An instrument combining a pulsed laser transmitter and optical receiver (usually a telescope) with an electronic signal processing unit used for the detection and ranging of various distant targets in the atmosphere, analogous to the principles of operation of microwave radar. Normally, the transmitter and receiver are coaligned and placed closely together to measure the laser energy backscattered by the target into the direction of the receiver. The use of lasers allows the light pulses to be exceptionally short, highly focused, and monochromatic, but laser light suffers from strong, range-limiting attenuation in many types of clouds. Depending on the spectral characteristics of the laser and detector and the number of receiver channels, a large variety of lidar applications for atmospheric research are in use. Simple one-channel laser ceilometers measure cloud-base heights and internal cloud structures; polarisation lidars measure cloud phase and hydrometeor type; differential absorption (DIAL) and Raman lidars measure the concentrations of selected molecular species; high spectral resolution lidars (HSRL) measure the separation of molecular and aerosol or cloud constituents; and Doppler lidars measure the radial velocity of aerosol or cloud targets. Lidar wavelengths range from the near-ultraviolet to the midinfrared (≈0. 3–12 μm).