De La Salle University - Manila

LIDAR

Basic Lidar Principle

A basic LIDAR (Light Detection and Ranging) system is comprised of a pulsed laser source, transmitting optics, a telescope, the receiving optics, and equipment for acquisition of data, as shown in Figure 1. Laser pulses are fired into the atmosphere, interacting with both the gas molecules and the aerosols present.

Figure 1. Schematic of a typical LIDAR System

When light strikes a medium it is scattered and/or absorbed. Elastic scattering is the type of scattering wherein the wavelengths of the scattered and the incident light are the same. Two types of elastic scattering are Rayleigh scattering and Mie scattering.

As shown in figure 2, when the scattering particles are much smaller that the laser wavelength, like gas molecules for example, Rayleigh scattering takes place, which has a backscatter intensity inversely proportional to the fourth power of the wavelength of the incident light. On the other hand, Mie scattering takes place when the scattering particles' sizes are comparable to that of the wavelength of the incident radiation. Particles of smoke, fog, and the water droplets or ice crystals that form clouds all contribute to Mie scattering [Measures, 1984].

Figure 2. Types of scattering and Absorption of radiation in the atmosphere.

The LIDAR Equation

The basic LIDAR principles discussed can be expressed in the Single-scattering LIDAR equation:

Where Pr is the instantaneous received power at time t, Po is the transmitted power at time to, c is the velocity of light, t is the pulse duration, b is the volume backscattering coefficient of the atmosphere, R is range, Ar is the receiving area and a is the volume extinction coefficient of the atmosphere (cross section per unit volume).

The Single scattering LIDAR Equation assumes that the incident laser radiation has only one collision with atmospheric particles and the scattered light are immediately detected by the receiving system.