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The generation of a random intensity distribution, called a speckle pattern, when light from a highly coherent source, such as a laser, is scattered by a rough surface or inhomogeneous medium. See Laser

The surfaces of most materials are extremely rough on the scale of an optical wavelength (approximately 5 × 10-7 m). When nearly monochromatic light is reflected from such a surface, the optical wave resulting at any moderately distant point consists of many coherent wavelets, each arising from a different microscopic element of the surface. Since the distances traveled by these various wavelets may differ by several wavelengths if the surface is truly rough, the interference of the wavelets of various phases results in the granular pattern of intensity called speckle. If a surface is imaged with a perfectly corrected optical system, diffraction causes a spread of the light at an image point, so that the intensity at a given image point results from the coherent addition of contributions from many independent surface areas. As long as the diffraction-limited point-spread function of the imaging system is broad by comparison with the microscopic surface variations, many dephased coherent contributions add at each image point to give a speckle pattern.

The basic random interference phenomenon underlying laser speckle exists for sources other than lasers. For example, it explains radar “clutter,” results for scattering of x-rays by liquids, and electron scattering by amorphous carbon films. Speckle theory also explains why twinkling may be observed for stars, but not for planets. See Coherence, Diffraction, Interference of waves

In metrology, the most obvious application of speckle is to the measurement of surface roughness. If a speckle pattern is produced by coherent light incident on a rough surface, then surely the speckle pattern, or at least the statistics of the speckle pattern, must depend upon the detailed surface properties. An application of growing importance in engineering is the use of speckle patterns in the study of object displacements, vibration, and distortion that arise in nondestructive testing of mechanical components.

Astronomical speckle interferometry is a technique for obtaining spatial information on astronomical objects at the diffraction-limited resolution of a telescope, despite the presence of atmospheric turbulence. Speckle interferometry techniques have proven to be an invaluable tool for astronomical research, allowing studies of a wide range of scientifically interesting problems. They have been widely used to determine the separation and position angle of binary stars, and for accurate diameter measurements of a large number of stars, planets, and asteroids. Speckle imaging techniques have successfully uncovered details in the morphology of a range of astronomical objects, including the Sun, planets, asteroids, cool giants and supergiants, young stellar objects, the supernova SN1987A in the Large Magellanic Cloud, Seyfert galaxies, and quasars. See Interferometry


A phenomenon in which the scattering of light from a highly coherent source, such as a laser, by a rough surface or inhomogeneous medium generates a random-intensity distribution of light that gives the surface or medium a granular appearance.
References in classic literature ?
On the speckled side of his face he has no eyebrow, and on the other side he has a bushy black one, which want of uniformity gives him a very singular and rather sinister appearance.
Joe even came back, from securing the anchor in the tree, speckled with bites, but he kept his temper, because he found it quite the natural thing for mosquitoes to treat him as they had done.
If Denham could have seen how visibly books of algebraic symbols, pages all speckled with dots and dashes and twisted bars, came before her eyes as they trod the Embankment, his secret joy in her attention might have been dispersed.
I've just composed a tune called 'The Speckled Alligator.
High up in the steeple, where the belfry is, and iron rails are ragged with rust, and sheets of lead and copper, shrivelled by the changing weather, crackle and heave beneath the unaccustomed tread; and birds stuff shabby nests into corners of old oaken joists and beams; and dust grows old and grey; and speckled spiders, indolent and fat with long security, swing idly to and fro in the vibration of the bells, and never loose their hold upon their thread-spun castles in the air, or climb up sailor-like in quick alarm, or drop upon the ground and ply a score of nimble legs to save one life
He gave me his books, and his dog, and his speckled hens, and his big knife, and said, 'Good-by, Polly,' and kissed me the last thing and then O Jimmy
Mother Atkinson, as all called their hostess, was the merriest there, and the busiest; for she kept flying up to wait on the children, to bring out some new dish, or to banish the live stock, who were of such a social turn that the colt came into the entry and demanded sugar; the cats sat about in people's laps, winking suggestively at the food; and speckled hens cleared the kitchen floor of crumbs, as they joined in the chat with a cheerful clucking.
Poiret's eyes were dim, his glance weak and lifeless, his skin discolored and wrinkled, gray in tone and speckled with bluish dots; his nose flat, his lips drawn inward to the mouth, where a few defective teeth still lingered.
They paused, however, at the bridge, and, leaning their elbows upon the stonework, they stood looking down at their own faces in the glassy stream, and at the swift flash of speckled trout against the tawny gravel.
But suddenly the bunch of feathers stopped whirling, and then, to her amazement, the girl saw Billina crouching upon the prostrate form of a speckled rooster.
Thus said the hawk to the nightingale with speckled neck, while he carried her high up among the clouds, gripped fast in his talons, and she, pierced by his crooked talons, cried pitifully.
It lives on the bare sand near the sea coast, and from its mottled colour, the brownish scales being speckled with white, yellowish red, and dirty blue, can hardly be distinguished from the surrounding surface.