  Koehler transillumination is a powerful imaging method, yet commercial
Koehler condensers are difficult to integrate into tabletop systems and make
learning the concepts of Koehler illumination difficult. We propose a simple 4f
Koehler illumination system that offers advantages with respect to building
simplicity, cost and compatibility with tabletop systems, which can be
integrated with open source Light Sheet Fluorescence Microscopes (LSFMs). With
those applications in mind as well as teaching, we provide instructions and
parts lists to assemble the system. The system's performance is comparable to
that of commercial condensers and significantly better than LED flashlights
such as those found in low-cost diagnostic devices and LSFMs with respect to
illumination homogeneity and sectioning capability.
