Detailed Explanation of the Laser Triangulation Ranging Principle

As a low-cost LiDAR solution, the laser triangulation ranging method achieves high accuracy and cost-effectiveness, making it the preferred solution for indoor service robot navigation. This article introduces the core components of LiDAR and explains in detail the principle behind LiDAR based on the triangulation method.

Four core components of LiDAR

LiDAR mainly consists of four core components: the laser emitter, receiver, signal processing unit, and rotating mechanism.

Laser emitter: The laser emitter is the light source of the LiDAR. During operation, it emits pulses. For example, SLAMTEC’s RPLIDAR A3 series lights up and switches off 16,000 times per second.
Receiver: After the emitted laser hits an obstacle, the reflected light is collected by the lens group and focused onto the receiver.
Signal processing unit: This unit controls the laser emission and processes the signals received by the receiver, calculating the distance of the target object.
Rotating mechanism: These three components form the measurement core. The rotating mechanism spins them at a stable speed to perform planar scanning, generating real-time environmental maps.

Principle of the Laser Triangulation Method

LiDAR ranging methods generally fall into three categories: pulse, coherent, and triangulation. The pulse and coherent approaches require highly advanced hardware and achieve very high precision, which makes them more common in military applications. The triangulation method, on the other hand, offers lower cost while delivering accuracy sufficient for most commercial and civilian applications, and thus has gained widespread use.

The triangulation method works as follows: A laser beam strikes the target at a fixed incident angle. The reflected or scattered beam is then focused through a lens onto a CCD (Charge-Coupled Device) position sensor. When the target moves along the laser’s path, the reflected spot on the CCD shifts. The displacement of the spot corresponds to the object’s movement distance. Using geometric triangulation principles, algorithms compute the object’s distance relative to the baseline.

Depending on the relationship between the incident laser beam and the surface normal of the target, triangulation can be classified into direct-incidence and oblique-incidence types.

Direct-Incidence Triangulation

Figure 1: Laser triangulation direct optical path diagram

As shown in Figure 1, when the laser beam is perpendicular to the target surface—meaning the incident beam is aligned with the surface normal—the method is referred to as direct-incidence triangulation.

Oblique-Incidence Triangulation

When the incident beam forms an angle of less than 90° with the surface normal, the setup is called oblique incidence. As illustrated in Figure 2, the laser strikes the surface at an angle, and the reflected light is collected through a lens and projected onto the photosensitive unit.

Figure 2: Laser triangulation oblique optical path diagram

In this optical geometry, AO represents the incident ray, AB is the distance between the laser source and the CCD center, and α is the angle of incidence. BF denotes the focal length of the lens. If the object is at an infinite distance, the reflected light falls at the extreme position D on the CCD. When the object is closer, the light spot shifts by a distance x (DE). Using the similarity of triangles (△ABO ~ △DEB), the displacement can be used to compute the distance to the object.

In practice, the CCD sensor is aligned so that one axis is parallel to the baseline AB (e.g., the y-axis). From the pixel coordinates (Px, Py) of the laser spot, along with the pixel size (CellSize) and correction value (DeviationValue), the displacement x can be calculated. Any relative displacement of the object with respect to the baseline AB results in a measurable change in x, which is then converted into the actual distance y of the object.

Final Notes

Both direct-incidence and oblique-incidence triangulation methods enable high-precision, non-contact measurement. However, oblique incidence generally achieves higher resolution.

SLAMTEC’s RPLIDAR series employs the oblique-incidence triangulation method, powered by its proprietary RPVision 3.0 ranging engine. This system achieves up to 16,000 measurements per second, a detection radius of 25 meters, and angular resolution as fine as 0.225°. During operation, the RPLIDAR emits modulated infrared laser signals. Reflected light is captured by its optical system, processed in real time by an embedded DSP processor, and output as both distance and angle information via communication interfaces.

RPLIDAR A3M1 operating principle diagram

Driven by its motorized mechanism, RPLIDAR continuously rotates clockwise, performing 360° omnidirectional scanning and ranging of the environment.

Keywords: Lidar,Technology Explained

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Detailed Explanation of the Laser Triangulation Ranging Principle

Four core components of LiDAR

Principle of the Laser Triangulation Method

Direct-Incidence Triangulation

Oblique-Incidence Triangulation

Final Notes

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