Entoptic phenomenon

Entoptic phenomena are visual effects whose source is within the eye itself. Occasionaly these are referred to as Entopic phenomena. In Helmholtz's words,

Under suitable conditions light falling on the eye may render visible certain objects within the eye itself. These perceptions are called entoptical.

When you are viewing entoptical effects, there is no way to show anyone else exactly what you are seeing, which gives them a certain mysterious and personal quality.

They are different from optical illusions, which are perceptual effects occurring within the brain. Most entopical phenomena have a direct, known physical cause.

Some examples of entoptical effects include the following:

Floaters or muscae volitantes are slowly drifting transparent blobs of varying size and shape, which are particularly noticeable when lying on the ground looking up at the sky. They are caused by imperfections in the fluid of the eye.

The blue field entoptic phenomenon has the appearance of tiny bright dots moving rapidly along squiggly lines in the visual field. It is much more noticeable when viewed against a field of pure blue light and is caused by white blood cells moving in the capillaries in front of the retina.

Haidinger's brush is a very subtle yellow-and-blue pattern that is seen when viewing a field of light that is polarized.

The Vascular figure is an image of the blood vessels in one's own eye. It can be seen by shining a bright light like a penlight onto the sclera (the white of the eye) in a darkened room. This figure is often seen by patients during an optometric exam when the doctor is using an ophthalmoscope. In the process of aligning the instrument so that he can view the blood vessels through the pupil, the light from the instrument often falls briefly on the sclera, so that the patient gets a quick glimpse of the vascular figure.