White light scanner

Interference will only be achieved when the path length delays of the interferometer are matched within the coherence time of the light source.

The axial resolution of the system is determined by the coherence length of the light source and is typically in the micrometer range.

The white light interferogram actually consists of the superposition of fringes generated by multiple wavelengths, obtaining peak fringe contrast as a function of scan position, that is, the red portion of the object beam interferes with the red portion of the reference beam, the blue interferes with the blue, and so forth.

The data are stored digitally and processed in a variety of ways depending on the system manufacturer, including being Fourier-transformed into frequency space, subject to cross-correlation methods, or analysis in the spatial domain.

If a Fourier transform is used, the original intensity data are expressed in terms of interference phase as a function of wavenumber.

In the actual measuring process, the optical path difference is steadily increased by scanning the objective vertically using a precision mechanical stage or piezoelectric positioner.

The intensity data as a function of the optical path difference are processed and converted to height information of the sample.

Figure 1. Lunate cells of Nepenthes khasiana visualized by Scanning White Light Interferometry (SWLI).
Figure 2. File:Twyman-Green interferometer set up as a white light scanner
Figure 3. White light interferometric microscope