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.
A thorough knowledge of the angular distribution of light scattered by an illuminated surface under different angles is
essential in numerous industrial and research applications. Traditionally, the angular distribution of a reflected or
transmitted light flux as function of the illumination angle, described by the Bidirectional Scattering Distribution
Function (BSDF), is measured with a point-by-point scanning goniophotometer yielding impractically long acquisition
times. Significantly faster measurements can be achieved by a device capable of simultaneously imaging the far-field
distribution of light scattered by a sample onto a two-dimensional sensor array. Such an angular-to-spatial mapping
function can be realized with a parallel catadioptric mapping goniophotometer (CMG).
In this contribution, we formally establish the design requirement for a reliable CMG. Based on heuristic considerations
we show that, to avoid degrading the angular-to-spatial function, the acceptance angle of the lens system inherent to a
CMG must be smaller than 60°. By means of a parametric study, we investigate the practical design limitations of a
CMG caused by the constraints imposed by the properties of a real lens system. Our study reveals that the values of the
key design parameters of a CMG fall within a relatively small range. This imposes the shape of the ellipsoidal reflector
and drastically restricts the room for a design trade-off between the sample size and the angular resolution. We provide a quantitative analysis for the key parameters of a CMG for two relevant cases.
We present a semi-analytical model of optical coherence tomography (OCT) taking into account multiple scattering. The model rests on the assumptions that the measured portion of the backscattered sample field is spatially coherent and that the sample is motionless relative to measurement time. This allows modeling an OCT signal as a sum of spatially coherent fields with random phase arguments-constant during measurement time-caused by multiple scattering. We calculate the mean OCT signal from classical results of statistical optics and a Monte Carlo simulation. Our model is shown to be in very good agreement with a whole range of experimental data gathered in a comprehensive study of cross-talk in wide-field OCT realized with spatially coherent illumination. The study consists of depth scan measurements of a mirror covered with an aqueous suspension of microspheres. We investigate the dependence of cross-talk on important optical system parameters, as well as on some relevant sample properties. We discuss the more complex OCT models based on the extended Huygens-Fresnel principle, which rest on different assumptions since they assume partially coherent interfering fields.
The crucial role played by the source's degree of spatial coherence in wide-field optical coherence tomography is shown experimentally. Spatially coherent illumination, as obtained with a pulsed laser, generates a considerable amount of coherent optical cross-talk. The latter can be suppressed with spatially incoherent illumination as provided by a thermal or a pseudothermal light source. Demonstration is made for a US air force resolution target covered with a scattering solution made of polystyrene microspheres suspended in water. The origin and nature of cross-talk signals are discussed, as well as specific limitations of spatially incoherent sources.
We demonstrate phase space tomography for the measurement of the transversal spatial coherence function of light after propagation through a scattering medium. The results of this approach are compared to measurements performed with shearing-interferometry. Implications for parallel Optical Coherence Tomography will be briefly discussed.
Parallel optical coherence tomography is demonstrated at video rate using a 58 by 58 smart-pixel detector array. A sample volume of 210x210x80 micrometers3 (corresponding to 58x58x58 voxels) was imaged at 25 Hz. A femtosecond mode-locked Ti:Sapphire laser in combination with a free space Michelson interferometer was employed to achieve a 3 micrometer longitudinal resolution. We used 20x microscope objectives in both sample arm and reference arm and measured a 8 micrometer transverse resolution. The sensitivity of the system was 76 dB.
In this paper a numerical a posteriori depth-variant dispersion compensation technique for PCI and OCT depth-scan signals is presented. This technique is based on numerical correlation of the depth-scan interferometer signal with a depth-variant kernel. Examples of dispersion compensated depth-scan signals obtained from microscope cover glasses are presented. First results show substantial resolution improvements.
Parallel optical coherence tomography in scattering samples is demonstrated using a 58 by 58 smart-pixel detector array. A femtosecond mode-locked Ti:Sapphire laser in combination with a free space Michelson interferometer was employed to achieve 4micrometers longitudinal resolution and 9mm transverse resolution on a 260x260 micrometers 2 field of view. We imaged a resolution target covered by an intralipid solution with different scattering coefficients as well as onion cells.
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