These extension tubes are designed for building Guerilla projectors. They fit the GuerillaBeam slide holder, as well as its lens and LED light adapters. One purpose of these extensions is to get the neccessary distance between lens and slide without making the slide holder longer. This is mostly the case for slide and cinema projector lenses. They are also useful to increase the distance between light and lens, which can help to not have the LED appear in the image.