Speckle imaging

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

Extraction of phase field from a single contoured correlation fringe pattern of ESPI.

Abstract: Speckle fringe patterns of ESPI are full of high-level speckle noise and normally are processed by phase shifting methods that require multi speckle fringe patterns. We propose a novel method to generate a speckle-noise-free fringe pattern from a single speckle fringe pattern and to extract the phase field from the new pattern. With the new method, the correlation between two speckle patterns is performed only within contour windows instead of rectangular windows and this contoured correlation results in a smooth, normalized fringe pattern without speckle noise. The new ESPI fringe patterns are speckle-noise-free and of comparable quality to that of moire and hologram, which is unimaginable with traditional ESPI methods. In addition to the smoothness, the resultant fringe pattern is normalized automatically so that the full phase field can be extracted from this single fringe pattern by the single-image phase-shifting method.

Decoherence of fiber supercontinuum light source for speckle-free imaging.

Abstract: Speckle-free imaging is attractive in laser-illuminated imaging systems. The evolutionary process of supercontinuum decoherence in extra-large mode area step-index multimode fiber is analyzed to provide high-quality broadband light source for speckle-free imaging. It is found that spectral bandwidth, number of spatial transverse modes, and decoherence among different modes all greatly contribute to speckle reduction. The combination of supercontinuum and extra-large mode area step-index multimode fiber can considerably increase the efficiency of decoherence process for speckle-free imaging. This work may enrich the research of speckle-free imaging and also provide guidance on speckle-free imaging using fiber-optics based light source.

Roughness parameters and surface deformation measured by coherence radar

Abstract: The 'coherence radar' was introduced as a method to measure the topology of optically rough surfaces. The basic principle is white light interferometry in individual speckles. We will discuss the potentials and limitations of the coherence radar to measure the microtopology, the roughness parameters, and the out of plane deformation of smooth and rough object surfaces. We have to distinguish objects with optically smooth (polished) surfaces and with optically rough surfaces. Measurements at polished surfaces with simple shapes (flats, spheres) are the domain of classical interferometry. We demonstrate new methods to evaluate white light interferograms and compare them to the standard Fourier evaluation. We achieve standard deviations of the measured signals of a few nanometers. We further demonstrate that we can determine the roughness parameters of a surface by the coherence radar. We use principally two approaches: with very high aperture the surface topology is laterally resolved. From the data we determine the roughness parameters according to standardized evaluation procedures, and compare them with mechanically acquired data. The second approach is by low aperture observation (unresolved topology). Here the coherence radar supplies a statistical distance signal from which we can determine the standard deviation of the surface height variations. We will further discuss a new method to measure the deformation of optically rough surfaces, based on the coherence radar. Unless than with standard speckle interferometry, the new method displays absolute deformation. For small out-of-plane deformation (correlated speckle), the potential sensitivity is in the nanometer regime. Large deformations (uncorrelated speckle) can be measured with an uncertainty equal to the surface roughness.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Effects of luminance and spatial noise on interferometric contrast sensitivity

Abstract: Optical properties of the eye contribute to the reduced visibility of spatial patterns at low luminance. To study the limits of spatial vision when optical factors are minimized, we measured contrast-sensitivity functions (CSF’s) for 543.5-nm laser interference fringes imaged directly on the retina. Measurements were made in the fovea at four luminance levels, ranging from 0.3 to 300 photopic trolands (Td). At each luminance the fraction of coherent light in the stimulus pattern was varied to assess the masking effects of laser speckle, which is visible as spatial noise in fields of coherent light. Compared with published CSF’s obtained under natural viewing conditions, interferometric CSF’s were similar in height but broader, with the range of visibility being extended to higher spatial frequencies. The masking effects of speckle were greatest at the highest luminance and were negligible at the lowest luminance. For low coherent fractions, contrast sensitivity improved over the entire luminance range at a rate consistent with a square-root law; with purely coherent light, sensitivity tended to level off at approximately 30 Td because of speckle masking. The results indicate that the optical quality of the eye reduces the spatial bandwidth of vision even at luminances near the foveal threshold. The change in interference fringe visibility with luminance is consistent with noise-limited behavior, and the masking effects of speckle noise diminish as luminance decreases.

Real-time simultaneous single snapshot of optical properties and blood flow using coherent spatial frequency domain imaging (cSFDI)

Abstract: In this work we present and validate a wide-field method for the real-time mapping of tissue absorption, scattering and blood flow properties over wide regions of tissue (15 cm x 15 cm) with high temporal resolution (50 frames per second). We achieve this by applying Fourier Domain demodulation techniques to coherent spatial frequency domain imaging to extract optical properties and speckle flow index from a single snapshot. Applying this technique to forearm reactive hyperemia protocols demonstrates the ability to resolve intrinsic physiological signals such as the heart beat waveform and the buildup of deoxyhemoglobin associated with oxygen consumption.