Interferometer for 3D measurement
An interferometer for high-precision acquisition of surface structures on the micro and nano scale is a major step forward in engineering.
Complex geometries can be captured and evaluated in their entirety without contact. Typical industrial examples include wear tracks,
roughness patterns, residual indents and other surface features that are difficult to describe with purely two-dimensional techniques.
- Interference
- Interferometer
- Coherence
What do coherence and interference mean?
Coherence
Coherence describes the property of a light beam or wave train to maintain a constant shape as it propagates through space. It is also a necessary condition for the formation of interference. The interacting waves have the same frequency and a constant phase shift with respect to each other.
Interference
Interference is the result of the superposition of coherent wave trains. In simple terms: two identical waves meet and their wave crests either add up to form a larger wave or cancel each other out. These two cases are referred to as constructive and destructive interference.
How does an interferometer enable fast and accurate 3D surface measurements?
A classical white-light interferometer based on the principle of a Michelson interferometer is an excellent basis for high-precision 3D surface measurement. In principle, a test object is illuminated by a light source [1] with (sufficiently) coherent light, and the reflected light from the measurement surface [4] is used for evaluation. A reference mirror [5], illuminated by the same beam via a beam splitter [3], provides the reference. If the measurement surface of the specimen is at the same distance from the beam splitter as the reference mirror, the coherent light beams interfere.
Via an optical system, the light is directed onto a camera sensor [6] as detector. In the overview image, the user sees a fringe pattern that depends on the height position of the surface. This indicates that the measurement surface is at the same distance as the known reference path of the system. This defines the reference plane in which height data of the test object can be recorded. To capture the three-dimensional shape of a surface, the specimen is moved through this reference plane. Either the sensor or the specimen can be moved along the vertical axis.
Typical applications include three-dimensional acquisition of the micro-geometry of components. Miniaturised parts can be inspected quickly for form deviations and dimensional accuracy. Larger measurement areas of around 50 mm² and small lateral sampling distances can be realised. In addition, roughness, curvature, tilt and waviness can be characterised, which makes the method useful for process traces, residual indent evaluation and other surface-sensitive validation work.
