LiDAR for Robots: From Theory to Your First Scan

A single ultrasonic sensor tells you how far away one thing is. A LiDAR (Light Detection And Ranging) sensor tells you how far away everything is — in all directions at once. This 360-degree distance map is the foundation of autonomous navigation.

How LiDAR Works

A LiDAR sensor emits laser light and measures the distance to whatever it hits. There are two common ways to do this: time-of-flight (measuring how long each pulse takes to return) and laser triangulation (measuring the angle at which the reflected beam lands on a sensor). By spinning the laser and measuring at thousands of angles per second, it builds a 2D map of distances in a horizontal plane.

The output is a 2D scan — a set of (angle, distance) readings that together describe the shape of the environment around the robot. In ROS2 this is published as a LaserScan message (which the code below subscribes to). The term “point cloud” properly refers to 3D LiDAR data (ROS2’s PointCloud2), not the 2D scan a single-plane LiDAR produces.

The RPLIDAR A1

The Slamtec RPLIDAR A1 is the most popular entry-level LiDAR for robotics. It uses laser triangulation (not time-of-flight) to measure distance. At ~$100, it offers:

• 360° scanning
• Up to 8,000 samples per second
• Range: 0.15–12 meters
• Angular resolution: ≤1°
• Scan rate: ~5.5 Hz (configurable up to ~10 Hz)
• USB interface (appears as a serial port)

How does it compare to the other distance sensors in this series? Here’s the short version:

Sensor	Coverage	Range	Rough cost	Best for
Ultrasonic (HC-SR04)	One narrow cone	~0.02–4 m	~$2	Single-direction obstacle checks
Depth camera	Forward field of view	~0.1–10 m	$150–600	Rich 3D in front of the robot
2D LiDAR (RPLIDAR A1)	Full 360° plane	0.15–12 m	~$100	Mapping & navigation (SLAM)

Connecting to ROS2

Install the ROS2 driver:

sudo apt install ros-humble-rplidar-ros

Launch the driver:

ros2 launch rplidar_ros rplidar_a1_launch.py

Visualize in RViz2:

ros2 run rviz2 rviz2
# Add a LaserScan display, set topic to /scan

Reading Scan Data in Python

import rclpy
from rclpy.node import Node
from sensor_msgs.msg import LaserScan
import numpy as np

class LidarReader(Node):
    def __init__(self):
        super().__init__('lidar_reader')
        self.sub = self.create_subscription(
            LaserScan, '/scan', self.scan_callback, 10
        )
    
    def scan_callback(self, msg):
        ranges = np.array(msg.ranges)
        # Filter out invalid readings (inf, nan, 0)
        valid = ranges[(ranges > msg.range_min) & (ranges < msg.range_max)]
        
        if len(valid) > 0:
            self.get_logger().info(
                f'Min distance: {valid.min():.2f}m, '
                f'Max distance: {valid.max():.2f}m, '
                f'Points: {len(valid)}'
            )

def main():
    rclpy.init()
    node = LidarReader()
    rclpy.spin(node)

if __name__ == '__main__':
    main()

LiDAR is the key sensor for SLAM (Simultaneous Localization and Mapping) — the technique that lets a robot build a map of its environment while navigating through it. We’ll cover SLAM in a future tutorial.