Wide-Range Thickness Determination of Oil Films on Water Based on the Ratio of Laser-Induced Fluorescence to Raman

In this article, we proposed a novel method to determine the oil film thickness on water with a wide range, which is based on the intensity ratio between the laser-induced oil film fluorescence and Raman scattering of the water without oil films. To verify the feasibility of this method, we presented a portable detection system that employs a laser diode at 405 nm and an intensified charged-coupled device (ICCD) detector. Using this system, the diesel oil films with variable thicknesses on water were measured at different distances. The measured thickness was in the range from 0.07 to 36.62 mm with an average relative error of 5.98% and 4.07% by parameter calibration method (PCM) and spectral fitting method (SFM), respectively. The experimental results showed that the effective thickness range measured using the proposed ratio method is 40 times greater than that measured by the traditional Raman suppression method (less than 0.82 mm). In addition, different thicknesses of crude oil films were also measured, and the effective thickness range was from 0.002 to 0.493 mm, which was 70 times greater than the traditional Raman method (less than 0.007 mm). In the range of 0.007-0.493 mm, the average relative error is less than 10% using the proposed ratio method. Furthermore, this method can greatly eliminate the influences from the detection distances, instrument factors, and external environments compared with the fluorometric method.