For those interested in the background hows and whys of what Dr Köhler achieved, the latest PUMA microscope video ( Köhler Illumination - How to build, collimate and use the Köhler Illuminator for the PUMA microscope - YouTube ) goes over Köhler’s original 1893 paper (written in German but I provide an English translation) and shows some of his original photomicrographs - the very first published Köhler-illuminated photomicrographs in history! In the subsequent parts of the video I show you how to build your own Köhler illuminator from scratch with 3D printing and fit it to a PUMA open source microscope. I hope you / your students find it helpful. If you have any comments on it please share them on the YouTube page. Please note that this is not a ‘how to set your diaphragms’ intro talk - if you need that type of instruction then you will be better served looking at Jennifer Waters excellent video on Köhler illumination on the Microcourses channel instead.