How do distance sensors work with angled surfaces?

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From my understanding of distance sensors, most of them typically rely on an emitted beam being reflected back to the sensor, which would work if the surface is perpendicular to the beam. However, if the target surface is at an angle, I imagine that emitted beams would simply reflect away from the sensor, and the sensor would not be able to measure any distance. I was wondering if some types of distance sensors are able to overcome this, and what makes them work for angled surfaces.

(For example, I see LiDAR being used to measure distances to many differently oriented surfaces, like entire forests, but I’m not sure how the emitted lasers are being reflected back and detected).

Thanks!

In: Physics
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If you had a perfectly smooth and level surface, this would be an issue. However, surfaces aren’t perfectly smooth so the light really gets scattered in pretty much all directions. Specular reflection is when you’re dealing with largely smooth surfaces, Diffuse reflection is when you’re dealing with bumpy surfaces. [Here’s a visual diagram](https://static.sciencelearn.org.nz/images/images/000/000/045/full/Types-of-reflection_Science_Learning_Hub.jpg?1598474835)

As an experiment. Take a laser pointer and point it at an extreme angle at a surface. Notice how you can still see the light reflected back at you just fine and you don’t see it reflected onto another surface. Now do the same with a (clean) mirror – you’ll likely barely see the light on the mirror’s surface, but see it strongly on wherever the light is reflecting to off of the mirror.

An angled surface still diffracts the light that is bouncing off of it, meaning it spreads in *all* directions, not just a perfect angle-in-angle-out bounce. This means that some of it is going to get back to the sensor.

Very rarely will you actually have a LiDAR sensor work on something that is perfectly perpendicular.

Additionally, if you’re bouncing off of something like a mirror, which will have perfect angle-in-angle-out bouncing of the light, there are some strategies that LiDAR systems can use to detect that they’re bouncing off a mirror, which includes using multiple distinguishable measurements that are calibrated at slightly different angles to detect the flip of the mirror.

The other explanations are good. As an extra, this is why you can make stealth planes/boats that cannot be seen using radar. These vehicles are made of flat surfaces that basically act like a mirror and do not scatter the radar. As long as no surface of the vehicle is perfectly pointing back to the radar receiver, the vehicle is basically invisible. (There will be a slight radar traces from edges and imperfections).