"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 bright blue light (such as the sky).^[1] This is a normal effect that can be perceived by almost everybody. The dots are due to the white blood cells that move in the capillaries in front of the retina of the eye, near the macula.

Blue light (optimal wavelength: 430 nm) is well absorbed by the red blood cells that fill the capillaries. The eye "edits out" the shadow lines of the capillaries 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 the 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.

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."