Light Detection And Ranging (LIDAR)
LIDAR (Light Detection And Ranging):
The laser system used for monitoring the environment is known as LIDAR. LIDAR is an acronym that stands for "Light Detection And Ranging".
Before the discovery of the laser, the study of the atmosphere was carried out using an optical beam, the source being the search light. One such experiment was performed by Hulbert in 1937 to study the turbidity of the atmosphere. After the discovery of the laser as a source of an optical highly coherent beam, the study of the atmosphere was revolutionised.
A pulsed laser beam is transmitted into the atmosphere. It is scattered by the particles present in the atmosphere. The scattered radiations are picked up by a receiver. The receiver removes the background sunlight by using different filters. The scattered light gives information regarding the particles present in the atmosphere. Although microwaves can also give these characteristics, the results from laser beams are better in resolution and clarity. The different particles present in the atmosphere in colloidal form can be studied by a LIDAR. A schematic diagram of such a setup is shown in the Figure Below.
A photo detector is used to measure the time dependence of the intensity of the back-scattered laser beam. The time variation can be easily converted into the height (range) from which the laser beam has been back scattered the figure below shows a plot of time dependence of back scattered laser beam, which corresponds to height in the case of clear atmosphere with no aerosols, i.e., back scattering is by pure molecular gases such as $N_{2}$, $O_{2}$, $Ar$ etc. These molecules have dimensions much smaller than optical wavelength.
The scattering is of Rayleigh type. The figure below shows a plot of time dependence of backscattered light in the atmosphere contained aerosols (colloidal particles). These particles have dimensions comparable with the wavelength of laser light. This is Mie scattering. The curve in the figure below has kinks at points A and B between heights h and h. These kinks are due to the fact that between points $A$ and $B$, there are aerosols that are responsible for a greater intensity than that for a clear atmosphere. This implies the presence of aerosols between heights $h_{1}$ and $h_{2}$. With LIDAR, it is also possible to study the concentration and sizes of the aerosols present in the atmosphere. These are very important in atmospheric pollution studies.
