Why does this happen? Blue light 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 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. This is called the blue field entoptic phenomenon. 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. This phenomenon is used to measure blood flow in the capillaries of the retina, known as Blue field entoptoscope.
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