The word is an acronym for Light
Detection and Ranging. The US military and NASA invented the
Lidar technology during the early 1960s for measuring distance
in space. Its first commercial usage did not occur until 1995.
It uses ultraviolet, visible, or near infrared light from
lasers. Radar (Radio Detection and Ranging) uses radio or
electromagnetic waves. Lidar used in cars is low
powered and classed as 'eye-safe' allowing it to be used with
few safety precautions.
Some refer to Lidar as laser radar,
however it is not. It is more precise than radar, because the
speed of light is a constant, so a laser can make extremely
precise measurements of distance by computing the time between
when the device emits a laser pulse and when it detects the
reflection. Sound travels about 1,000 feet (300 meters) per
second and light travels about 984,000,000 feet per second
(300,000,000 meters). Also, radar wavelengths suffer from
atmospheric conditions, such as humidity, fog, rain, snow, and
temperature, but do perform better in smokey or dusty
conditions.
A laser unit fires a short pulse of light. The pulse rebounds
off a point, such as the rear of the car in front and is
detected by a sensor in the laser unit. A computer connected
to the unit measures the time between the initial pulse and
the light return and, using the speed of light, calculates the
distance the light has traveled. It creates a high-resolution
3D map of the surrounding environment. The best sensors can
see details of a few centimeters at distances of more than 330
feet or 100 meters.
Currently most autonomous cars use some combination of Lidar,
Radar, and camera. Lidar is precise, Radar is good at motion,
and cameras are good for depiction. Each technology has
strengths and weaknesses, so automakers and others are trying
to find the best combination of strengths at the lowest cost.