Wavelength shifter
A wavelength shifter is a photofluorescent material that absorbs higher frequency photons and emits lower frequency photons. In most cases, the material absorbs one photon, and emits multiple lower-energy photons.
These materials can be used to increase the efficiency of a photovoltaic cell (solar cell) by changing one "too-high" energy photon into multiple "just-right" energy photons. Optical fibers doped with a wavelength shifter are typically used to collect scintillation light in particle physics experiments.