7. Water Analysis Using Lidar

Detection of oil spills (1/4)

Oil type classification

Oil belongs to the substances which absorb light and re-emit part of it as fluorescence. The degree of absorbance is strong in the ultraviolet and decreases monotonically with increasing wavelengths in the visible range (blue, green yellow, red). Since absorbance and fluorescence are connected with each other, the intensity of fluorescence emission decreases with increasing wavelength as well. Strong fluorescence emission makes it easy to detect oil. Therefore, pulse lasers are utilised with emission wavelengths in the ultraviolett at about 250 to 350 nm.

The photographs below, taken in the laboratory, depict the bright fluorescence emission of lubricating oil when illuminated with UV laser radiation at 355 nm. This effect makes it possible to detect oil on the sea surface with a laser fluorosensor operated aboard an aircraft.

Left: a thick layer of lubricating oil is illuminated with UV laser radiation at 355 nm wavelength. The laser radiation is invisible to the eye and to the photocamera. The oil surface emits a bright blue fluorescence, the deeper oil is visible with its light brown colour. Right: thin films of lubricating oil on a water surface are illuminated with the laser radiation and emit blue fluorescence. The oil-free water does not fluoresce and remains dark on the image.

Laser radiation is characterised by a very small bandwidth: the spectrum is a sharp line. This is called monochromatic radiation, see chapter 2 of the tutorial on Remote Sensing Using Lasers. The fluorescence of oil has a comparably much larger bandwidth. As a rule, fluorescence emission spectra always have higher wavelengths than those of the exciting light. Their shape, i.e., the intensity as a function of wavelength, differs for different oil types.

Light oils such as petrol and Diesel fluoresce at ultraviolet or blue wavelengths. Heavy oils which appear dark brown or black and absorb light strongly fluoresce less intensively, and their emission covers the entire visible spectrum.

Fluorescence spectra of different oil types upon irradiation with UV light at 308 nm wavelength. The curves show the intensity of the fluorescence versus the wavelength between 320 nm (UV) an 685 nm (far red). The curves are normalised to the same total (i.e., integrated over-all wavelengths) fluorescence intensity in order to highlight the shape of the spectra. The absolute fluorescence intensity of heavy oils is much weaker than that of light oils and could not be visualised on the graph without normalisation.

The graph above depicts the differences of fluorescence spectra of several oil types. These oils can be detected with a laser fluorosensor, and classified by analysing their fluorescence spectra. In particular, less harmful vegetable oils and fish oil can be distinguished from mineral oils.

Oil classification ↓

Please think about methods to classify oil types with fluorescence spectroscopy by using the fluorescence spectra shown in the graph above.

Instructions:

A commonly used method employs colour contrast functions g which are given by ratios of intensity data at wavelengths where the data of the objects under consideration show a high variance. Fluorescence spectra of oils vary strongly in the UV, blue, and red. A suitable colour contrast function could be the ratio of fluorescence intensities F at 350 nm and 550 nm:

$g = \frac{F (350)}{F (550)}$
You can estimate the required data from the graph above. Alternatively, more precise data can be taken from a Data catalogue of optical properties of oils.
It might be advantageous to apply colour contrast functions which use data at three wavelengths, e.g.:

$g = \frac{F (500) \cdot F (600)}{{(F (400))}^{2}}$
Please consider also other combinations of wavelengths which might be suitable. Check their usefulness for oil classification. Find out which spectral ranges are particularly useful or useless for the definition of a colour contrast function.

You will find more information on the molecular properties of fluorescence in supplement 2, chapter 7 of the tutorial on Remote Sensing Using Lasers.