Basics of White Light Interferometry
Polytec´s white-light interferometers utilize special optical configurations and short-coherence-length light sources that optimize the interaction between the reflected light from the sample and the reference beam
The measurement itself is based on the principle of the Michelson interferometer, whereby the optical configuration (image above) contains a light source with a coherence length in the in the micron range (short coherence length implies a "broad" spectral source). The light is collimated from the light source and then split into two beams: an object beam and a reference beam. The object beam reflects from the object being measured and the reference beam reflects off of a reference mirror. The reflected light from each beam is captured and recombined at the beam splitter. The superimposed beams are imaged by a CCD camera for processing.
If the optical path for an object point in the measurement arm is the same as the optical path in the reference arm, then for all wavelengths in the spectrum of the light source, there is constructive interference and the camera pixel of the respective object point has a high intensity. For object points having a different optical path, the assigned camera pixel has a much lower intensity. Consequently, the camera output can be processed pixel by pixel to determine which object points are at the same height. In the interferometer, only the reference arm or the object is moved relative to the beam splitter, so when traversing the evaluation length, interferences are formed pixel by pixel as the object height is scanned. After a measurement run, the images from the camera are compiled, analyzed and the topographical structure of the sample is digitized.
Depending on the customer’s application requirements, Polytec has specific TopMap models that optimize the measurement.
Instruments with a telecentric configuration allow the topography of large surfaces to be measured quickly in one pass.
If high lateral resolution is required, microscope systems are more suitable, where the optical configuration including the reference arm is integrated into the objective.