LiDAR
How it works
An emitter (laser diode or fiber laser, typically 905 nm or 1550 nm) sends a short pulse or a modulated wave. The light reflects off an object and returns to a detector (e.g. an avalanche photodiode APD or SPAD). Timing electronics compute the time of flight: directly by measuring pulse delay (dToF) or via the phase of a modulated signal (iToF); in the FMCW variant it compares the frequencies of the transmitted and received signals, obtaining range and radial velocity at once. Distance is computed as d = c·t/2. A scanning mechanism (rotating mirror, MEMS, phased array, or flash illumination) sweeps the beam across the field of view. The collected measurements (range + angles + intensity) are assembled into a 3D point cloud that downstream algorithms use for object detection, mapping, and SLAM.
Problem solved
Robots and vehicles need accurate, metric information about distance and the 3D shape of their surroundings. Passive cameras do not measure depth directly and struggle under poor or changing lighting, while radar has low angular resolution. LiDAR provides dense, accurate 3D geometry independent of ambient light.
Components
Light source (laser diode or fiber laser), typically 905 nm or 1550 nm, generating pulses or a modulated wave.
Official
Steers the beam across the field of view: rotating mirror, MEMS, optical phased array, or flash illumination (no scanning).
Official
Photodetector capturing the returning light, e.g. an avalanche photodiode (APD) or single-photon (SPAD).
Official
Computes time of flight (dToF) or phase/frequency (iToF/FMCW) and converts it to distance d = c·t/2.
Assembles range, angle, and intensity measurements into a 3D point cloud used by perception, mapping, and SLAM.
Official
Implementation
Rain, fog, snow, dust, and exhaust scatter the beam, reducing range and introducing spurious points.
Dark, wet, or mirror-like objects return little light to the detector, causing gaps in the point cloud.
Sequential scanning while the sensor moves warps the point cloud unless de-skewed.
With many lidars present, pulses from other units can be falsely detected.
Evolution
Theodore Maiman demonstrates the first working (ruby) laser, enabling active light-based range measurement.
Hughes Aircraft Company builds an early laser rangefinder/tracking system.
The term "lidar" appears in print; the "Colidar Mark II" rangefinder is produced.
Apollo 15 astronauts use a laser altimeter to map the lunar surface, popularizing the technology.
The competition exposes the limits of single-line lidar and cameras for autonomous driving, motivating 3D lidar.
Velodyne (David Hall) introduces a rotating 64-beam sensor producing a 360° 3D map (~1M points/s) — a breakthrough for autonomous vehicles.
Hyperparameters (configurable axes)
Typically 905 nm (cheaper, eye-safety limited) or 1550 nm (longer range, more eye-safe).
Maximum measurement distance; depends on laser power, detector sensitivity, and target reflectivity.
Angular density of points; determines the ability to resolve small and distant objects.
Measurement throughput; modern sensors reach hundreds of thousands to millions of points/s.
Angular coverage horizontally and vertically; mechanical units give 360° horizontal, solid-state usually a sector.
How the beam is swept: mechanical, MEMS, phased array, or flash.