid refractive index based on Fourier transform spectrum analysis technology was proposed. Based on the equal-thickness interferometry optical path composed of a split-tip liquid container, the common-path interferometric measurement system was built recorded by a charge coupled device (CCD) camera. Taking the Raspberry Pi hardware system as platform and the Python language as programming basis, the accurate extraction of the number of interference fringes from the CCD plane was realized through Fourier transform spectrum analysis and cubic spline interpolation. Moreover, a graphical user interface (GUI) was designed to realize the dynamic visualization for the measurement process. The measurement process only needed to determine the initial fringe number before injecting any liquids, then the real-time measurement of the refractive index of flowing liquid could be realized without additional preset parameters. In the experiment, the refractive indices of three liquids were determined and the time stability of the measurement system was verified. The results show that the measurement system can achieve better time stability as the initial fringe number is greater than 180, and the measured relative errors of the refractive indices are all within 0.754%.