What’s the difference between 3D depth sensing with structured light scanning and LiDAR? What’s better for which use case and why should we care? Read on and learn more about 3D depth mapping!

Structured Light Scanning

A great example of structured light is Apple’s Face ID. A dot projector flashes thousands of infrared dots and are read by an infrared camera. The dot readings are then used to build a depth map since closer dots will appear larger, and farther away dots will appear smaller.

Credit goes to TechInsider for the screenshots

This works really well for building very accurate depth maps at a close range (like 1 to 5 feet). Which makes sense for Face ID - the reading of your face needs to be very accurate and you’ll generally hold your phone a couple of feet away from your face.

But as the the target object gets farther away, the accuracy of the structured light depth map gets worse and worse. It’s harder for the device to accurately read and interpret the projected dots.

To summarize structured light scanning:

• Works by projecting light in a pattern and using readings of the light to build a depth map
• + Excellent short range performance
• + Produces very high fidelity depth maps
• - Poor longer range performance: Accuracy of depth map rapidly degrades the farther away the object

LiDAR

On the other hand there’s LiDAR, which stands for LIght Detection And Ranging, or Laser Imaging, Detection, And Ranging, or is a mash-up of LIght and RADAR, depending on who you ask. These devices use the Time-of-Flight principle to gather distance data in a scene.

Time-of-Flight involves an emitter to project light (usually infrared) and a receiver to capture reflections of the light. The device can then calculate how far the light travelled by measuring the time between the emission and reflection - or the time of flight

Credit to Wikipedia user RCraig09 for this image.

LiDAR devices generally have better distance performance than structured light devices, but at the cost of detail accuracy. 3D LiDAR cameras for consumers generally perform best at distances of 0.5 to 10 meters, hence their use in environment mapping, AR, person & object detection, among other things.

Here’s how the same scene looks with the regular camera, the LiDAR camera, and the Face ID camera (structured light) camera on the iPhone 12 Pro:

Notice how it’s really easy to tell the couch, Christmas tree, and presents apart in the LiDAR image, but it’s hard to get a sense of anything in the structured light photo.

To summarize LiDAR:

• Uses the Time of Flight principle to capture distance data
• + Excellent medium range performance
• - Not meant for fine details, better for more general distance and scene measurements

Wrap

And there you have it. Both methods have their advantages and drawbacks. As with anything, it’s important to know which tool works best for the job at hand!

Follow along as we explore more 3D depth mapping topics and share more about the Tau LiDAR Camera!