The Basics of White Light Interferometry


White-light interferometers from Polytec utilize short-coherence-length light sources and special optical configurations to optimize the interaction between the reflected light from the sample and the reference beam.

Measurement Principle

The principle of the Michelson interferometer is utilized for taking the measurement, whereby the optical configuration (image above) consists of a light source with a coherence length 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: a reference beam and an object beam. The object beam is reflected from the object being measured and the reference beam is reflected off of a reference mirror. The reflected light from the two beams is captured and combined again at the beam splitter. A CCD camera is used for imaging the superimposed beams 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 every individual wavelength in the spectrum of the light source, constructive interference occurs and the camera pixel of the respective object point has a high intensity. The assigned camera pixel has a much lower intensity in the case of object points having a different optical path. Consequently, the camera output can be processed pixel wise so as to determine which object points are at the same height. In the interferometer, just the object or the reference arm is moved relative to the beam splitter. Thus, when traversing the evaluation length, interferences are formed pixel wise when the object height is scanned. After taking measurements, the camera images are compiled and analyzed, and the topographical structure of the sample is digitized.

Polytec’s TopMap Models

Polytec has specific TopMap models to optimize the measurement in order to cater to the customer’s application requirements.

Instruments having a telecentric configuration enable quick measurement of the topography of large surfaces in one pass.

When high lateral resolution is needed, microscope systems are preferred, where the optical configuration including the reference arm is integrated into the objective.

This information has been sourced, reviewed and adapted from materials provided by Polytec.

For more information on this source, please visit Polytec.


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