Blue field entoptic phenomenon
The blue field entoptic phenomenon or Scheerer's phenomenon (after the German ophthalmologist Richard Scheerer, who first drew clinical attention to it in 1924) is the appearance of tiny bright dots (nicknamed blue-sky sprites) moving quickly along squiggly lines in the visual field, especially when looking into bright blue light such as the sky. The dots are short-lived, visible for a second or less, and traveling short distances along seemingly random, curvy paths. Some of them follow the same path as predecessors. The dots may be elongated along the path like tiny worms. The dots appear in the central field of view, within 10 to 15 degrees from the fixation point. The left and right eye see different dots; someone looking with both eyes sees a mixture.
Most people are able to see this phenomenon. However, it is rather weak, and many people don’t notice it until asked to pay attention.
The dots are white blood cells moving in the capillaries in front of the retina of the eye. Blue light (optimal wavelength: 430 nm) is 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, create gaps in the blood column, and these gaps appear as bright dots. The gaps are elongated because a spherical leukocyte is too wide for the capillary. Red blood cells pile up behind the leukocyte, showing up like a dark tail. This behavior of the blood cells in the capillaries of the retina has been observed directly in human subjects by adaptive optics scanning laser ophthalmoscopy, a new real time imaging technique for examing retinal blood flow. The dots won’t appear at the very centre of the visual field, because there are no blood vessels there (foveal avascular zone).
Blue field entoptoscopy 
In a technique known as blue field entoptoscopy, the effect is used to estimate 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.
Difference from other entoptic phenomena 
Scheerer’s phenomenon can be easily distinguished from floaters (muscae volitantes). Scheerer’s phenomenon consists of corpuscles of identical diameter and visual sharpness, of a simple dot or worm-like shape, brighter than the background. If the eye stops moving, the dots keep darting around. If the eye moves, the dots follow instantaneously, because they are contained in the retina. In contrast, floaters are specks or threads of variable diameter and variable visual sharpness, some of complex shape, darker than the background. If the eye stops moving, the floaters settle down. If the eye moves, the floaters follow sluggishly, because they are contained in the vitreous humor which is subject to inertia. There are also several phosphenes that need to be distinguished from Scheerer’s phenomenon.
See also 
- Blom, Jan Dirk (2010), A Dictionary of Hallucinations, Springer (Blue-Field Entoptic Phenomenon entry)
- Scheerer, Richard (1924), "Die entoptische Sichtbarkeit der Blutbewegungen im Auge und ihre klinische Bedeutung", Klinische Monatsblätter für Augenheilkunde (in German) 73: 67–107
- Hafez, Ali (2009). "Blue Field Entoptic Simulation". In R.N. Weinreb. Ocular Blood Flow in Glaucoma. Kugler Publications. p. 27.
- Sinclair, S.H., Azar-Cavanagh, M., Soper, K.A., Tuma, R.F., & Mayrovitz, H.N. (1989). "Investigation of the source of the blue field entoptic phenomenon.". Investigative Ophthalmology & Visual Science 30 (4): 668–673. PMID 2703307.
- Snodderly, D.M., Weinhaus, R.S., & Choi, J.C. (1992). "Neural-vascular relationships in central retina of Macaque monkeys (Macaca fascicularis)". Journal of Neuroscience 12 (4): 1169–1193.
- Akihito Uji (2012). "The Source of Moving Particles in Parafoveal Capillaries Detected by Adaptive Optics Scanning Laser Ophthalmoscopy". Investigative Ophthalmology & Visual Science 53 (1): 171–178.