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 pattern is formed on the retina. If the observer has perfect vision, the image of the surface is also formed on the retina, and movement of the head will result in the speckle pattern and the surface moving together so that the speckle pattern remains sataionary with respect to the background.

If the observer is near-sighted the image of the surface is formed in front of the retina. Since the speckle pattern is perceived by the brain to be on the retina, the effect if of [parallax]]; the speckle patterns appears to be nearer than the surface and hence moves in the opposite direction to the surface.

If the observer is far-sighted, the speckles apprea to move in the same direction as the head has moves, since in thsi case the surface image is focused behind the retina.

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