LiDAR Time-of-Flight Sensor

Light leaves, bounces off a wall 100 metres away, and returns — and the sensor measures that 667-nanosecond round trip to within tens of picoseconds.

Fires short pulses of near-infrared laser light and measures the time each pulse takes to return after reflecting from a surface. Since light travels at a known speed, that elapsed time directly encodes distance. Scanning the laser beam — mechanically, with a MEMS mirror, or via a phased array — turns single-distance samples into dense 3D point clouds used for autonomous navigation, robot perception, mapping, and obstacle avoidance.

A driver circuit fires a laser diode in bursts of 2–20 nanoseconds. The outgoing pulse is timed by a reference clock. Reflected photons arrive at a detector — typically an avalanche photodiode (APD) or single-photon avalanche diode (SPAD) — which generates an electrical pulse on photon arrival. A time-to-digital converter (TDC) measures the interval between emission and detection with a resolution of tens of picoseconds. For a 100-metre round trip at the speed of light, the flight time is approximately 667 ns; a 10-ps timing error corresponds to a 1.5-mm range error. SPADs can trigger on single photons, enabling detection even at very long range or low reflectivity, but require quenching circuits to reset between detections. Modern sensors fire many laser channels simultaneously or in rapid sequence to build full 3D scans at 10–20 Hz.

In plain terms

Imagine standing in a dark cave and shouting. You count the milliseconds until you hear the echo — and from that you know how far the wall is. LiDAR does exactly the same thing with flashes of light instead of sound, but millions of times per second and with enough precision to measure the distance to something the size of a tennis ball a football field away.

Also called: pulsed ToF LiDAR, direct time-of-flight LiDAR, dToF LiDAR, laser rangefinder.

Why it matters

Range accuracy of ±2.5 cm at 90 m (Ouster OS1) is achievable in full sunlight, something cameras cannot match.
A rotating multi-beam sensor generates over 5 million range points per second, providing a real-time 3D model of the environment with no ambient lighting requirement.
SPADs with picosecond timing jitter allow single-photon sensitivity, enabling detection of objects with only 10% reflectivity at 200 m (Hesai AT128, 905 nm).
Eye-safe Class 1 operation at 905 nm or 1550 nm is mandated by IEC 60825-1, allowing deployment on public roads and around people.
Unlike radar, LiDAR produces angular resolution fine enough to distinguish a bicycle from a pedestrian at 50 metres.

Where it lives

Autonomous vehicle perception stacks (Waymo, Cruise, Zoox) — primary 3D environment sensor
Hesai AT128 automotive-grade solid-state LiDAR (200 m range at 10% reflectivity, mass production 2022)
Ouster OS1/OS2 sensors for robotics, mapping, and smart infrastructure
Apollo 15 Lunar Laser Altimeter (1971) — first space-borne laser ranging, ~2–3 m accuracy from 100 km orbit
NASA Mars rover wheel-odometry and terrain assessment sensors
Warehouse autonomous mobile robots (AMRs) for pallet and obstacle detection

Key numbers

Round-trip flight time (100 m): ~667 ns (Speed of light 3×10⁸ m/s, two-way path)
TDC timing resolution: ~55 ps single-shot (TI LIDAR reference design; ≈8 mm range equivalent)
Range accuracy (OS1): ±2.5 cm (Ouster OS1 at 90 m, Lambertian targets, 1024×10 Hz mode)
Max range (Hesai AT128): 200 m at 10% reflectivity (905 nm, Class 1 eye-safe, automotive grade)
Point rate: 1.3–5.2 million pts/s (Ouster OS1 32–128 channel; Hesai AT128 up to 1.53 M pts/s single-return)
Laser wavelength: 865 nm or 905 nm / 1550 nm (Most automotive sensors; 1550 nm used for longer-range applications)

Surprising facts

Light completes the 200-metre round trip to a target and back in just 1.33 microseconds — yet the TDC measuring that interval resolves individual picoseconds, a precision improvement of more than a million to one.
A Texas Instruments reference LiDAR design achieves a single-shot ToF accuracy of 55 picoseconds, corresponding to a distance error of about 8 mm — from a circuit that costs a few dollars in silicon.
The Velodyne HDL-64E, introduced for the 2007 DARPA Urban Challenge and invented by David Hall, packed 64 laser-detector pairs into a spinning head that produced 2.56 million range points per second — a density that changed the trajectory of autonomous driving research overnight.
Apollo 15 used a laser altimeter to map the Moon's surface in 1971, one measurement per second, from 100 km altitude — the same time-of-flight principle scales from a 1-Hz lunar altimeter to a 5-million-point-per-second robot sensor.

A short history

The first LiDAR prototype was built in 1961 at Hughes Aircraft Company — the same laboratory where Theodore Maiman had demonstrated the first working laser only a year earlier. Hughes introduced the Hughes Mark II Colidar commercially in 1962. Pulsed laser ranging for space applications came of age with the Apollo 15 Lunar Laser Altimeter in 1971, which measured the Moon's surface elevation profile from 100 km orbit at roughly one point per second and 2–3 m accuracy. Terrestrial LiDAR evolved steadily through airborne topographic mapping in the 1990s. The pivotal moment for robotics came in 2005, when David Hall designed the first spinning multi-beam 3D sensor for Velodyne, used in the DARPA Grand Challenge. The 64-beam HDL-64E debuted at the 2007 DARPA Urban Challenge and became the defining sensor of a decade of autonomous-driving research. Simultaneously, solid-state and MEMS-mirror designs reduced cost and size; Hesai's AT128 automotive-grade sensor entered mass production in 2022, combining 200-metre range with 1.53-million-points-per-second measurement frequency.

Sources

Ouster OS1 datasheet Rev7 — range, accuracy, wavelength, point rate, field of view (https://data.ouster.io/downloads/datasheets/datasheet-rev7-v3p0-os1.pdf)
Texas Instruments — LIDAR Pulsed Time-of-Flight Reference Design (55 ps accuracy, TDC resolution) (https://www.ti.com/lit/pdf/tiducm1)
Hesai — AT128 Hybrid Solid-State LiDAR release (200 m at 10%, 1.53 M pts/s, 905 nm) (https://www.hesaitech.com/hesai-releases-long-range-automotive-grade-hybrid-solid-state-lidar-at128/)
Todd Neff — LiDAR History Timeline (Hughes 1961 prototype, Mark II Colidar 1962, Apollo 15 1971) (https://toddneff.com/books/lidarhistory/extras/lidarhistory-timeline/)
DARPA Urban Challenge 2007 — Austin Robot Technology technical report (Velodyne HDL-64E first documented use) (https://www.cs.utexas.edu/~pstone/Papers/bib2html-links/urbanchallenge07.pdf)