Speckle imaging

About: Speckle imaging is a research topic. Over the lifetime, 3730 publications have been published within this topic receiving 62354 citations.

Computer-aided speckle interferometry using spectral amplitude fringes

Abstract: A fully automatic speckle metrology technique is developed. Two speckle patterns of a specimen, one before and one after the specimen deformation, are captured by a video camera. An equivalent double-exposure speckle pattern is obtained by superimposing the two digital images. The superimposed speckle pattern is then segmented into a series of small subimages. For each subimage a fast-Fourier transform is applied and a computer-generated Young’s fringe pattern is obtained. The fringe pattern, which characterizes the local displacement vector, is analyzed by a second fast-Fourier transform. The local displacement vector is determined by a cardinal interpolation and a crest searching around a signal peak in the second spectral domain. An artificial rigid shift between the two images is introduced in the cases of extremely large or small displacements. From analysis of all subimage pairs of the whole superimposed speckle pattern a complete two-dimensional displacement field is deduced. Experimental results using laser as well as white-light speckle patterns are demonstrated.

Limited degree-of-freedom adaptive optics and image reconstruction.

Abstract: The use of limited degree-of-freedom adaptive optics in conjunction with statistical averaging and a linear image reconstruction algorithm is addressed. Image reconstruction is traded for full predetection compensation. It is shown through analytic calculations that the average optical transfer function (OTF) is significant for high spatial frequencies in the case of imaging through atmospheric turbulence with an adaptive optics system composed of a Hartmann-type wave-front sensor and a deformable mirror possessing far fewer actuators than one per atmospheric coherence diameter (r(0)). Statistical averaging is used to overcome the effects of measurement noise and randomness in individual realizations of the OTF. The imaging concept and signal-to-noise considerations are presented.

Effects of speckle on resolution

Abstract: The effect of speckle in the imaging of diffusely illuminated gratings and continuous-tone objects was studied. It was found that when imaging diffuse gratings, the aperture of a coherently illuminated system must be 2.6 times as large as that of an incoherent system to obtain comparable resolution. This factor must be increased to five when imaging continuous-tone objects and to a factor of seven if the coherent image is subsequently low-pass filtered. A coherent system can achieve resolution which is comparable to an incoherent system of equal aperture if the coherent image is smoothed so that the mean to standard deviation ratio is ten or more.

Speckle-free rear-projection screen using two close screens in slow relative motion

Abstract: Speckle in rear-projection screens, such as those used in microfiche viewers, reduces resolution and can cause viewer fatigue. Many speckle-reduction techniques have been tried with varying degrees of success, including rapid movement of a single screen in its plane, the use of two stationary spaced screens, liquid-crystal screens, and the use of two closely spaced screens in slow relative orbital motion. We describe a simplified, two-screen support that permits only linear screen motion, and show that such motion provides total speckle elimination. An analysis of speckle in such screens is presented which accounts for the observed remarkable sensitivity of the speckle pattern to minute screen displacements. To confirm this analysis we have measured the speckle pattern’s correlation as a function of screen displacement using the speckle interferometry method of Francon. We consider the effects of a spatially and spectrally extended incandescent light source, and conclude that these effects are negligible in our application compared to the effect of screen motion. We show that these three speckle-reducing effects (screen motion, spatial extent of the source, and spectral extent of the source) can be considered together by introducing the notion of space-time correlation cells.

Triple-correlation subplane reconstruction of photon-address stellar images.

Abstract: The performance of a photon-address, subplane implementation of the triple-correlation (TC) algorithm is evaluated for application to near-real-time, stellar speckle imaging at low-light levels. A simple least-squares relaxation algorithm for recovering object phase from the bispectrum is proposed and found to be consistently better than the usual recursive method. Photon-address speckle data from six simulated objects of different degrees of complexity, and from the binary stars β Del and μ Ori, were used in this study. For real-time applications for which computational efficiency is critical, the relaxed two-plane TC algorithm offers excellent performance and rugged-ness with respect to object complexity.