Eye testing using speckle

Laser speckle can be employed as a method for conducting a very sensitive eye test .

When a surface which is illuminated by a laser beam is viewed by an observer, a speckle interference pattern is formed on the retina. If the observer suffers from defective vision, the image of the surface is not formed on the retina. In the case of near-sighted people, it is formed in front of the retina, while for far-sighted people, it is focused behind the retina.

Motion of the head will cause the speckles to appear to move relative to the background. Near-sighted observers will perceive the speckles moving in a direction opposite to the motion of the head while the far-sighted will perceive the speckles to move in the same direction as the head. The more defective the vision, the faster the apparent motion. With perfect vision, the speckles do not move.

This technique is so sensitive that it can be used to determine changes in the ability of someone to focus through the day.