Lidar << LY dahr >> is an electronic technology used to detect and locate moving or fixed objects. The word lidar is short for light detection and ranging. Lidar works on a principle similar to that of radar and is sometimes called optical radar or laser radar. Lidar can be used to determine the direction, distance, shape, and speed of objects hundreds of feet or meters away. Many experimental self-driving automobiles rely on lidar for detailed information about their surroundings.
How lidar works.
A lidar unit includes a laser and a light sensor, called a photodetector. The laser shines a beam of light into the unit’s surroundings. When the light strikes an object, some of the light is reflected. When reflected light returns to the lidar unit, it is received by the photodetector. Because light has a constant speed, a computer can calculate how far an object is from the lidar unit based on how long it took the light to return to the photodetector.
Manufacturers produce many types of lidar units with different resolutions and fields of view. Resolution is a measure of the sharpness of an image. In some units, the laser emits a broad flash to scan a wide field of view. In other units, the laser spins, or makes use of a moving mirror , to make sweeping measurements in many directions. A lidar unit may also include multiple photodetectors. Some lidar units can make more than 1 million measurements each second, producing a detailed picture of their surroundings.
History.
Lidar was first developed by the Hughes Aircraft Company in 1961. The last three crewed missions to the moon— Apollo 15, 16, and 17—used lidar to measure the elevation of the moon’s surface.
In 2004 and 2005, the United States Defense Advanced Research Projects Agency (DARPA) hosted a pair of contests called Grand Challenges. The contests were designed to encourage the development of self-driving military vehicles. The American businessman and inventor David Hall created a lidar system for use in his 2005 entry. Hall mounted lidar sensors on a spinning platform on top of his vehicle, giving it a 360-degree view of its environment. His design impressed contest officials and other contestants. After the Grand Challenge, Hall began developing and manufacturing lidar units through his electronics company, called Velodyne.
Uses.
Lidar units attached to airplanes or drones can scan the ground to make high-resolution maps . Such maps can reveal archaeological ruins or seasonal variations in plant life. However, lidar cannot penetrate clouds, precipitation (rain or snow), fog, or dust, so weather conditions must be clear and calm. Laser-producing beams of some wavelengths can penetrate water, enabling specialized lidar to gather data on sea floors or riverbeds. Lidar is also used in meteorology to study features of the upper atmosphere or to detect dust.
Many companies manufacture lidar units for experimental self-driving vehicles, working to reduce the units’ size and cost. Several companies have also worked to develop solid-state lidar units that can point the laser beam in different directions. This allows the unit to scan in multiple directions without the need for spinning mirrors or other moving parts. One way to do this is through the use of a technology called an optical phased array (OPA). In an OPA, the light beam from a laser is split into several smaller beams, each of which passes through a special switch called a phase shifter. A computer instructs the phase shifter to change the phase (alignment) of each beam. The beams then leave the OPA through tightly spaced antennas arranged in a line or a small square. Through the canceling and reinforcing effects of interference , these multiple beams combine to form a single beam. The angle of this combined beam varies depending on the phases of the beams that make it up.
Solid-state lidar units cannot produce a 360-degree field of view like spinning lidar units can. But because they have no moving parts, solid-state units may be cheaper to produce and more durable than spinning units.
