What an iPhone lidar can show about the speed of light

However, since this uses the total flight time of the ball, it gives the total distance the ball has traveled – to the wall and back. If you take that distance and divide it by 2, you get the distance from my hand to the wall, which in this case would be 10 meters.

I like this BallDAR method because it’s a good idea to throw a ball and time it. But lidar is essentially the same idea: instead of using a ball that moves back and forth, lidar uses light. (This is the “li” part of lidar.)

In theory, you could create a do-it-yourself version of lidar using a flashlight or even a laser pointer. Just point your laser at an object and as soon as you turn on the laser, you start a stopwatch. The light travels outwards, hits the wall and is then reflected back. Once you see this laser dot on the wall, stop the stopwatch. Then all you need is the speed of light to calculate the distance.

There is, of course, a practical problem: light travels Yes, really quickly. Its speed is 3 x 108th meters per second. That’s over 670 million miles per hour. If you measure a distance of 10 meters (like in the BallDAR example) the flight time would be about 0.000000067 seconds or 67 nanoseconds.

If you want to get lidar working, you need a really fast stopwatch. Galileo did in fact attempt something like this with his experiment to determine the speed of light. Of course, he didn’t have a laser or a nice stopwatch, but that didn’t stop him from trying. (He couldn’t actually get a measurement.)

Most lidar versions use a single laser with a detector. When a short pulse is sent, a computer measures the time it takes for a signal to get back to the device. Then it’s a simple calculation to find the distance the light has travelled.

But that only measures a single distance. It’s not enough to create one of those awesome 3D lidar surface images that show the shapes of objects. To get that you need more data.

If you know where the laser is pointing, you can determine the range and bearing one Point on the surface of an object. Next you just have to repeat this with the laser pointing in a slightly different direction, usually using a spinning mirror. Keep it up and you can get one whole lot of points. After collecting thousands of them, these dots will merge into an image shaped like the surface of the object you are scanning.

But using a laser and a spinning mirror is not only expensive, it’s also too bulky to fit in your phone. How does lidar work on an iPhone? I just want to say, “It’s magical” – because it seems that way to me. All I know is that instead of using a beam of light to measure distance, the iPhone uses a grid of dots emitted by the phone in the near-infrared range (like the light from your infrared TV remote). These multiple light beams are due to an array of vertical cavity surface emitting lasers or VCSELs. It’s basically many lasers on a single chip, and that makes it possible to put lidar on a smartphone.

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