Abstract

The myriad of modern applications for lasers includes the remote sensing and investigation of distant objects. When this work is conducted outdoors, at ranges of a few tens of metres to many hundreds of kilometres, the subject is usually called “laser radar”. It must be admitted that this is something of a misnomer—the term radar itself derives from “radio detection and ranging”, which was of course developed over 60 years ago for detection of aircraft by long-wavelength radio waves. However, we are presently considering the use of very much shorter wavelengths—that is, light waves in the visible and near-visible region. In consequence, the terms “lidar” for “light detection and ranging” and “ladar” for “laser detection and ranging” have also been introduced. While some attempts have been made to differentiate the usage of the three expressions “laser radar”, “lidar” and “ladar”, they are in fact generally used freely and interchangeably. However, one can do a great deal more than just “detect and range” with lasers. In consequence, two other terms are also used for more specific applications: laser Doppler velocimetry (LDV)—use of the Doppler principle for remote velocity measurements, and differential absorption by laser (DIAL)—chemical detection with lasers tuned on and off resonance absorption.