Electromagnetic sunscreen model: design of experiments on particle specifications.

We report a numerical study on sunscreen design and optimization. Thanks to the combined use of electromagnetic modeling and design of experiments, we are able to screen the most relevant parameters of mineral filters and to optimize sunscreens. Several electromagnetic modeling methods are used depending on the type of particles, density of particles, etc. Both the sun protection factor (SPF) and the UVB/UVA ratio are considered. We show that the design of experiments' model should include interactions between materials and other parameters. We conclude that the material of the particles is a key parameter for the SPF and the UVB/UVA ratio. Among the materials considered, none is optimal for both. The SPF is also highly dependent on the size of the particles.

Considering sources and detectors distributions for quantitative photoacoustic tomography.

Photoacoustic tomography (PAT) is a hybrid imaging modality that takes advantage of high optical contrast brought by optical imaging and high spatial resolution brought by ultrasound imaging. However, the quantification in photoacoustic imaging is challenging. Multiple optical illumination approach has proven to achieve uncoupling of diffusion and absorption effects. In this paper, this protocol is adopted and synthetic photoacoustic data, blurred with some noise, were generated. The influence of the distribution of optical sources and transducers on the reconstruction of the absorption and diffusion coefficients maps is studied. Specific situations with limited view angles were examined. The results show multiple illuminations with a wide field improve the reconstructions.

Electromagnetic sunscreen model: implementation and comparison between several methods: step-film model, differential method, Mie scattering, and scattering by a set of parallel cylinders.

Sunscreens protect from UV radiation, a carcinogen also responsible for sunburns and age-associated dryness. In order to anticipate the transmission of light through UV protection containing scattering particles, we implement electromagnetic models, using numerical methods for solving Maxwell's equations. After having our models validated, we compare several calculation methods: differential method, scattering by a set of parallel cylinders, or Mie scattering. The field of application and benefits of each method are studied and examples using the appropriate method are described.

Depth probing of diffuse tissues controlled with elliptically polarized light.

Polarization gating is a popular technique in biomedical optics. It is widely used to inspect the surface of the tissues (under colinear or cocircular detection) or instead to probe the volume (cross-linear detection), without information on the probed depth. Elliptical polarization is introduced to explore the possibility of probing diffuse tissues at selective depths. A thorough Monte Carlo simulation study shows complete correlation between the probed depths and the ellipticity of the polarized light, for a medium with known optical properties. Within a wide range of optical parameters, a linear relation between the backscattered intensity and the depth extension of the probed volume was found whatever the polarization used, but with a controlled extension depending on the ellipticity.

Elliptically polarized light for depth resolved optical imaging.

It is shown that using elliptically polarized light permits selecting well-defined subsurface volumes in a turbid medium. This suggests the possibility of probing biological tissues at specific depths. First, we present the method and preliminary results obtained on an Intralipid phantom. We next report on the method's performance on a biological phantom (chicken breast) and, finally, on the exposed cortex of an anesthetized rat.

Light scattering from edematous human corneal grafts' microstructure: experimental study and electromagnetic modelization.

Along with the lens, the cornea is the only transparent tissue in the human body. However, the development of an edema involves structural disturbances increasing light scattering and leading to the opacification of the cornea. Several mechanisms of transparency loss have been studied in the literature, but the whole phenomenon is complex and the part played by each scatterer is still unclear. We propose here to study human corneal grafts combining microscopic OCT imagery with far-field measurement of the scattered light in the reflected half-space. We introduce afterwards numerical calculations based on electromagnetic equations solved with first order approximation to link the observed microscopic-scale structural modifications with the intensity level of the scattered light, and to try and quantify the relationship between them.

Optical component interface scatter characterization by selective polarization extinction.

A procedure for the selective extinction of the scattered light based on "null ellipsometry" [R. M. A. Azzam and N. M. Bashara, Ellipsometry and Polarized Light (North-Holland, 1977)] is presented. The technique allows scattering measurement from individual layers of a multilayer component by extinguishing the scattered light from the other layer interfaces. The technique is easily applicable to multilayer components with nearly identical surface profiles at every interface and little significant bulk scattering. Analysis is provided to determine the conditions required to extinguish the light from the excluded interfaces isolating the scattered light from the desired interface. An analysis of sensitivity of the extinction conditions to experimental parameters and to layer optical thickness is also provided. Experimental data collected using the technique are compared with measurements made using a white-light optical surface profilometry.

Membrane characterization by microscopic and scattering methods: multiscale structure.

Several microscopic and scattering techniques at different observation scales (from atomic to macroscopic) were used to characterize both surface and bulk properties of four new flat-sheet polyethersulfone (PES) membranes (10, 30, 100 and 300 kDa) and new 100 kDa hollow fibers (PVDF). Scanning Electron Microscopy (SEM) with "in lens" detection was used to obtain information on the pore sizes of the skin layers at the atomic scale. White Light Interferometry (WLI) and Atomic Force Microscopy (AFM) using different scales (for WLI: windows: 900 × 900 µm2 and 360 × 360 µm2; number of points: 1024; for AFM: windows: 50 × 50 µm2 and 5 × 5 µm2; number of points: 512) showed that the membrane roughness increases markedly with the observation scale and that there is a continuity between the different scan sizes for the determination of the RMS roughness. High angular resolution ellipsometric measurements were used to obtain the signature of each cut-off and the origin of the scattering was identified as coming from the membrane bulk.

Human graft cornea and laser incisions imaging with micrometer scale resolution full-field optical coherence tomography.

Micrometer scale resolution full-field optical coherence tomography (FF-OCT) is developed for imaging human graft corneas. Three-dimensional (3-D) images with ultrahigh resolution (respectively, 1 and 1.5 µm in the axial and transverse directions), comparable to traditional histological sections, are obtained allowing the visualization of the cells and the precise structure of the different layers that compose the tissue. The sensitivity of our device enables imaging the entire thickness of the cornea, even in edematous corneas more than 800 µm thick. Furthermore, we provide tomographic 3-D images of laser incisions inside the tissue at various depths without slicing the studied corneas. The effects of laser ablations can be observed, along various optical sections, directly in the bulk of the sample with high accuracy, providing information on the interface quality and also imaging tiny changes of the tissue structure. FF-OCT appears to be a powerful tool for subcellular imaging of the corneal structure and pathologies on the entire thickness of the tissue as well as interface quality and changes in the collagen structure due to laser incisions on ex vivo human cornea.

Partial polarization of light induced by random defects at surfaces or bulks.

Partial polarization may be the result of a scattering process from a fully polarized incident beam. It is shown how the "loss of polarization" is connected with the nature of scatterers (surface roughness, bulk heterogeneity) and on the receiver solid angle. These effects are theoretically predicted and confirmed via multiscale polarization measurements in the speckle pattern of rough surfaces. "Full" polarization can be recovered when reducing the receiver aperture.

From angle-resolved ellipsometry of light scattering to imaging in random media.

A procedure is described to allow selective cancellation of polarized scattering within optical substrates and multilayers. It is shown how bulk scattering (respectively surface) can be directly eliminated while the remaining roughness (respectively bulk) signal is still measurable. The same procedure can be applied to isolate a single interface or bulk within a stack or to detect slight departure from perfect correlation within multilayers. Experiments and a procedure for selective imaging in random media are described.

Efficiency of polarimetric z probing in optical multilayers.

Numerical calculation is performed to validate the principles of a single optical technique devoted to real time probing or imaging of submultilayers within interferential coatings.

Selective probing and imaging in random media based on the elimination of polarized scattering.

An ellipsometric technique based on angle-resolved light scattering is addressed to open applications in the field of imaging in random media. The first experimental demonstration is given to prove the selective extinction of different scattering sources such as surface roughness and bulk heterogeneity in optical components and liquids. The results are compared with theory.

Z-probing of optical multilayers: theory.

A single optical technique is presented that allows direct and selective probing or imaging in the thickness of multilayers. It is based on tunable interferences of polarized light. Adequate vertical resolution is provided with this method.

Optical characterization of an unknown single layer: Institut Fresnel contribution to the Optical Interference Coatings 2004 Topical Meeting Measurement Problem.

We present the characterizations performed at the Institut Fresnel for the Measurement Problem of the Optical Interference Coatings 2004 Topical Meeting. A single layer coated on a fused-silica substrate of unknown composition and parameters is analyzed in terms of optogeometrical parameters, uniformity, and scattering. We determine the refractive index and the average thickness of the coating, then provide the localized determination of the thickness with a 2 mm spatial resolution. Topography measurements include atomic force microscopy and angle-resolved scattering measurements. These results are completed thanks to a Taylor Hobson noncontact 3D surface profiler.

Light-scattering characterization of transparent substrates.

Angle-resolved light scattering has been used for decades to quantify the surface roughness of optical components. However, because this technique is affected by the contribution of both interfaces of the sample, it cannot be applied to transparent substrates. We show how to overcome this issue and apply these principles to the characterization of superpolished samples.

Multidielectric quarter-wave coatings on microspheres: a study in colorimetric space.

Spectral properties of quarter-wave stacks deposited on microspheres are investigated with Mie theory. These properties are converted into colorimetric spaces to deal with visual applications. An ion-beam-sputtering process is then implemented to produce elementary stacks on the spheres, and the resulting powders are characterized.

Ellipsometry of reflected and scattered fields for the analysis of substrate optical quality.

Specular ellipsometry is a well-known and efficient technique to characterize surfaces and coatings. This technique has been extended to the measurement of scattered light. We present an experimental setup, using a polarization modulator, which permits us to characterize transition layers and roughness without a calibration procedure. Experimental results are presented concerning transition layers for damage threshold applications and for rough surfaces or bulks.

Angle-resolved ellipsometry of scattering patterns from arbitrary surfaces and bulks.

Accurate angular phase data are extracted from angle-resolved scattering measurements made with polarized light using a technique developed in the laboratory. This Ellipsometry of Angle-Resolved Scattering (E.A.R.S.) technique makes it possible to distinguish surface scattering from bulk scattering independent of the scattering levels for different types of samples. Phase data are also investigated in the speckle pattern.

Elimination of polarized light scattered by surface roughness or bulk heterogeneity.

An interferential technique is described to eliminate polarized scattering from optical substrates and coatings. Conditions of annulment are respectively given for surface roughness and for bulk heterogeneity, at each direction of space. At low-level scattering, the method offers a complete discrimination of surface and bulk effects, whatever the micro-structural parameters. Arbitrary scattering levels can be treated in a similar way, but require the knowledge of microstructure.