The imaging principle used with the ApoTome is referred to in the literature as "grid projection" or "structured illumination". The fundamental theory was described in publications on interferometry as early as in the nineteen-sixties. A grid of stripes of defined width is projected onto the focal plane of the objective and shifted laterally in three defined steps relative to the sample (see illustration). A CCD camera takes a picture in each grid position. The three "raw" images are combined into a resultant image by on-line computation. The resultant image is an optical section through the sample, from which all image slices originate. Free of artifacts, with out-of-focus information removed. This image has an improved signal-to-noise ratio, and approximately doubles resolution in axial (Z) direction. Making an optical section is a prerequisite for computing 3D visualizations of specimens.