Igor Markhvida

Influence of geometry on polychromatic speckle contrast

Understanding speckle behavior is very important in speckle metrology application. The contrast of a polychromatic speckle depends not only on surface roughness and the coherence length of a light source, as shown in previous works, but also on optical geometry. We applied the Fresnel approach of diffraction theory for the free-space geometry and derived a simple analytical relationship between contrast, coherence length, size of illuminated spot, and distances between source, object, and observation plane. The effect of contrast reduction is found to be significant for low-coherence light sources.

Using a zone model to incorporate the influence of geometry on polychromatic speckle contrast

A simple physical zone model was developed to explain the formation of polychromatic speckle patterns within the Fresnel region. This model represents a reasonable compromise between complex theoretical formulation and simple estimations for practical needs, and allows the speckle contrast to be calculated as a function of geometric parameters for the optics and coherence length of the light source. The model was experimentally verified, and the results are consistent with our previous rigorous theoretical formulation.

Eliminating the effect of bulk scattering when measuring skin surface roughness using speckle contrast: a skin phantom study

We have been investigating the quantification of skin surface roughness by polychromatic speckle contrast. Speckle contrast, being a measure of light coherence, decreases as coherence decays when low coherent light is reflected from a rough surface. The main constraint of applying the technique to skin is the presence of bulk scattering along with surface reflection. Bulk scattering also decays coherence and is a source of noise. To examine the effect of bulk contribution, we studied speckle patterns generated by silicone phantoms with controllable roughness and optical parameters in the range of human skin. We discovered that using the theoretical curve plotting speckle contrast vs. surface roughness as a calibration curve overestimates the phantom surface roughness. We propose to use the effective calibration curve for the proper skin roughness measurements. The effective calibration curve was obtained experimentally taking the advantage of its weak dependence on phantoms attenuation coefficients.

Optical discrimination of surface reflection from volume backscattering in speckle contrast for skin roughness measurements

Background: The intermixing of light reflected from tissue surface and scattered from tissue volume complicates skin surface roughness assessment by laser speckle technique, a non-invasive optical method based on the analysis of the contrast of a speckle pattern. Objective: In this study we investigated optical discrimination methods to separate the two contributions in a speckle pattern. Methods: Three discrimination methods, spatial, polarization and spectral filtering, were implemented to suppress light from skin internal volume in a laser speckle device. In order to determine the effectiveness of the discrimination methods, speckle patterns were obtained from healthy volunteers, and polychromatic speckle contrast was computed before and after each filtering procedure. Results: Speckle contrast increased after discrimination filtering. A simple formula was derived to calculate the speckle contrast associated with light scattered from the skin surface. This corrected speckle contrast was proposed to be used for skin roughness assessment.

Backscattering of linearly polarized light from turbid tissue-like scattering medium with rough surface.

Abstract. In the framework of further development of a unified computational tool for the needs of biomedical optics, we introduce an electric field Monte Carlo (MC) model for simulation of backscattering of coherent linearly polarized light from a turbid tissue-like scattering medium with a rough surface. We consider the laser speckle patterns formation and the role of surface roughness in the depolarization of linearly polarized light backscattered from the medium. The mutual phase shifts due to the photons’ pathlength difference within the medium and due to reflection/refraction on the rough surface of the medium are taken into account. The validation of the model includes the creation of the phantoms of various roughness and optical properties, measurements of co- and cross-polarized components of the backscattered/reflected light, its analysis and extensive computer modeling accelerated by parallel computing on the NVIDIA graphics processing units using compute unified device architecture (CUDA). The analysis of the spatial intensity distribution is based on second-order statistics that shows a strong correlation with the surface roughness, both with the results of modeling and experiment. The results of modeling show a good agreement with the results of experimental measurements on phantoms mimicking human skin. The developed MC approach can be used for the direct simulation of light scattered by the turbid scattering medium with various roughness of the surface.

Polarization Speckles and Skin Applications

Interference and polarization techniques provide the highest sensitivity and precision in optical metrology. The recently introduced polarization speckle method, incorporating both techniques, is a promising biomedical tool. In this chapter we demonstrate how the method could be used in dermatology. Measuring the polarization properties of coherent backscattered light, we obtain the skin surface roughness, which is an important diagnostic parameter in the clinical recognition of some skin cancers, such as melanoma and their differential diagnosis from common benign lesions, such as pigmented seborrheic keratoses. The principle of the technique has been validated in two clinical studies, and the theory between skin surface roughness and depolarization was investigated using electric field Monte Carlo simulations. The goal of this research is to enhance the capacity of portable devices in the hands of primary care providers to enhance the accuracy of cancer diagnoses in a cost-effective manner.

Depolarization of light by rough surface of scattering phantoms

The growing interest in biomedical optics to the polarimetric methods push researchers to better understand of light depolarization during scattering in and on the surface of biological tissues. Here we study the depolarization of light propagated in silicone phantoms. The phantoms with variety of surface roughness and bulk optical properties are designed to imitate human skin. Free-space speckle patterns in parallel (III) and perpendicular (I⊥) direction in respect to incident polarization are used to get the depolarization ratio of backscattered light DR = (III - I⊥)/( III + I⊥). The Monte Carlo model developed in house is also applied to compare simulated DR with experimentally measured. DR dependence on roughness, concentration and size of scattering particles is analysed. A weak depolarization and negligible response to scattering of the medium are observed for phantoms with smooth surfaces, whereas for the surface roughness in order to the mean free path the depolarization ratio decreases and reveals dependence on the bulk scattering coefficient. In is shown that the surface roughness could be a key factor triggering the ability of tissues’ characterization by depolarization ratio.

Geometry influence on polychromatic speckle contrast

Speckle contrast is widely used in various applications. In this work we develop a simple model to examine the influence of optical geometry on contrast reduction for polychromatic speckle the Fresnel diffraction zone. The model is based on the known fact that the sum of N independent speckle patterns decreases the contrast of the resultant pattern. The model shows how to construct zones in such a way that each zone creates an independent speckle. Theoretical grounds and experimental validation are presented. Practical applications of the derived formulae are discussed. The contrast reduction due to geometry is found to be significant for broad light low-coherent beams.

Abel inversion: optical preprocessing

The Abel inversion is a tomographic problem which we meet in a wide range of areas from astronomy, plasma and jet research to cell microscopy. The Abel inversion restores a radial distribution of refractive index, absorption index or other values for axial symmetric objects. It uses derivative operation that is why the theoretical solving is incorrect. Complex mathematical methods are usually used in practice. We apply an interferometric technique to provide the derivative procedure. The final calculation is approximately twenty times faster than usual techniques. A noise/signal ratio reduces too. An example of biological application is described.

Aspects of speckle contrast metrology

Recent revitalization of interests in applying speckle techniques gives rise to the concern of measurement accuracy. In particular, speckle contrast, an important metric in numerous optical techniques, is affected by many factors related to light sources, propagation media, and receivers. As a result, proper experimental design is required to minimize measurement errors. This article considers errors introduced by the discrepancy of incidence and observation angles, by the limited number of available speckles, and by intensity saturation.