Figure 1. The principle of illumination in Ultramicroscopy: Light (e.g. 488 nm Argon laser) is being focused quoin-like by a cylinder lens.

In the area where the beam is focused maximally a quoin-like description is not appropriate and the profile of the illumination beam stays relatively constant. The sample is immersed in the same liquid used to gain transparency. This assures that the illumination beam is not diffracted on the surface of the sample and propagates straight, even inside the sample. By moving the sample, different z-positions can be illuminated and recorded afterwards. In this way we acquired three-dimensional tomographical data.