Lidars - Introduction to DIAL

How does it work?

1)  Light is absorbed by many of the atmospheric components.  Absorption of light by molecules is the basis for DIfferential Absorption Lidar (DIAL) which is used to measure the atmospheric concentrations of gases such as water vapor and ozone.

2)  As shown in Fig. a, water vapor has many discrete absorption lines.  In water vapor DIAL, laser pulses are   transmitted at two wavelengths, one on a water vapor absorption line, (lambda on), and another off-line, (lambda off).

3)  If the two wavelengths are close together, then for both wavelengths the scattering by molecules and particles is essentially equal. The difference in the returns between the two wavelengths is then due entirely to absorption by water vapor molecules.[Fig. b]

4)  Thus, measurement of the ratio of the backscatter at the two wavelengths as a function of range can be used to calculate the water vapor concentration profile.