Laser profilers measure 2D range profiles and operate in different environments, like spaceborne, airborne and indoor. It is the simplest application of the LIght Detection And Ranging technique. It transmits a short pulse of energy (visible or near-infrared radiation) and detects 'echo', by measuring the time delay. Knowing the speed of propagation of the pulse (speed of light), the range from the instrument to the surface can be measured.
Laser profiling uses successive reflectorless laser range measurements (1D distance measurement) on adjacent points along a path, which results in a 2D profile or cross-section of the ground. A laser profiler can be terrestrial, or ground-based, or it can be mounted on an airborne or spaceborne platform. In the case of ground-based measurements, the platform is fixed but the angle of illumination changes, allowing for the cross section of the terrain to be mapped. An airborne laser profiler can transmit a continuous stream of pulses along its flight path. As a result, if the position of the platform is known, e.g. from GPS/IMU system, a surface profile along the flight path can be reconstructed using the successively recorded vertical distances between the platform and the points on the ground. The use of an additional rotational mirror allow to scan the Earth in an additional dimension, providing 3D information of the mapped surface. This is the principle of a laser scanner.
There are two principal types of laser profiling techniques: the first one is based on analog detection and the second on photon counting. In analog detection, the signal power is converted into an output voltage providing a signal strength as function of time. The analog-to-digital conversion yields either a full waveform that allows retrieving the entire time-structure of the return signal strength- and therefore the full vertical structure of the target-, or discrete returns when the signal strength exceed a certain threshold. The full waveform information is especially useful when analyzing vegetation, as every vegetation layer (canopy, stems, branches) and the ground return pulses, allowing the determination of e.g. canopy height, ground surface topography but also a deeper analysis of the canopy structure. Photon counting techniques record the arrival of single photons. The counting of photons is combined with their time-of-flight. The accumulation of single photons at a specific range is similar to the signal strength of analog detection and allows retrieving the height and structure of specific targets.