The optical configuration is shown schematically in Fig. . Excitation light from the LED passes through a short-pass filter, which absorbs long-wavelength photons that overlap spectrally with the fluorophore emission. The filtered light from the LED passes through a detection zone containing the sample under test, where it is partially absorbed and reradiated by the fluorophores. The emergent light from the sample -a mixture of unabsorbed light from the LED and longer-wavelength light from the emitting fluorophores -passes through a long-pass filter, which heavily attenuates the excitation light but allows the emitted light to pass and strike the photodetector. The sample is introduced into the detection zone using a 10-mm cuvette or a low-volume (< 100 μl) flow cell. Interchangeable 3D-printed holders allow for easy swapping between cuvette-and flow-based detection. (The greater sample volume of the cuvette typically provides slightly better sensitivity but requires sample volumes of at least 2 ml compared to ~100 μl for the flow cell). The holders may be readily adapted to other sample formats.