Look up at a bright, blue sky and you may notice tiny dots of moving light. You aren’t imagining these spots. They are created by your own white blood cells flowing through your eyes. What you are experiencing is a very normal occurrence called the blue field entoptic phenomenon.
Blood flows to your eyes through capillaries that pass over the retina — the tissue at the back of your eye that acts as a receptor for all light. Red blood cells, which make up more than 90 percent of your blood, absorb blue light. As these cells move through the capillaries in front of your retina, your eye and brain adjust so that you don’t see shadows or dark spots.
White blood cells flow through the capillaries much less frequently than red blood cells. These larger blood cells let blue light through to your retina. The retina sends a signal of increased brightness to the brain, and, to a viewer looking at the sky or any large monochromatic area, it looks like a tiny spot of white light is moving through the space.
The dots may look like little worms as the bigger white blood cells stretch and elongate to pass through the capillaries. You may also see a dark tail with the dot of light, which is a bunch-up of red blood cells behind the slower-moving white blood cell.
These “blue-sky sprites” normally disappear after a second or less. Their movement may seem squiggly, following the path of the capillaries in your eyes. And their speed varies in time with your pulse, accelerating with your heartbeat.
Don’t confuse the normal blue field entoptic phenomenon (BFEP) with floaters or flashes—which can interfere with your vision and signal a serious eye problem. BFEP lights are all the same size and shape. If the eye stops moving, the spots keep whizzing around.
In contrast, floaters, which are clumps in your vitreous, the gel-like fluid in your eye, differ in size and brightness. If the eye stops moving, the floaters settle down. If the eye moves, the floaters follow more slowly.
Flashes look like lightning streaks or stars, can last for 10 to 20 minutes and may come and go for several weeks or months. If you notice a change in your vision or have a sudden onset of flashes or floaters, call an ophthalmologist right away. New or increased flashes or floaters can be a sign of a retinal detachment.