How lidar maps the world in 3D using only lasers

HostYou have probably seen those strange, spinning buckets on top of self-driving cars. They look like something out of a science fiction movie, and they're the main way those cars actually see the world around them. It's not like a normal camera at all, because it uses lasers to build a map of every single thing in sight. But how do you actually turn a beam of light into a 3D picture?

GuestIt's basically a very high-speed game of catch. The tool shoots out a tiny burst of light, it hits a tree or a wall, and then it bounces back to a sensor. Since we know exactly how fast light travels, we can work out how far away that thing is by timing how long the round trip took. If the light comes back in a tiny fraction of a second, we know the object is close. If it takes a bit longer, it's further away. By doing this over and over, the system builds a list of distances for everything around it.

HostBut light is the fastest thing there is. Nothing in the world moves quicker. How do you even time something that fast? A normal stopwatch wouldn't even see it happen.

GuestYou're right, a normal watch is way too slow for this. The sensors inside these kits have to be incredibly quick. They can track time down to a trillionth of a second. It's like having a clock that can count every single grain of sand on a beach in the time it takes you to blink. That speed is what lets us get the measurements down to the inch. If the timing was off by even a tiny sliver, a car would think the curb is five feet away when it's actually right under the tire. It needs that level of exactness to stay on the road.

HostOkay, so you get one dot and one distance. But the maps these things make look like glowing ghost worlds where you can see every branch on a tree. How do you get from one dot to a whole 3D world?

GuestWe don't just shoot one laser. We shoot millions of them every second. The laser usually sits behind a mirror that's spinning around at a very high speed. As that mirror spins, it throws the laser beams in every direction—up, down, and all the way around in a circle. It's like a lawn sprinkler, but instead of water, it's spraying light. Each of those millions of beams comes back with its own distance. When you put all those points together, you get a 3D shape of everything the light touched. We call it a point cloud because it looks like a thick mist of dots that perfectly matches the shape of a building or a car.

HostSo it's just a camera that brings its own flash?

GuestNot really. A camera just takes in the light that's already there, like the sun or a street lamp. It sees colors and shadows, but it's not great at knowing exactly how deep a room is. This laser tool is more like being a bat. A bat screams and listens for the echo to know where a wall is. This tool does the same thing with light. It's an active way of seeing. It doesn't care if it's pitch black outside because it brings its own light to the party.

HostThat makes sense for a car, but I have heard about people using this to find lost cities in the jungle. If you're flying a plane over a thick forest, wouldn't the lasers just hit the tops of the trees and stop? You would just see a big green carpet of leaves.

GuestThat's actually one of the best tricks this tool has. Most of the light does hit the leaves, but some tiny bits of that light find the small gaps between the branches. They make it all the way down to the forest floor and bounce back up. The sensor is smart enough to record every single echo it hears from one burst of light. It sees the first bounce from the treetop, some middle bounces from the branches, and a final bounce from the dirt. We can then just tell the computer to hide all the tree bounces. Suddenly, the trees disappear and you're left looking at the bare ground, which might show the shapes of old walls or roads that have been hidden for a thousand years.

HostWait, if it can see through gaps in trees, can it see through other stuff? Like, can a self-driving car see through a brick wall or another car to see what's coming?

GuestNo, it's not X-ray vision. If the light can't get through, it can't see what's behind it. If a big truck is in front of a car, the laser hits the back of the truck and stops. The car is basically blind to whatever is in front of that truck. That's a big hurdle. And it gets even weirder with things like glass or water. Since those are clear, the laser might go right through them or bounce off at a strange angle. A car might see a big glass window and think it's an empty road because the light didn't bounce straight back to the sensor.

HostThat sounds like it could be a mess. If the car thinks a window is an open path, it's going to crash. How do they deal with those blind spots?

GuestThey have to use other tools too. They don't just rely on the lasers. They use cameras to see the colors of traffic lights and radar to track how fast other cars are moving. It's about using all the senses at once. But the lasers are the ones that give that solid, 3D shape of the world. They tell the car exactly where the solid stuff is so it doesn't have to guess.

HostIs there anything the lasers just can't see at all? What if it's a very dark, black car? Does the light still come back?

GuestIt's much harder. Dark things soak up light. Think about how a black shirt gets hot in the sun because it's drinking in all that light energy. A very dark, matte black car can be like a black hole for these lasers. It might only send back a tiny, weak whisper of light. The sensors have to be sensitive enough to catch those faint messages. And then you have weather. Raindrops, snowflakes, or even thick fog can act like a million tiny mirrors. The laser hits the rain and bounces back too early, and the car might think there's a solid wall of water in front of it.

HostSo it's the best way we have to turn a flat view into a real space, but it still has to deal with the same things our eyes do.

GuestScientists are already working on new lasers that can see through fog and rain by using different kinds of light that don't bounce off water as easily.

HostThe spinning buckets on those cars might look a bit clunky, but they're the reason we can finally turn a flat view of the road into a world with real depth and height.