Carrying out measurements with a measuring tape is time-consuming, prone to errors, and unsuitable for inaccessible areas of buildings. 3D laser scanners are an efficient alternative. There are many different varieties of this measuring method.
For active triangulation, in other words, measuring distances based on the angles of a triangle, 3D scanners have a laser beam source and a camera positioned at a certain distance. When the laser beam encounters a surface, the camera records the points of light. Laser, camera, and point form a triangle, the angles of which allow the distance to the surface to be determined precisely in the sub-millimeter range. Instead of points, triangulation scanners also project stripes or lines. Area of application: smaller objects at distances of up to a few meters.
For distances up to 100 m, experts recommend scanners that use phase comparison methods. These emit modulated infrared light of various wavelengths and receive light reflected from the object. The number of complete intervals and the respective remaining lengths provide very accurate information about the distance. Pulse methods measure distances up to 1 kilometer based on laser pulses between the beam source and the receiver.
Modern laser scanners have color sensors and deliver HD images with resolutions of up to 165 megapixels. Scan rates are around 1 million points per second and distances can be up to 350 meters.
As well as scanners, stereophotogrammetry is another widespread method in this area. Two cameras record measurement images of an object from different aspects. The images are used to derive extremely precise, color-accurate surface models.
Laser scans for the reconstruction of Notre Dame
Scientists used a laser scanner to document the Paris landmark accurately several years before the fire. The data could provide valuable assistance during the reconstruction.