The blue field entoptic phenomenon or Scheerer's phenomenon (after the German ophthalmologist Richard Scheerer, who first drew clinical attention to it in 1924[1]) is the appearance of tiny bright dots (called "blue-sky sprites") moving quickly along squiggly lines in the visual field, especially when looking into bright blue light (such as the sky).[2] This is a normal effect that can be perceived by almost everybody. The dots are white blood cells in the capillaries in front of the retina of the eye, near the macula.[3]
Blue light (optimal wavelength: 430 nm) is well absorbed by the red blood cells that fill the capillaries. The eye and brain "edit out" the shadow lines of the capillaries, partially by dark adaptation of the photoreceptors lying beneath the capillaries. The white blood cells, which are much rarer than the red ones and do not absorb blue light well, create gaps in the blood column, and these gaps appear as bright dots. They won't appear at the very center of the visual field, because there are no blood vessels in the fovea. The effect is rather weak, and many people don't notice it at all. It is strongest when looking at a smooth blue surface. Some times worm like objects can also be seen floating in front of the visual.
In a technique known as blue field entoptoscopy, the effect is used to measure the blood flow in the retinal capillaries. The patient is alternatingly shown blue light and a computer generated picture of moving dots; by adjusting the speed and density of these dots, the patient tries to match the computer generated picture as well as possible to the perceived entoptic dots. This then allows calculation of the blood flow in the capillaries. This test is important in diseases such as diabetes which can cause retinopathy.
Scheerer's phenomenon should not be confused with "floaters" (muscae volitantes). Scheerer's phenomenon is distinguished by the appearance of multiple, identical-looking bright dots that move around rapidly in random directions. Floaters are variable in appearance; although they sometimes are dots, they often have the appearance of threads or shreds of crumpled cellophane. Floaters remain almost stationary or drift slowly and do not follow well-defined paths. They are due to debris floating in the vitreous humor of the eye. There are also several phosphenes that need to be distinguished from Scheerer's phenomenon.
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