Laser speckle also known as eye testing using 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 Speckle, speckle pattern, or speckle noise is a granular noise texture degrading the quality as a consequence of interference among wavefronts in coherent imaging systems, such as radar, synthetic aperture radar (SAR), medical ultrasound and optic ...

is formed on the

retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then ...

. 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 Near-sightedness, also known as myopia and short-sightedness, is an eye disease where light focuses in front of, instead of on, the retina. As a result, distant objects appear blurry while close objects appear normal. Other symptoms may include ...

, 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 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. Due to foreshortening, nearby objects ...

; 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 Far-sightedness, also known as long-sightedness, hypermetropia, or hyperopia, is a condition of the Human eye, eye where distant objects are seen clearly but near objects appear blurred. This blurred effect is due to incoming light being focused ...

, 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.

References

Ophthalmology {{Ophthalmology-stub