Laser Triangulation (LT)
One of the most widely used active acquisition methods is Laser Triangulation (LT). The method is based on an instrument that carries a laser source and an optical detector. The laser source emits light in the form of a spot, a line or a pattern on the surface of the object while the optical detector captures the deformations of the light pattern due to the surface’s morphology. The depth is computed by using the triangulation principle. Laser scanners are known for their high accuracy in geometry measurements (<50µm) and dense sampling (<100µm). Current LT systems are able to offer perfect match between distance measurements and colour information. The method being used proposes the combination of three laser beams (each with a wavelength close to one of the three primary colours) into an optical fibre. The acquisition system is able to capture both geometry and colour using the same composite laser beam while being unaffected by ambient lighting and shadows.
Diagram illustrating the principles of laser triangulation (LT) based range devices.
Structured Light (SL)
Structured Light (SL) - also known as fringe projection systems - is another popular active acquisition method that is based on projecting a sequence of different alternated dark and bright stripes onto the surface of an object and extracting the 3D geometry by monitoring the deformations of each pattern. By examining the edges of each line in the pattern, the distance from the scanner to the object’s surface is calculated by trigonometric triangulation. Significant research has been carried out on the projection of fringe patterns that are suitable for maximizing the measurement resolution. Current research is focused on developing SL systems that are able to capture 3D surfaces in real-time. This is achieved by increasing the speed of projection patterns and capturing algorithms.
Diagram illustrating the principles of structured light (SL) measurement devices.