The light of someone's life might not be just another person, but light in the literal sense. According to a recent study by researchers from University of Calgary, every living system emits light without requiring external excitation due to a biological phenomenon known as ultraweak photon emission (UPE).
The paper itself says “The phenomenon of biological ultraweak photon emission (UPE), that is, extremely low-intensity emission (10–103 photons cm–2 s–1) in the spectral range of 200–1000 nm, has been observed in all living systems that have been examined.” Human vision falls within that spectral range, but the brightness is several orders magnitude too faint to see with the naked eye, even in absolute darkness.
The paper itself says “The phenomenon of biological ultraweak photon emission (UPE), that is, extremely low-intensity emission (10–103 photons cm–2 s–1) in the spectral range of 200–1000 nm, has been observed in all living systems that have been examined.” Human vision falls within that spectral range, but the brightness is several orders magnitude too faint to see with the naked eye, even in absolute darkness.