2.15 Laser Radar

Devices which measures the physical characteristics such as distance, density, velocity, shape etc., using scattering, returned time, intensity, frequency and/or polarization of light are called optical sensors. However as the actual light used by the optical sensor is mostly laser, it is usually called laser radar or lidar (light detection and ranging).

Laser radar is an active sensor which is used to measure air pollution, physical characteristics of atmospheric constituents in the stratosphere and its spatial distribution. The theory of laser radar is also utilized to measure distance, so that for this application it is called laser distancemeter or laser altimeter.

The main measurement object is the atmosphere although laser radar is also used to measure water depth, thickness of oil film or vividness of chlorophyll in vegetation.

[ Theory of Lidar ]
Figure 2.15.1 shows a schematic diagram of a lidar system. The power of the received light Pr (R) reflected from a distance of R can be expressed as follows.

Pr (R) = Po K Ar q (R) T (R) Y (R) / R + Pb
where Po : intensity of transmitted light
K : efficiency of optical system
Ar : aperture
q : half wavelength
b (R) : backscattering coefficient
T (R) : transmittance of atmosphere
Y (R) : geometric efficiency
Pb : light noises of background

Received light is converted to an electric signal which is displayed or recorded after A/D conversion. The effective distance of lidar depends on the relationship between the received light intensity and the noise level.

Lidar can be classified with respect to its physical characteristics, interactive effects, physical quantities etc., as shown in Table 2.15.1. In this table, Mie Lidar is the most established sensor,with a which signal intensity large enough to measure Mie scattering due to aerosols.

Fluorescence lidar, Roman lidar and differential absorption lidar Figure 2.15.3 are utilized for measurement of density of gaseous body, while Doppler lidar is used for measurement of velocity. Polarization effects of lidar is utilized for measurement of shape.

There are several display modes for example, a scope with horizontal axis of distance and with vertical axis of intensity, PPI (plane position indication ) with gray level in polar coordinate system, RHI (range high indication) with a display of the vertical profile, THI (time height indication) with a horizontal axis of elapsed time and with a vertical axis of altitude.


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