The Origins of Ploem's Filter Cube: A Pandora's Box

Ploem's filter cube, central to modern epifluorescence microscopy, integrates three matched optical components: an excitation filter, an emission filter, and a dichroic mirror. Together, these elements enable the separation of excitation and emission light paths, producing high-contrast fluorescence images. Despite their importance, the origins of these optical filters and the concept of a matched filter set remain unattributed. This review traces the development of these principles to discoveries made half a century before the fluorescence microscope. Through a narrative historical analysis, we identify George Stokes, Wilhelm Haidinger, and David Brewster as key contributors whose studies of light, colour, and polarisation established the foundations of fluorescence filtering. We also show that the matched filter concept can be dated to 173 years ago and that early dichroic films predated Ploem's cube by more than a century. By uncovering these origins, this work highlights how early optical discoveries shaped the design of modern fluorescence microscopy.