Eye testing using speckle

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

The speckle interference pattern forms on your retinas regardless of where your eyes are actually focused. This is in contrast to the wall or screen, from which the laser is scattering, which will only be in focus if the lenses in your eyes are adjusted correctly. Usually, people are either near- or far-sighted. In the former case, the image of the background will be focused slightly in front of the retina while in the latter case the background will be focused behind the retina.

With imperfect vision, transverse motion of the head will cause the speckles to appear to move relative to the background. If you are near-sighted, the speckles appear to move in a direction opposite to the motion of your head. If far-sighted the speckles appear to move in the same direction. The worse your 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.

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