During the confocal measurement with the SIM method, both LEDs (LED A and LED B) are actuated alternately, which causes the focus to move through the topography. When LED A is activated, the grid is imaged by transmission into the focal plane (see “LED A on“). LED B is then activated and the grid is imaged into the focal plane by reflection on the chrome-plated bars using beam splitters (see “Illumination B on“). The image sensor thus captures the surface with the imaged grid, whereby two 180° phase-shifted images are generated during movement through the topography.
The measuring result is obtained by calculating the difference in contrast between the two images. A Point Spread Function (PSF) is generated for each measuring point, from which the z coordinate is determined by Gaussian fit. Subsequently, a multi-stage quality filter is applied to ensure that only measuring points of high data quality are included in the evaluation.