Multiple scattering in wide-field optical coherence tomography

Optical Coherence Tomography (OCT), a well-established imaging method based on low-coherence interferometry, provides cross-sectional images of the internal structure of biological samples with a resolution in the micrometer range. OCT was successfully applied on various tissues such as for instance the retina, the skin or a tooth. In highly scattering tissues like the skin, probing depth is limited to approximately 2mm, mainly due to insufficient rejection of multiply scattered light. Presently, the contribution of multiple scattering in OCT is not fully understood. Therefore, there is a strong and urgent need to develop models allowing a reliable evaluation of the system's limitations as well as the improvement of the imaging capabilities. It is generally believed that a relevant model should account for loss of correlation between the reference and the sample field due to multiple scattering. We developed a new comprehensive model of OCT. Our preliminary study revealed that the reference and sample fields are actually fully correlated. This important result allowed us to model the OCT signal as a sum of stationary random phasors and treated it as a statistical signal. The mean of this signal can be calculated thanks to classical results of statistical optics and to a Monte Carlo simulation. Unlike other existing models, our model accounts for the source autocorrelation function. The model proved to be in excellent agreement with a whole range of experimental data gathered in a comprehensive study of cross-talk in wide-field OCT. Moreover, our results put in question the applicability of widely used models of OCT based on the "extended Huygens-Fresnel principle", which assume a partial correlation between interfering fields due to multiple scattering. The construction of conventional OCT images is based on lateral scanning of a beam focused within the sample. To increase image acquisition speed and eliminate the need for lateral scanning, wide-field OCT was recently developed. Our experimental and theoretical investigations of the potential and limitations of wide-field OCT revealed the crucial role played by the spatial coherence of the light source. Spatially coherent illumination generates considerable coherent optical cross-talk, which prevents shot-noise-limited detection and diffraction-limited imaging in scattering samples. The dependence on several parameters of the optical system and of the sample properties was investigated in a comprehensive study. Cross-talk increases with the wide-field diameter, numerical aperture, source coherence length, and sample optical density; and strongly depends on sample anisotropy. We showed that spatially incoherent illumination realized with a thermal light source permits cross-talk suppression in wide-field OCT, i.e. rejection of multiply scattered light to a level comparable to that of point scanning OCT. We performed a theoretical study which revealed that the power per spatial mode radiated by thermal light sources is too low to permit a high signal-to-noise ratio while maintaining a fast acquisition speed. Therefore, wide-field OCT realized with either spatially coherent or spatially incoherent illumination suffers from inherent fundamental limitation. This led us to investigate the possibility of exploiting a spatially incoherent light source brighter than a thermal light source. We came to the conclusion that such a "pseudothermal" light source can potentially lead to wide-field OCT systems devoid of cross-talk and an image acquisition speed higher than that of a point scanning OCT system. However, the attractive properties of pseudothermal light sources could be gained at the expense of the simplicity and the economical advantages offered by thermal light sources. Furthermore, fast acquisition speed also relies on a performing "smart pixel detector array". Presently, such detectors do not have sufficient sensitivity and their frequency readout is too low as shown in our feasibility study. We performed a theoretical investigation of the potential of thermal light sources in terms of axial resolution and power per mode. The former revealed that the maximum power per mode is radiated at a wavelength higher than the spectral peak of a blackbody radiator. This led to the important practical conclusion that, at 6000 K, the maximum power is collected in the therapeutic spectral window in OCT (600 - 1300 nm), while at 3000 K this peak is shifted out of the therapeutic window leading to significant power losses. More generally, our work provides a design tool for choosing the optimal thermal light source for a given therapeutic window in terms of signal-to-noise ratio. Currently available sources at 6000 K consist of high pressure gas arc lamps providing a spectrum endowed with spectral lines deleterious for OCT. By suppressing a portion of the spectrum devoid of spectral lines of a mercury arc lamp, we obtained amongst the highest axial resolution so far reported in OCT. Furthermore, the importance of the speckle statistics in OCT incited us to clarify the origin of a difference between two theoretical results reported in the literature. Indeed, two calculations of the amplitude distribution of speckles in OCT, each of them based on a different mathematical formulation, yield different results. We showed that a modification of an initial assumption in one of the formulation leads to equivalent results. In conclusion, this thesis provides a deeper understanding of the potential and limitations of widefield OCT, leading to important design rules. Moreover, it presents a new comprehensive model of OCT putting in question other widely used models

Limits and artefacts of reflective imaging goniophotometers for complex solar façade systems

The design of systems for solar light collection, modulation and/or distribution requires a thorough knowledge of their optical properties. The angular distribution of the scattered incident light flux, described by the Bidirectional Scattering Distribution Function (BSDF), can be measured with a step-by-step scanning goniophotometer but requires considerable time, especially when aiming at high angular resolution over a wide range and numerous incidence angles like in typical solar applications. Considerably faster measurements can be achieved with a so-called imaging goniophotometer, which simultaneously measures light fluxes in all scattered directions by dispatching them over different portions of a two-dimensional sensor array. In this contribution, we revisit the widely accepted principle of a reflective imaging goniophotometer (RIG), which is based on a hemispherical (or ellipsoidal) mirror and a fisheye camera. Specifically developed ray-tracing tools allowed us to obtain accurate figures relative to the influence of key design parameters on angular resolution. Our calculations reveal that the measurement accuracy is too low for samples larger than a few tens of millimeters. Most importantly, we found significant limitations and artefacts in the angular-to-spatial mapping function inherent to the RIG principle, which generally severely bias BSDF measurements

Concept, Design and Performance of a Shape Variable Mashrabiya as a Shading and Daylighting System for Arid Climates

The design of a solar protection system that can minimize solar gains while maximizing daylight and view to the outside is particularly challenging in arid climates, such as in the Middle-East, where sand, wind and corrosion impose specific constraints. We propose a system that provides a trade-off for three requirements: (i) maximize diffuse sunlight and view to the outside, (ii) efficiently block direct sunlight and (iii) transform a fraction of it into diffuse light for indoor daylighting. Compliance with this last requirement provides a solution for the common problem of insufficient daylighting even in the presence of abundant solar radiation, which often forces occupants to fully close their shading system and use electric lighting. In addition, our design potentially copes well with these extreme environmental conditions and preserves local architectural character (mashrabiya-inspired design). In this paper, we establish quantitative specifications for these three requirements, provide the working principle of our shading and daylighting system and its design, which consists of a shape variable mashrabiya (SVM). We calculate and analyze the annual daylighting performance of our SVM and benchmark it against the performance of Venetian blinds and diffuse sunlight alone. Finally, we provide the minimum reflectance required for the SVM to comply with our third requirement. We built a mock-up of our SVM to investigate the validity of our simulation model

Lighting and Energy Performance of an Adaptive Shading and Daylighting System for Arid Climates

Finding the proper trade-off between blocking direct sunlight, ensuring sufficient indoor daylighting and view out is a particularly delicate task especially in arid climates, due to harsh environmental conditions. As a tentative answer to this challenge, an adaptive shading and daylighting system (Shape Variable Mashrabiya – SVM) has been developed by the authors, described in an earlier paper. In this paper, we analyze how the SVM may affect annual lighting and global primary energy performance of an office building in Abu Dhabi: the SVM was applied to east and west façades and compared to external Venetian blinds, reflective and selective glazing

Origin and nature of measurement bias in catadioptric parallel goniophotometers

We briefly categorize and compare parallel goniophotometers, which are instruments capable of simultaneously measuring the far-field distribution of light scattered by a surface or emitted by a source over a large solid angle. Little is known about the accuracy and reliability of an appealing category, the catadioptric parallel goniophotometers (CPGs), which exploit a curved reflector and a lens system. We analyzed the working principle common to all the different design configurations of a CPG and established the specifications implicitly imposed on the lens system. Based on heuristic considerations, we show that the properties of a real (thick) lens system are not fully compatible with these specifications. This causes a bias to the measurements that increases with the acceptance angle of the lens system. Depending on the angular field, the measured sample area can be drastically reduced and shifted relative to the center of the sample. To gain insights into the nature and importance of the measurement bias, it was calculated with our model implemented in MATLAB for the CPG configuration incorporating a lens system with a very large acceptance angle (fisheye lens). Our results demonstrate that, due to the spatio-angular-filtering properties of the fisheye lens, this category of CPGs is so severely biased as to give unusable measurements. In addition, our findings raise the question of the importance of the bias in the other types of CPGs that rely on a lens system with a lower acceptance angle

Design of a light confining concentrator for a solar photochemical reactor and upper bound to the method

Optical concentration obtained by light confinement bears unique features that can increase the efficiency of a photochemical reactor. A suitable implementation of this method for a solar reactor is a series of parallel tubular receivers sealed in a slab-shape reflective cavity, in which light is trapped thanks to a self-adaptive optical filtering mechanism. To predict the concentration in such a generic configuration, we had previously established an analytical model based on idealistic assumptions, which are not valid in our real configuration. Here, we use analytical calculations and numerical ray-trace simulations to investigate how the finite size of the latter impacts the prediction of our model and extrapolate design guidelines for minimal departure from ideality. We apply these guidelines to design an optical concentrator maximizing flux density on tubular receivers and discuss the upper bound to the method, as well as the benefits from its unique features. Accounting for practical and technological limitations, this method can provide optical concentration in the order of ten suns in our generic configuration

NOMAD - Network of Optimization Modalities in Architectural Design

Two pairs of identical experimentation modules are being developed within the framework of a collaborative initiative named NOMAD – Network of Optimization Modalities in Architectural Design - between the Interdisciplinary Laboratory of Performance-Integrated Design (LIPID) and the Laboratory of Architecture and Sustainable Technologies (LAST), both at EPFL. NOMAD relates to a series of projects related to the optimization of the building envelope in a sustainability context. Based on comparative evaluations in two climates (cool-temperate versus hot-arid), these projects will most notably rely on a dual infrastructure: the NOMAD modules. These modules will be located in Lausanne, Switzerland and in the Middle East (Ras-Al-Kaimah, United Arab Emirates (U.A.E.). They are currently in design development and will be operational in the fall of 2012 for the launching of the Master in Energy Management and Sustainability Caravans. One pair of modules will be located on the main EPFL campus in Lausanne, Switzerland and the other pair will be installed on the site of the EPFL Middle East campus in Ras-Al-Kaimah, U.A.E. The modules’ dimensions will be 3 m wide x 3 m high x 9 m deep so as to allow deep plan layouts, and include an active insulation skin for temperature and heat exchange control. Having the modules in pairs offers the option either rely on a reliable reference case in any measurement campaign, to hide the measurement setup from the users in field studies, or to conduct two experiments simultaneously to get most out of periods where optimal climate conditions are scarce. This dual climate approach allows us to open up an innovative and original research framework, able to generate new façade concepts with high environmental standards, and to develop bio-climatic strategies based on multiple criteria. This unique dual research facility will thus be particularly adapted to investigate climate-related issues in building technology and to enhance the educational potential in sustainable architecture at EPFL and EPFL Middle East