Topographic confocal Raman imaging enables the imaging of samples with rough or inclined surfaces without surface smoothing or any other sample preparation. The sample topography is first scanned, then confocal Raman imaging is performed with the surface held in constant focus guided by the topographic information.
The topographic sensor operates according to the confocal chromatic sensor principle: A white-light point source is focused onto the sample with a lens system that has a strong linear chromatic error (hyperchromatic lens system). Every color has therefore a different focal distance. The light reflected from the sample is focused through a pinhole onto a dedicated spectrometer on top of the sensor. As only one specific color is in focus at the sample surface, only this light can pass through a confocal pinhole. The detected wavelength is therefore directly related to the surface topography. Scanning the sample in the x-y-plane reveals a topographic map of the sample. This map can then be followed in a subsequent Raman image so that the Raman laser is always kept in focus with the sample surface (or at any distance below the surface if required).
The profilometric capabilities of the topographic sensor allows scan ranges of up to 150x100 mm with a spatial resolution on the order of 40-120 nm vertically and 10-25 µm laterally.