I am trying to learn something about LIDAR sensors or Laser Range finders. But I am always getting confused how could it get works.

What I understood about the principle is it emits a laser (invisible laser) which gets reflected back to sensor and mcu calculates the time of flight based on reflection which all happens in a fraction of seconds.

But how the ray always get reflected back to the sensor?? I mean if it falls on a flat 90 degree plane, it gets reflected back. But on a 45 degree it gets reflected in a different direction and the sensor wont get it.

enter image description here

So how does it really works??

  • \$\begingroup\$ There are two types of reflections: diffuse and specular. You are thinking of specular but most reflections are diffuse. \$\endgroup\$ – DKNguyen May 29 at 20:21
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    \$\begingroup\$ Stealth aircraft rely on that exact trick. Their polygonal shape is intended to minimize reflecting light back in the direction it came from. They also try to absorb the incoming light etc but the angling is a big part of it. \$\endgroup\$ – Flater May 30 at 0:04

If the surface you reflect it of is very well polished and mirror-like, it won't actually work. Most surfaces however have some roughness to them which will result in at least some of the light being reflected back to the rangefinder.

In fact, a lot of LIDAR systems use rotating mirrors to scan. If rangefinders also worked on mirrors, that wouldn't be possible since the rangefinder would just detect its own mirror the entire time.

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  • \$\begingroup\$ en.wikipedia.org/wiki/Diffuse_reflection \$\endgroup\$ – Dampmaskin May 29 at 7:35
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    \$\begingroup\$ To demonstrate diffuse reflection, take a laser pointer and point it at various surfaces. If you can see the dot, then a sensor can likely see it. A laser rangefinder uses IR, not visible light, but the principle is the same. \$\endgroup\$ – Mattman944 May 29 at 8:24
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    \$\begingroup\$ Actually for ex. surveyors use rangefinders which need a good reflector which reflects the laser beam back to the transmitter. The retroreflector principle makes it possible en.wikipedia.org/wiki/Retroreflector The laser can have safe power but long range measurements are still possible. Measurings happen fast with much better accuracy than GPS would give. A typical surveyor's prism: [1]: i.stack.imgur.com/qk3WT.png \$\endgroup\$ – user287001 May 29 at 8:47
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    \$\begingroup\$ @SandeepThomas That means the laser operates in the infrared spectrum instead of visible light. The reflectivity of some materials also differs significantly between IR and visible light. \$\endgroup\$ – TooTea May 29 at 19:23
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    \$\begingroup\$ To extend @Mattman944's great DIY example, throw some sand at your mirror and notice how the dot becomes more noticeable even with an angled mirror. Imperfections in the surface and environmental dirt that sticks to the surface will help your ranger finder or lidar to "see" better. \$\endgroup\$ – Flater May 30 at 0:09

To expand on an answer already here, I wanted to include this graphic, taken from an website about the optical properties of different types of art material:

enter image description here

The vast majority of materials that a lidar system encounters will have reflectivity that looks like item #3 in the above graphic. Even though only a small fraction of the incident light is reflected in a direction that hits the lidar, it's still discernible by the sensors. To do this, a bunch of different signal-to-noise ratio boosting techniques are used. For instance

  • The laser transmitter shapes the intensity of its pulses over time so the receiver can more easily distinguish them.
    (This also helps the receiver get picosecond resolution on the time-of-flight, even if the sample rate is on the nanosecond level.
  • The laser transmitter only transmits photons of a very precise wavelength, so that the receiver can more easily pick up the tiny amounts of reflected light.
  • The photo-detectors that listen for the reflected light have a small optical bandwidth (so they'll just react to light in a specific frequency range).

However, as you suspect, a lot of materials don't have this diffuse reflection property, like windows, mirrors, or very polished metal surfaces. Indoor robots do have issues in spaces that have lots of glass or windows. I used to work as an assistant in a university robotics lab, and a grad student in my lab published a paper specifically about how to deal with this issue. All this to say that your intuition about this issue isn't wrong, we just get around it by some sophisticated DSP, and by 'luck' (I guess) in the fact that most materials diffuse light.

I guess humans would be pretty bad at seeing if all of our environments reflected light like mirrors do. (or we'd get pretty good at echolocation lol)

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    \$\begingroup\$ Vision would be weeded out through natural selection and everyone would use sonar or touch. \$\endgroup\$ – DKNguyen May 29 at 20:23

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