# A Review of Oil Spill Remote Sensing
## 3 Use of Active Sensors for Oil Spill Detection and Mapping  
### Laser Fluorosensors  
Laser fluorosensors use the phenomenon that oil aromatic compounds interact with ultraviolet light, absorb the light energy, and release the extra energy as visible light [33]. The absorption and emission wavelengths are unique to oil. Other substances in water, such as chlorophyll, fluoresce at distinctive wavelengths thus giving oil a unique signature. Various types of oil have distinct fluorescent intensities and spectral properties, resulting in the ability to differentiate different oil classes. The laser fluorosensor is best at discriminating between light, medium, and heavy oil types. Laser fluorosensors employ a UV laser operating between 308 and 355 nm [33]. UV lasers in the 300-355 nm region, such as the XeCl excimer laser (308 nm), the nitrogen laser (337 nm), the XeF excimer laser (351 nm), and the frequency-tripled Nd:YAG laser (355 nm), are available off-the-shelf. With an excimer laser excitation at 308 nm organic matter fluoresces at 420 nm. This is called \"Gelbstoff\" or yellow matter, which can be compensated for in fluorosensor output. Chlorophyll fluoresces at 685 nm. Crude oil fluoresces from 400 to 650 nm with the 308 nm excitation (Figure 5).  
Some fluorosensors have detector activation at the precise time that the light returns from the target surface. This technique is called \"gating\". This gating technique magnifies differentiation of the target oil from other possible interferences. Some fluorosensors can also gate their detectors to target areas below the sea surface. This enables detection in the water column. Work has been carried out with Orimulsion, a denser-than-water oil product [33]. This study was conducted using a gated fluorosensor that measured signal returns under the target sea surface. In this study, oil detection was possible as far as 2 m and easily at 1 m below a water surface. Laser fluorosensors are sampling instruments. The surface sampling rate is controlled by a laser repetition rate and the aircraft ground speed. At aircraft speeds of 120 knots and at a laser repetition rate of 100 Hz, a fluorescent sample is taken about every 60 cm along the flight path.  
Figure 5: Fluorescence spectra of several light crude oils after activation with a 308 nm laser (Environment Canada). Spectra were obtained in a laboratory.  
Raman scattering occurs when energy is exchanged between incident laser light and water [33]. Water molecules absorb UV light as rotational-vibrational energy and emit this energy in the form of light at a wavelength that is a difference between the incident radiation and the vibration-rotational energy of the molecule. The water Raman signal appears at 344 nm when the incident wavelength is 308 nm. The water Raman wavelength is an advantageous tool to maintain fluorosensor wavelength calibration.  
Laser fluorosensors are very useful oil spill sensors as they contribute a unique method of discriminating between oiled and unoiled seaweeds as well as detecting oil on shorelines. Tests on oiled shorelines demonstrated that laser fluorosensors can be very helpful [33]. Laser fluorosensors have a good track record in use and now are specified as essential in remote sensing packages. Laser fluorosensors provide a form of chemical analysis information to the user. The typical fluorosensor yields a plethora of user information as illustrated in Figure 6. Commercial instruments are now available and development of the technology continues.