Investigation of a self-calibration method for global rainbow refractometry

Accurate measurement and control of key parameters of droplets in the complex atomization or spray flow field play an instructive role in the mechanism study of multiphase flows and optimization of specific industrial process. Among numerous advanced optical measurement techniques for characterizing droplets, global rainbow refractometry is able to measure the size distribution and refractive index of droplets simultaneously. Temperature, concentration, solution components and any other relevant parameter can be determined according to the relationship between refractive index and target parameter. Since the information of refractive index and droplet size contained in the rainbow intensity distribution is highly sensitive to the scattering angle, accuracy of calibration for the relationship between CCD pixel column and scattering angle greatly affects the measurement accuracy. A novel self-calibrated global rainbow refractometry based on dual-wavelength laser is proposed in this paper. The angular calibration and the refractive index measurement under two wavelengths can be realised simultaneously without additional measure devices. The actual refractive indices of droplets under two wavelengths and calibration equation can be quickly and simultaneously obtained by searching the minimum of root mean square (RMS) errors between the hypothesizing refractive indices and the ones from inversion. The results of simulation and experiment show the feasibility and high precision of self calibration method for global refractometry. This self-calibrated global rainbow system was also successfully applied to measure the refractive index of water–ethanol solutions with volume concentration from 10% to 60%. What’s more, this design can be extended to calibration for one-dimensional rainbow technique.