Chemistry Reference
In-Depth Information
Coaxial
Telescope
Target sample
Laser
Spectrometer
Oblique (biaxial)
Telescope
Target sample
Laser
Spectrometer
Figure 6.3
Stand-off Raman spectroscopy geometries available.
6.1.2.4
Light Detection and Ranging (LIDAR) and Differential Absorption LIDAR (DIAL)
LIDAR is based on similar principles than RADAR but it uses light to collect useful information [28]. The
simplest LIDAR approach is used to determine the distance to a solid or hard target and, because of that, it
has been used for coastal mapping [29] and to measure vegetation heights [30] among others.
On the other hand, differential absorption LIDAR (DIAL) can measure chemical concentrations in the
atmosphere. The DIAL system remotely assesses concentrations of an atmospheric pollutant from the ratio
between the backscattered light on and off a spectroscopic absorption line of the pollutant. The range
resolution is obtained by emitting short pulses of light, combined with time-resolved acquisition of the
elastically backscattered light. DIAL system consists of a dual laser system, a telescopic receiver and a
detector system. It can be deployed in monostatic or bistatic configurations, depending on the position of the
laser and the detector system. The monostatic configuration is usually employed for contaminant profiling
and it can be performed in either a coaxial configuration, where the laser beam is transmitted within the
receiver front, or in a biaxial mode, in which the laser is adjacent to the receiving unit. Tunable laser systems
can be used as light source allowing for a fast change in the wavelengths to measure a wider range of
contaminants.
