Eye testing using speckle
Encyclopedia
Laser speckle can be employed as a method for conducting a very sensitive eye test.
When a surface is illuminated by a laser beam and 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 stationary 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 is of parallax
; the speckle pattern appears to be nearer to the eye than the surface and hence moves in the same direction as the surface, but faster than the surface. If the observer is far-sighted
, the speckles appear to move in the opposite direction as the surface, since in this case the surface image is focused behind the retina. The apparent speed of motion of the speckles increases with the magnitude of the defect of the eye.
This technique is so sensitive that it can be used to determine changes in the ability of someone to focus through the day.
When a surface is illuminated by a laser beam and is viewed by an observer, a speckle pattern
Speckle pattern
A speckle pattern is a random intensity pattern produced by the mutual interference of a set of wavefronts. This phenomenon has been investigated by scientists since the time of Newton, but speckles have come into prominence since the invention of the laser and have now found a variety of...
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 stationary with respect to the background.
If the observer is near-sighted
Myopia
Myopia , "shortsightedness" ) is a refractive defect of the eye in which collimated light produces image focus in front of the retina under conditions of accommodation. In simpler terms, myopia is a condition of the eye where the light that comes in does not directly focus on the retina but in...
, 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 is of parallax
Parallax
Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. The term is derived from the Greek παράλλαξις , meaning "alteration"...
; the speckle pattern appears to be nearer to the eye than the surface and hence moves in the same direction as the surface, but faster than the surface. If the observer is far-sighted
Hyperopia
Hyperopia, also known as farsightedness, longsightedness or hypermetropia, is a defect of vision caused by an imperfection in the eye , causing difficulty focusing on near objects, and in extreme cases causing a sufferer to be unable to focus on objects at any distance...
, the speckles appear to move in the opposite direction as the surface, since in this case the surface image is focused behind the retina. The apparent speed of motion of the speckles increases with the magnitude of the defect of the eye.
This technique is so sensitive that it can be used to determine changes in the ability of someone to focus through the day.