What should you know about how laser scanners work?
Laser scanning is based on emitting, receiving and evaluating a light signal. It makes use of the fact that most materials are capable of reflecting a light signal. In laser scanning, various parameters of the emitted light signal and of the type of reception can be varied. This in turn gives rise to the distinction between the pulse measurement method and the phase comparison method.
Every laser scanner has a movable scan head that is responsible for emitting the signal. It also has optics that register the reflected light. Depending on the application, so-called deflection mirrors are installed in the laser scanners. In land surveying, laser scanning usually involves three such deflection mirrors, which rotate extremely quickly. This is the only way to capture not only the length and width but also the depth of cuts in the terrain. For two-dimensional surveys, scan heads with two deflection mirrors are sufficient.
What characterises the pulse measurement method?
The pulse measurement method takes its name from the fact that identical light pulses are emitted at short intervals. This method measures the time delay between the emission and the reception of the reflected light signal. Here, laser scanning relies on the fact that light travels through air at a constant speed.
This results in the formula for calculating the distance: the speed of light multiplied by half the time that has elapsed between emitting and receiving the signal. In terrestrial measurement, the data is then transferred to a georeferenced coordinate system. In Europe, the Gauss–Krüger coordinate system is usually used for this, while in North America the Universal Transverse Mercator system is used.
The pulse measurement method has the decisive advantage that up to 10,000 measuring points can be recorded per second.
They are displayed as a point cloud. Using special algorithms, the structures of the surfaces can be calculated from this point cloud and converted into a three-dimensional graphic.
The second advantage of the pulse measurement method lies in the fact that distances of up to one kilometre can be determined with an accuracy of a few millimetres. This is why this type of laser scanning is preferred for outdoor use (land surveying, road construction and bridge construction).
How laser scanning works: what is different about the phase comparison method?
The name of this method is apt, as laser beams of different wavelengths are directed at the objects to be surveyed. These phases change with distance. It is precisely these changes that are recorded and analysed in the phase comparison method. The respective distance is then calculated from these phase changes.
However, this particular form of laser scanning has one disadvantage: it can only be used to measure distances of up to 50 metres. It is therefore suitable, for example, for surveying components in reverse engineering or for surveying smaller structures. The great advantage of the phase comparison method is its enormous working speed. Depending on the laser scanner model, between 100,000 and 500,000 measuring points can be transferred to the point cloud. This means that even objects with very delicate structures can be surveyed precisely within a short time.
Conclusion: the phase comparison method is used for laser scanning small objects. It works considerably faster than the pulse measurement method, which is used primarily for surveying parts of the Earth’s surface. Both variants of laser scanning guarantee a high degree of precision.
The images show the LSA scan of Kulmbach Town Hall.
You can see screenshots from FARO Scene and Autodesk ReCap.
