speckle

speckle [′spek·əl]

(optics)

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.

---

Speckle

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