Lidar has been used for decades for things like atmospheric research. More recently, people hear about it in advanced driver assistance systems and autonomous cars. It’s even showing up in mobile devices for augmented reality. But what is it? Here are five things to know about lidar.
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- It’s a combination of the words light and radar. It’s been turned into acronyms like “light detection and ranging,” but the upshot is it sends out light. A detector measures the light’s return after reflecting off objects and it uses the time it takes to do so to calculate depth and distance.
- The light is lasers. The lasers in lidar systems can use various types of light from ultraviolet, through the visible spectrum, into the near infrared at a variety of wavelengths. The lasers are often semiconductor diode lasers, or more recently (and more affordable), vertical cavity surface-emitting lasers.
- The detectors are often photoelectric cells. Short-range systems use silicon photodiodes. Longer-range systems with longer-wavelength lasers use avalanche photodiodes, which can detect lower light levels and be built into a chip to create a multi-pixel photon counter. The iPad uses single-photon avalanche diodes or SPADs from Sony.
- Lidar systems send out millions of pulses of light per second. Most quickly move the light on a gimbal in a pattern that can quickly cover the available area. Other, simpler systems that don’t need as long of a range use more, but less powerful, lasers in a fixed position to try to cover the available area. Either way, it results in a point cloud of detected returning light.
- What can’t lidar do? Well, it can’t read road signs or anything else that’s flat. It doesn’t see through fog, rain, or clouds. It sees the fog, clouds, and rain, but not what’s behind them–at least not very well.
Lidar is great at 3D mapping the world around you which makes it useful in lots of applications, but it works best when combined with other sensors. GPS can tell you where you actually are and turn the 3D images into a 3D map. Accelerometers can tell you how long before you hit that thing the lidar sensor sees. These technologies are getting cheaper to make, so you’ll see them in more things.
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