Simple and open 4f Koehler transmitted illumination system for low-cost microscopic imaging and teaching

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.