For light oil, the lifetime in shorter wavelength range is about 10nsec and that in longer wavelength range is 25nsec. This light oil seems to have two types of fluorophores with differing lifetime. For A-heavy fuel oil, the profile is more complex, and C-heavy fuel oil has a lifetime shorter than that of others. The fluorescence properties of weathered oil are different from that of fresh oil and greatly affected by the ambient conditions, a lot of case study will be needed. It is hoped that the chemistry of the individual fluorophores may be found so that we can determine the source of the fluorescence and its evolution.
Figure. 2 Fluorescence characteristics of fuel oil measured by a streak scope
3.2 Extinction of medium
The single scattering extinction coefficient for a collimated beam is expressed by the sum of the absorption coefficient and the scattering coefficient. But the effective water attenuation may vary with depth, because of changes in angular distribution of laser light as it is multiply scattered and deflected from collimated light at the surface to relatively diffuse light several attenuation lengths underwater. And also the scattering coefficient of the medium is greatly affected by the ambient conditions, so an in-situ measurement is necessary.
An observed image of light trajectory through the medium provides a convenient way to measure the extinction of the medium. Figure 3(a) shows the experimental set up. The laser beam illuminates the surface at an angle of 12.5 degrees from the vertical and a CCD camera with the gatable image intensifier observe the surface. Figure 3 (b) shows the image with the optical band pass filter of 405±5 nm, passing the Stokes Raman scattering of the water (406.9nm) excited by the third harmonic of the YAG laser. The temporal gate width was set at 13nsec, illustrated the beam trajectory where the water Raman signal to the bottom and the fluorescence signal at the surface induced by the reflected laser beam can be identified.
Figure 3 (c) shows the profile of the Raman signal along the laser beam, with known oil thickness. The signal at 350 in position includes the fluorescence of the oil film. From data, the extinction coefficient of the water and the light oil film were obtained to be 0.0019mm-1, 1.35mm-1, respectively.
