Topography with Laser Line

In the previous lessons, we considered images acquired by a regular camera, meaning we had no direct information about the 3D shape of the inspected objects.

In many inspection scenarios, we need to obtain the object’s topography. We might want to measure its volume or check for bumps and holes. Some options for acquiring topography information include:

• Stereoscopic vision system: Multiple cameras grab images of the same scene. When properly calibrated, a stereoscopic vision system can reconstruct the 3D coordinates of matched points. It will typically produce a sparse point cloud.

• Time-of-flight cameras: A projector sends light pulses on the scene, and a camera detects the pulse’s reflections. The delay between the pulse emission and its detection allows us to evaluate a dense topography map. Such a system can be a viable option when a spatial resolution of a few centimeters is acceptable.

• Structured lighting systems: A projector projects a pattern on the scene, typically a series of shifted fringes. By modulating the phase of the fringes, it is possible to reconstruct a dense topography map as long as the topography remains within the wavelength of the fringe pattern.

In the context of objects carried on a conveyor, the simplest system for 3D acquisition is the laser line. Let’s describe this system in more detail.

The laser line setup geometry

Although many configurations are possible, the most common one is depicted in the image below.