Lidar measures the world by timing how long reflected light takes to return
Fire a laser pulse, wait for the reflection, multiply the round-trip time by the speed of light and halve it. That simple trick, first tried for tracking satellites in 1961, now maps terrain from aircraft at 30-centimetre resolution, charts shallow seabeds and helps self-driving machines see where they are going.
The idea predates the laser. In 1930 E. H. Synge imagined probing the atmosphere with powerful searchlights. Only after the laser was invented did Hughes Aircraft, led by Malcolm Stitch, build a working system in 1961, called Colidar, for coherent light detecting and ranging, a deliberate echo of radar. Two years later the Colidar Mark II, a large rifle-like rangefinder for military targeting, could reach 11 kilometres with an accuracy of 4.5 metres.
Meteorologists were early adopters, using it to measure clouds and pollution. Public fame came in 1971, when Apollo 15 astronauts mapped the Moon's surface with a laser altimeter. Since then even the spelling has fractured: LIDAR, LiDAR and lidar all appear, sometimes in the same government document.
Choosing a wavelength is the real craft. Systems range from far infrared down to ultraviolet, and each target favours a different colour. Airborne terrain mappers typically use 1,064-nanometre lasers, while underwater depth surveys drop to 532 nanometres, which passes through water with far less loss. Lasers at 1,550 nanometres stay eye-safe at higher power because water absorbs them before they reach the retina, and night-vision goggles cannot see them, which appeals to the military. Cheaper consumer gear sticks to 600 to 1,000 nanometres with power limits.
Scanning is the other headache. Tiny spinning micromirrors are cheap but sweep only one plane and dislike vibration, and fully solid-state steering with phased arrays, standard in radar since the 1940s, is extremely hard to achieve with light. Flash lidar sidesteps both by lighting the whole scene at once and reading the echo like a camera. On aircraft or satellites, a GPS receiver and an inertial unit track exactly where the sensor sits.
Source: Lidar
