Where it sits
Every phosphene is proof that the visual system can produce its own input.
Tell it apart
| Aspect | Phosphenes | Floaters |
|---|---|---|
| Physical object | None | Yes |
| Cause | Neural firing | A shadow |
| Moves with the eye | No | Drifts |
| Producible on purpose | Yes | No |
Floaters are real shadows, cast on the retina by debris drifting in the vitreous. Phosphenes are light with no light behind it. One is an object; the other is a firing.
What it doesn't mean
They are real neural activations, and they have been used as instruments. Phosphenes have been employed to TEST RETINAL FUNCTION BEFORE CATARACT SURGERY, and Isaac Newton documented pressing the side of his own eye to produce a coloured ring.
It is documented and measured. Apollo 16 and 17 crews wore a helmet-mounted emulsion detector; on Apollo 17 the observer reported seventeen flashes, and analysis of a third of the detector plate found two definite matches between a heavy cosmic-ray nucleus crossing an eye and a reported flash. A 2006 survey collected reports from 59 astronauts in low Earth orbit. On lunar missions, dark-adapted astronauts reported a flash roughly every 2.9 minutes.
What it really means
Press gently on a closed eyelid and you will see light. Patches, rings, a shifting grid.
No light reached your retina. The pressure stimulated the cells directly, they fired, and the brain — which has no way of asking where a signal came from — reported the only thing those cells ever report: light.
That is a phosphene, from Greek phos + phainein, roughly light-showing. It is a 19th-century clinical coinage rather than an ancient word, assembled from Greek parts by physicians. Newton documented the effect on himself, pressing the side of his own eye to produce a coloured ring.
They are not imaginary and they are not useless. Phosphenes have been used as instruments — to test retinal function before cataract surgery, when the lens is too clouded to see through. Stimulate, ask what the patient perceives, and you learn whether the retina behind the obstruction still works.
The most striking case happens in orbit.
Astronauts report flashes of light in complete darkness, with their eyes closed. The explanation is that cosmic ray particles are passing through the eye and the visual system directly, triggering the same false report. And this has been measured, not merely reported: Apollo 16 and 17 crews wore a helmet-mounted emulsion detector; on Apollo 17 the observer reported seventeen flashes, and analysis of a third of the detector plate found two definite matches between a heavy cosmic-ray nucleus crossing an eye and a reported flash. A 2006 survey collected reports from 59 astronauts in low Earth orbit.
On lunar missions, dark-adapted astronauts reported a flash roughly every 2.9 minutes.
One piece of provenance is too strange to leave out. The word is credited to Henri Savigny — a French physician who was the surviving surgeon of the wrecked frigate Méduse, the shipwreck behind one of the most famous paintings in Europe. The man who came back from that raft went on to name the light you see when there is no light.
This page is educational and not a diagnosis. Ordinary pressure phosphenes are unremarkable, but sudden new flashes — especially with new floaters or a shadow across your vision — can indicate a retinal problem, and are worth taking to a doctor or clinician promptly.
Where it comes from
Greek — light
Greek — to show
Literally 'light-showing'. The word is a 19th-century clinical coinage, not an ancient Greek word — it was assembled from Greek parts by physicians.
- 1929
The neurologist Otfrid Foerster reports phosphenes produced by direct electrical stimulation of the brain.
- 1968
Brindley and Lewin implant an 80-electrode array on a blind woman's visual cortex and generate spots of light that combine into simple patterns; the work is later extended toward Braille-pattern visual prostheses.
In real life
- Rubbing her tired eyes at the end of a long day, she watched a slow chequerboard of colour bloom in the dark: phosphenes, made by pressure rather than light.
- As children they used to press their palms against their closed eyes to see who could summon the brightest phosphenes.
- Phosphenes is the name for the sparks you can make yourself by pressing on your eyelids; sudden new flashes that you haven't caused are a different matter, and worth a prompt word with a doctor.
Questions
Q01Why do I see colours when I rub my eyes?
Mechanical pressure stimulates retinal cells directly, producing pressure phosphenes — dark fields, coloured patches, bright circles or scintillating grids. The eye reports light because the cells fired, not because light arrived.
Q02Do astronauts really see flashes in space?
Yes — a well-documented cosmic-ray visual phenomenon. Apollo 16 and 17 crews wore a helmet detector to record it, and a 2006 survey collected reports from 59 astronauts.
Q03When should I be concerned?
This page is educational and not a diagnosis, but sudden new flashes — especially with new floaters or a shadow across your vision — can indicate a retinal problem and are worth taking to a doctor or clinician promptly.
Sources
- Phosphene — Wikipedia.
- Cosmic ray visual phenomena — Wikipedia.
- “Correlation of Individual Cosmic Ray Nuclei With the Observation of Light Flashes By Apollo Astronauts” — NASA Technical Reports Server.
- phosphene (etymology) — Wiktionary.