Optical Coherence Tomography (OCT)
The OCT method has long been established for medical applications, but is still little known for industrial process monitoring. Developments of new sensor concepts in recent years have led to the fact that OCT technology, as a very robust and reliable method, is increasingly being used in the production process.
The OCT image is composed of axial interferograms that touch each other laterally. The interferograms are based on white light interferometry. In this procedure, the flight time of photons reflected or scattered by the object surface is compared with the flight time of the photons in a reference beam. The interferograms from the two arms produce a linear pattern of structures with different brightness, from which the relative optical path can be mapped as an axial depth profile.
With the raster method, the light beam is guided transversally in one or two directions, resulting in a flat tomogram or a three-dimensional volume. Unlike conventional light microscopy, OCT decouples transversal resolution from z-direction resolution. The transverse resolution is determined by the numerical aperture of the optics used, whereas the longitudinal spatial resolution in the depth of the material depends on the spectral width of the light used.
An ASP chip (Active Sensor Pixel) derived from 300 x 300 “interferometers” can be used to detect scattered or reflected light. Each pixel is equipped with its own lens and signal pre-processing. The array, developed and patented in Switzerland, enables image capture at up to 1 million fps (frames per second).
Based on Michelson Interferometer
Light from the source (A) is divided into two beams on the prism (B). One light beam is directed to a mirror (C) and the other is directed to the object (D). The light returns to the prism (B) by reflection and interferes there as a function of the optical condition in the measuring object (D). The sensor (E) measures the bright / dark appearances of the interferences. These signals are then analysed with algorithms and software.