Speckle (interference)

Speckle, speckle pattern, or speckle noise designates the granular structure observed in coherent light, resulting from random interference. Speckle patterns are used in a wide range of metrology techniques, as they generally allow high sensitivity and simple setups. They can also be a limiting factor in imaging systems, such as radar, synthetic aperture radar (SAR), medical ultrasound and optical coherence tomography.[1][2][3][4] Speckle is not external noise; rather, it is an inherent fluctuation in diffuse reflections, because the scatterers are not identical for each cell, and the coherent illumination wave is highly sensitive to small variations in phase changes.[5]

Speckle patterns arise when coherent light is randomised. The simplest case of such randomisation is when light reflects off an optically rough surface. Optically rough means that the surface profile contains fluctuations larger than the wavelength. Most common surfaces are rough to visible light, such as paper, wood, or paint.

The vast majority of surfaces, synthetic or natural, are extremely rough on the scale of the wavelength. We see the origin of this phenomenon if we model our reflectivity function as an array of scatterers. Because of the finite resolution, at any time we are receiving from a distribution of scatterers within the resolution cell. These scattered signals add coherently; that is, they add constructively and destructively depending on the relative phases of each scattered waveform. Speckle results from these patterns of constructive and destructive interference shown as bright and dark dots in the image.[6]

Speckle in conventional radar increases the mean grey level of a local area.[7] Speckle in SAR is generally serious, causing difficulties for image interpretation.[7][8] It is caused by coherent processing of backscattered signals from multiple distributed targets. In SAR oceanography, for example, speckle is caused by signals from elementary scatterers, the gravity-capillary ripples, and manifests as a pedestal image, beneath the image of the sea waves.[9][10]

The speckle can also represent some useful information, particularly when it is linked to the laser speckle and to the dynamic speckle phenomenon, where the changes of the spatial speckle pattern over time can be used as a measurement of the surface's activity, such as which is useful for measuring displacement fields via digital image correlation.

  1. ^ Dainty, C., ed. (1984). Laser Speckle and Related Phenomena (2nd ed.). Springer-Verlag. ISBN 978-0-387-13169-6.
  2. ^ Goodman, J. W. (1976). "Some fundamental properties of speckle". JOSA. 66 (11): 1145–1150. Bibcode:1976JOSA...66.1145G. doi:10.1364/josa.66.001145.
  3. ^ Hua, Tao; Xie, Huimin; Wang, Simon; Hu, Zhenxing; Chen, Pengwan; Zhang, Qingming (2011). "Evaluation of the quality of a speckle pattern in the digital image correlation method by mean subset fluctuation". Optics & Laser Technology. 43 (1): 9–13. Bibcode:2011OptLT..43....9H. doi:10.1016/j.optlastec.2010.04.010.
  4. ^ Lecompte, D.; Smits, A.; Bossuyt, Sven; Sol, H.; Vantomme, J.; Hemelrijck, D. Van; Habraken, A.M. (2006). "Quality assessment of speckle patterns for digital image correlation". Optics and Lasers in Engineering. 44 (11): 1132–1145. Bibcode:2006OptLE..44.1132L. doi:10.1016/j.optlaseng.2005.10.004. hdl:2268/15779.
  5. ^ Moreira, Alberto; Prats-Iraola, Pau; Younis, Marwan; Krieger, Gerhard; Hajnsek, Irena; Papathanassiou, Konstantinos P. (2013). "A Tutorial on Synthetic Aperture Radar" (PDF). IEEE Geoscience and Remote Sensing Magazine. 1: 6–43. Bibcode:2013IGRSM...1a...6M. doi:10.1109/MGRS.2013.2248301. S2CID 7487291.
  6. ^ M. Forouzanfar and H. Abrishami-Moghaddam, Ultrasound Speckle Reduction in the Complex Wavelet Domain, in Principles of Waveform Diversity and Design, M. Wicks, E. Mokole, S. Blunt, R. Schneible, and V. Amuso (eds.), SciTech Publishing, 2010, Section B - Part V: Remote Sensing, pp. 558-77.
  7. ^ a b Brandt Tso & Paul Mather (2009). Classification Methods for Remotely Sensed Data (2nd ed.). CRC Press. pp. 37–38. ISBN 9781420090727.
  8. ^ Giorgio Franceschetti & Riccardo Lanari (1999). Synthetic aperture radar processing. Electronic engineering systems series. CRC Press. pp. 145 et seq. ISBN 9780849378997.
  9. ^ Mikhail B. Kanevsky (2008). Radar imaging of the ocean waves. Elsevier. p. 138. ISBN 9780444532091.
  10. ^ Alexander Ya Pasmurov & Julius S. Zinoviev (2005). Radar imaging and holography. IEE radar, sonar and navigation series. Vol. 19. IET. p. 175. ISBN 9780863415029.