Blue field entoptic phenomenon

How it works

The retina is fed by a fine net of capillaries that sit in front of the light-sensing cells. Blood flows through these vessels all the time. Red blood cells absorb blue light, so under blue light the capillaries act like faint dark lines¹.

White blood cells are larger than red cells and they do not absorb blue light in the same way. When a white cell passes through a capillary, it leaves a short gap in the column of red cells behind it. Blue light passes through that gap and reaches the retina. You see it as a bright moving dot¹.

The dots follow the paths of the capillaries, so they move along short curved tracks. Each dot lasts about a second. Many dots often appear in the same rhythm as your heartbeat, because blood flow speeds up with each pulse¹. The dots often look slightly elongated with a dark tail. The tail is a small pile-up of red cells behind the white cell¹.

The effect is strongest in blue light with a wavelength of about 430 nanometres¹. This is why a clear blue sky is the classic place to see it. The sprites are absent from the very centre of vision, because the fovea has no capillaries of its own².

Eye doctors use the same effect in a test called blue field entoptoscopy. The patient looks at a blue field and reports what the dots do. It can help check blood flow in the retina, for example in people with diabetes¹.

Try it

1. Go outside on a clear day. Look at a patch of blue sky away from the sun. Do not look at the sun.
2. Relax your eyes. Do not focus on anything.
3. Watch for small bright dots that dart along short wiggly paths and vanish.
4. Feel your pulse at the same time. Notice that the dots speed up with each beat.
5. If the sky is grey, a bright blue screen or a blue light works too.

History and culture

The effect is named after Richard Scheerer, a German eye doctor who described it in 1924¹. Most people have seen it as children and assumed everyone else saw the same thing. The XKCD comic “Visual Field” places the sprites on a map of the eye next to the blind spot and Haidinger’s brush³.