The light flashes astronauts perceive when high-energy charged particles pass through the eye, producing streaks, spots and bursts of light even with the eyes closed. First reported on Apollo missions and studied directly by the ALFMED experiment, the Apollo Light Flash Moving Emulsion Detector, which paired a particle detector with an astronaut’s observations.
The phenomenon recurs across the NASA spaceflight transcripts in PURSUE: it is a well-understood, conventional explanation for some of the visual oddities astronauts described, and a useful baseline when reading Apollo, Gemini and Skylab debriefs for genuinely unexplained content.