Phosphenes

Visual sensations, such as flashes of light, that are not caused by an external source of light entering the eye, but arise as a result of mechanical, electrical or magnetic stimulation of the eyeball.

They don't impair vision and they are transitory.

They can occur for instance when a person is moving in a strong static magnetic field.

Source: GreenFacts

GreenFacts Summary on Static Fields:

• 4.2 What determines the strength of static magnetic fields inside the body?

Static Fields