Photon-recycling lensless fluorometer

Fluorometers typically employ multiple lenses and a single-pass optical geometry. We present a lensless fluorometer with a simple reflective sample chamber to recycle the unused excitation photons by reflecting them back into the sample volume while simultaneously redirecting emission photons toward the detector, thereby significantly enhancing the signal. By using commonly available aluminum bars, we experimentally achieve a 4.3× signal enhancement compared to a single-pass chamber in a lensless configuration. Combined with the documented 2× advantage of lensless systems, this design is projected to outperform standard lensed fluorometers by more than 8×. The enhanced performance allows for an 8× reduction in the excitation power while maintaining the required sub-parts-per-billion detection limit for tryptophan-like-fluorescence in drinking water. The power reduction significantly simplifies thermal and power management, which is essential for in-field and future handheld operation. We present a theoretical analysis that is supported by experimental results to validate this approach. Our system demonstrates a highly sensitive, power-efficient, and simplified platform for fluorescence measurement, opening new avenues for handheld and remote water quality sensing.