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Surface Roughness Evaluation
Published in Rajpal S. Sirohi, Speckle Metrology, 2020
Speckle can be regarded as an interference pattern produced by coherent light reflected from different parts of the illuminated surface. If the surface is rough compared with the wavelength of the light, rays reflected from different parts of the surface within a resolution cell (the size of which is determined by the limiting aperture of the observing system) will traverse different optical path lengths in reaching the eye of the observer (or the observing screen in the case of far-field speckle), and the resulting intensity at a given point on the object (or on the observing screen) will be determined by the coherent addition of the complex amplitudes associated with each of these rays. If the resultant amplitude is zero, a dark “speckle” will be seen at the point, whereas if all the rays arrive at the point in phase, an intensity maximum will be observed.
Speckle Metrology
Published in Rajpal S. Sirohi, Optical Methods of Measurement, 2018
The speckle phenomenon itself is essentially an interference phenomenon. However, when a reference beam is added to the speckle pattern to code its phase, the technique is then termed speckle interferometry. Speckle interferometry was first applied to measure in-plane displacement by Leendertz. The sensitivity of the measurement could be increased over that of speckle photography, which was limited by the speckle size. The basic theory was borrowed from holographic interferometry, since the phase difference introduced by deformation is governed by the same equation, namely, δ = (k2 – k1) · d. When the object is illuminated with two beams with directions symmetric with respect to the object normal (also the optical axis) and observation is made along the optical axis, the arrangement generates fringes that are contours of constant in-plane displacement. The fringes are called correlation fringes—the reason for this name will become obvious in later sections.
Laser and LED systems for industrial metrology and spectroscopy for industrial uses
Published in P. Dakin John, G. W. Brown Robert, Handbook of Optoelectronics, 2017
This is a method for determining movement of objects from the changes in the optical “speckle” pattern observed when they are illuminated by a coherent laser. The speckle is due to patterns of constructive (bright regions) or destructive (dark regions) interference, when an object is illuminated by laser light from a diffusely scattered screen. The seemingly random changes in the pattern when an object moves can be detected by cameras and processed to provide information on dimensional changes. The changing pattern gives an indication of stress or strain in mechanical systems. Extremely small dimensional changes, of order less than the wavelength of the light, cause dramatic changes in intensity of the pattern, which can be processed to recover information on the nature of the stresses and strains. Apart from the analytical data, the pictorial information provided can form a very valuable 2D image of static or dynamic distortion of the surface of structures, for example, a strained mechanical part, a medical prosthesis, or the outside surface of a whole automobile or aircraft engine.
Simulation-based verification of the shape measurement mechanism of micro structures beyond the diffraction limit using speckle interferometry
Published in Journal of Modern Optics, 2022
A speckle is a grainy interference fringe that depends on the aperture size of the observation optics which is produced when highly interfering light is irradiated on a rough surface, and the scattered light from the object surfaces interferes with each other [1–3]. In interferometry, speckle has historically been regarded as a type of noise element [4], and research has been actively performed to suppress the effects of speckle noise [5–9]. By developing filtering techniques for noise suppression [9], the usefulness of speckle interferometry is increasing in the field of engineering [10–15] using high-resolution optical techniques, such as the fringe scanning technique [3], to effectively analyse the phase distribution recorded in the speckle. As a result, speckle interferometry has been developed as a technique which can measure the deformation and displacement of objects with rough surfaces with a resolution of tens of nanometres [16,17]. Furthermore, it has been reported that three-dimensional (3D) shape measurement of microstructures is possible by detecting the phase of light recorded in the speckle with high resolution [18].
Automated surface water detection from space: a Canada-wide, open-source, automated, near-real time solution
Published in Canadian Water Resources Journal / Revue canadienne des ressources hydriques, 2020
Koreen Millard, Nicholas Brown, Douglas Stiff, Alain Pietroniro
Next, raw DN values are converted to calibrated amplitude values using gains provided in the SigmaNought (σ0) Look-Up Table (LUT) which is unique to each dataset. Speckle, due to the coherent interference of waves reflected from many elementary scatterers, is then filtered (Lee et al., 1999). In an effort to maintain an open source solution, a 5 × 5 Enhanced Lee Filter was implemented using GDAL libraries (GDAL/OGR, 2019). The image was then orthorectified using GDAL to correct any terrain distortions and the Canadian Digital Elevation Model (CDEM, 2015). Finally, following Bolanos et al. (2016), a texture measure was created for each polarization to identify areas of high homogeneity. For this, the energy metric (Dekker, 2003) was used with a 5 × 5 window: where E is Energy, x is the DN value and i, j are row, column.
Hybrid method combining orthogonal projection Fourier transform profilometry and laser speckle imaging for 3D visualization of flow profile
Published in Journal of Modern Optics, 2020
Ori Izhak Rosenberg, David Abookasis
Laser speckle imaging detects dynamic blood flow changes within the medium by analyzing the random interference patterns resulting from scattered laser light with different path lengths [15-17]. When a surface illuminated by coherent light (i.e. laser) is imaged by a camera, a random interference pattern known as a granule or speckle is formed. The produced speckle pattern can be analyzed to evaluate flow by simple statistical methods. The size of an individual speckle is not determined by the structures of the surface producing it or by the scattering center's distribution. It is determined by the f-number (F#) of the optical system (lens aperture) used to observe the speckle pattern [24]. The relationship between the speckle size (δ) and F# of the lens system is given by [16,20], where M is the optical magnification and λ is the wavelength of the laser source. Note that: (1) by controlling the F# of the lens system the optimal speckle size can be chosen and (2) based on Nyquist criterion, δ should be at least two times larger than the camera pixel size. If the scattering particles are in motion, a time-varying speckle pattern is generated at each pixel of the image and by analyzing the temporal or spatial intensity variations of this pattern, information about the movement can be estimated. The speckle pattern recorded by the camera is blurred in a manner dependent on the movement of scattering particles. The blurring can be expressed quantitatively as speckle contrast by a simple statistical calculation: