Most laser fluorosensors used for oil spill detection employ a laser excitation source operating in the ultraviolet region of 300–355 nm [4]. The fluorescence response of crude oil when excited with an ultraviolet laser ranges from 400 to 650 nm with peak centers in the 480 nm region. A typical laser fluorosensor system with excimer laser and scanner is shown in Fig. 7.1. These excitation wavelengths are a compromise in that they excite all three classes of oil (light, medium, and heavy) with reasonable efficiency (shorter wavelength lasers would excite lighter oils efficiently but would be rather poor at exciting crude and heavy refined oils). There are several reasonably priced, commercially available, ultraviolet lasers in the 300–355 nm region including the XeCl excimer laser (308 nm), nitrogen laser (337 nm), XeF excimer laser at 351 nm, and frequency tripled Nd:YAG at 355 nm. With excitation in this wavelength region, there exists a broad organic matter fluorescent return, centered at 420 nm. This is referred to as Gelbstoff or yellow matter, which must be accounted for. While Gelbstoff disappears if the oil thickness is greater than 10–20 μm (i.e., where the oil is optically thick), it can be an interfering signal when attempting to detect thin films of light oils on water. Chlorophyll yields a sharp peak at 685 nm.

The detection systems in most laser fluorosensor systems usually involve the collection of laser-induced fluorescence by a telescope, the focusing of the fluorescence onto the entrance slit of a grating spectrometer, and then onto intensified diode arrays. The fluorescence spectrum is then recorded at a number of selected wavelengths or over a wide spectral range covering the ultraviolet through the visible.

The majority of modern laser fluorosensors are equipped with range-gated detection systems. Range-gating is simply the turning on of the detection system at precisely the time at which the laser-induced fluorescence is expected to return to the laser fluorosensor. This is accomplished by turning the detection system on and off at a precise time based on the known altitude. To accomplish this, the timing of the laser pulse is monitored prior to exiting the aircraft and the elastic backscatter from the surface is then monitored to determine the precise aircraft altitude, which is then used to control the range-gating electronics. This allows the detector to observe only the fluorescence induced by the excitation laser and neglect the majority of the background solar radiation.

As noted earlier, active sensors need to deliver sufficient energy onto the target, the surface of the earth containing oil contamination, to excite sufficient fluorescence to allow for the detection and classification of the oil. With most airborne laser fluorosensor systems, this means illuminating a field-of-view (FOV) of about 1 × 3 mrad, giving a footprint on the surface of about 0.1 m × 0.3 m at 100 m altitude. This does not allow for a large amount of the surface to be interrogated by each laser pulse. With higher-powered ultraviolet lasers, one can fly at higher altitudes and enlarge the footprint of the sensor. The repetition rate of the laser and the ground speed of the aircraft are also major factors in the sampling of the surface where the oil contamination is being examined. At ground speeds of 100–140 knots (nautical miles per hour) at a laser repetition rate of 100 Hz, a fluorescence spectrum is collected approximately every 60 cm long the flight path (at 100 m altitude). Some laser fluorosensors only “look” directly below the aircraft and collect fluorescence spectra in a straight line; this is referred to as a “fixed” FOV system. As spilled oil often piles up in narrow bands at the high tide line, detection of this oil with a fixed FOV system is not optimal. This means that the oil might not be detected because the sensor is striking the surface of the earth on either side of the high tide line. To compensate for this tendency of the oil to accumulate in a narrow band, it is preferable to change the laser FOV by employing a scanner. The scanner can either be moved in a conical (circular) fashion or back and forth across the surface to increase the likelihood of striking the oil contamination. There are conical scanning laser fluorosensor systems developed in Germany [15] and Canada [16]. An example of a conical scanner is shown in Fig. 7.2.

The display of spectral data is essential for the effective operation of modern laser fluorosensors. Laser fluorosensor display monitors often include a representation of sensor parameters, such as laser pulse energy, operating altitude, laser backscatter energy, reference spectra, and live or real-time fluorescence spectra. The observation of each of these parameters is extremely useful to the sensor operator. In particular, real-time spectra are useful to provide an indication of the interaction of the laser beam with the surface. By observing the live spectrum, the operator has an indication of the water clarity through observation of the water Raman scattering spectrum. A trained sensor operator can easily recognize the presence of oil contamination via the characteristic spectrum of light refined, crude, or heavy refined petroleum products. Fig. 7.3 shows typical laser-induced fluorescence spectra of a crude oil. The lack of a proper spectral signature can indicate a problem with the fluorosensor system such as low laser power, low laser backscatter signal, incorrect range-gate timing, or laser misalignment. It is impossible to display all of the spectral data collected with a high sampling rate laser fluorosensor system, a subsample of the spectral data is all that is needed for an experienced operator to monitor system operations.

An accurate map display of oil contamination location(s) is essential for the rapid mitigation of the environmental effects of spilled oil. Displays of oil contamination superimposed over aircraft flight lines are useful for spill responders who participate in oil spill remote sensing over flights. Fig. 7.4 shows the operator's display from Environment Canada's SLEAF system. Fig. 7.5 shows the operator's display with areas of oil contamination illustrated as colored bars perpendicular to the flight path. Similar information is presented in Fig. 7.6 which overlays the scanner pattern on the flight path along with the oil contamination information (different colors for clean, light, medium, or heavy oil classifications). Spill response organizations and personnel are not interested in sensor parameters or spectral data. However, geo-referenced maps showing oil locations are necessary. These maps, or at least the geo-referenced oil contamination locations, should be in a format that can be transmitted electronically and compatible with commonly used geographical information systems. These maps will help in the rapid and efficient deployment of spill response resources and equipment to the location(s) where oil contamination is the heaviest and the cleanup of contamination is most needed.