The blue field entoptic phenomenon or Scheerer's phenomenon is the appearance of tiny bright dots moving quickly along squiggly lines in the visual field, especially when looking into blue light (such as the sky). These dots are due to the white blood cells that move in the capillaries in front of the retina of the eye, near the macula. The dots move somewhat in sync with the heart beat.

Blue light (optimal wavelength: 430 nm) is well absorbed by the red blood cells that fill the capillaries. The brain "edits out" the dark lines that would result from this absorption. The white blood cells, which are much rarer than the red ones and don't absorb the blue light well, create gaps in the blood column, and these gaps appear as bright dots.

In a technique known as blue field entoptoscopy, the effect is used to measure the blood flow in the retinal capillaries, which is important in diseases such as diabetes. The patient is alternatingly shown blue light and a computer generated picture of moving dots; by adjusting the speed and density of the computer generated dots, the patient tries to match the computer generated picture as best as possible to the perceived entoptic dots. This then allows calculation of the blood flow in the capillaries.

Scheerer's phenomenon should not be confused with "floaters" or muscae volitantes, which are larger and darker structures that usually move slower and along straight lines; they are due to debris floating in the vitreous humor of the eye.

References

• Scheerer R., Die entoptische Sichtbarkeit der Blutbewegungen im Auge und ihre klinische Bedeutung. Klinisches Monatsblatt Augenheilkunde 1924;73:67-107
• Sinclair et al. "Investigation of the source of the blue field entoptic phenomenon.", Investigative Ophthalmology & Visual Science. April 1989;30(4):668-673