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lab / optical 3D measurement
Interferometer for optical 3D measurement
An interferometer for highly accurate detection of surface structures on the micro and nano level is a particular advance in technology. Geometries can be recorded and evaluated in their entirety without contact. The possibility to capture fast and easily the entire geometry of an object is a big step forward compared to two-dimensional measurements.
Fig.: optical 3D measured surface
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Explanation of the concepts of coherence and interference
Coherence:
The coherence describes the property of a light beam or wave to behave consistently in its shape when moving through space. It is also a condition for the formation of interference. The waves that meet have the same frequency and a constant phase shift to each other.
Fig.: coherence wave
Fig.: non coherence wave
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Interference:
Interference is the result of the superposition of coherent waves on each other. Two identical waves meet and cancel or superimpose each other. These two cases are described as destructive and constructive interference.
Fig.: constructive interference
Fig.: destructive interference
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Interferometer - fast 3D surface measurements with high accuracy
A classical white-light interferometer based on the principle of a Michelson interferometer can be used as a basis for high-precision 3D measurements. In principle, a measuring object is illuminated with coherent light by a light source [1] and the reflecting light of the measuring surface [4] is used for evaluation. A reference mirror [5], which is exposed by the same light beam via a beam splitter [3], helps here. If the measuring surface of a sample is at the same distance from the beam splitter as the reference mirror, then interference of the coherent light beams occurs. Via an optical system, the light is further guided to a camera chip [6] as a detector. The user sees a wave-like pattern in the overview image depending on the surface position. This is the indication that the measuring surface is at the same distance from the known reference path length of the system. This defines the reference plane in which height data of the measurement object can be measured. To measure the three-dimensional shape of a surface, the measuring object moves through this reference plane. In doing so, either the sensor or the sample can position itself in the height.
Fig.: Beam path in an interferometer with reference mirror
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Typical applications are the three-dimensional detection of the micro-geometry of components. Miniaturized components can be inspected in high accuracy within a short time. Larger measuring areas of approx. 50 mm^2 and low lateral point distances can be realized in this context. Furthermore, roughness, curvature, skewness and waviness can be characterized.