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Illumination

Illuminate features & make defects visible

Principle of laser triangulation

Triangulation is a precise and versatile measurement technique widely used in surface metrology. A laser line plays a central role. This laser line is directed at an oblique angle onto the surface to be measured. The laser produces a clearly visible line on the surface.

In areas where the object is lower, the laser beam hits the surface at a slightly different point. This line offset is caused by differences in the height of the object being measured. The offset of the bright laser line in the image can now be captured and analysed by a camera.

The camera detects the reflected laser light and gives the exact position of the laser line. The topography of the surface can be accurately derived from the changes in position of the laser line.

Laser and camera setup for laser triangulation

3D measurement technology using laser triangulation

The principle of laser triangulation is perfected by the use of 3D triangulation cameras and lasers. The test object is continuously moved under a camera-laser system and a large number of triangulation profiles are generated.

Complex software algorithms are then used to calculate a three-dimensional image of the body from the deflection of the individual line profiles.

High quality laser triangulation systems are metrically calibrated to provide metrically correct measurements in 3D space.

For more information, see the 3D laser triangulation systems section.

Mathematical basis for height calculation

If the angle between the camera and the laser and the line offset are known, the component height can be calculated using simple trigonometric formulae (right-angled triangle).

If the laser hits the object very flat, even small differences in height will result in a large offset of the lines, but the height range of detection is small. In practice, differences of even tenths of a millimetre can easily be detected without any special effort. For particularly small measurement ranges, the height can even be determined in the micrometre range. With steep angles, the measurement accuracy is low, but large differences in height (such as metres) can be detected.

Square shape

Red laser line with small angle of incidence on component with square edges.

Rounded edges

The same mechanical setup, component now with rounded profile edges.

Laser and speckle phenomena

Problems in interpreting laser lines and other projection patterns can be caused by so-called "speckles". This term describes the granular, glittery speckles of light that appear on rough surfaces such as paper, wood, plastics and metals. This is caused by the coherent light of the laser, where the light waves all oscillate in the same direction and with the same frequency and phase. The irregularities and textures of the surface cause the coherent laser light to be reflected and the light phases to be shifted. This overlap creates a three-dimensional interference that is increasingly imperceptible to the human eye, which is constantly moving. However, it is still clearly visible in the camera image.

A laser line is no longer shown as a sharp line, but as a blurred line. In measurement applications or 3D triangulation systems, this can affect subsequent image processing.

A laser line no longer appears as a sharp line, but as a blurred, spotted line. This can interfere with subsequent image processing in metrology applications or 3D triangulation systems.

Superluminescent laser diodes can be used to produce a lower temporal coherence of light, resulting in fewer speckles. This in turn leads to more homogeneous and sharper lines and therefore better analysis. For high resolution applications it is essential to use lasers with a low speckle content!

With laser triangulation and the acquisition of full point clouds, the problem of speckle is less important: since the acquisition is usually not static (as a single profile) but continuous in motion, speckle effects are greatly reduced and therefore play a subordinate role.

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