Ground-based measurements of the weather-driven sky radiance distribution in the Southern Hemisphere.

The angular distribution of the sky radiance determines the energy generation of solar power technologies as well as the ultraviolet (UV) doses delivered to the biosphere. The sky-diffuse radiance distribution depends on the wavelength, the solar elevation, and the atmospheric conditions. Here, we report on ground-based measurements of the all-sky radiance at three sites in the Southern Hemisphere across a transect of about 5,000 km: Santiago (33°S, a mid-latitude city of 6 million inhabitants with endemic poor air quality), King George Island (62°S, at the northern tip of the Antarctic Peninsula, one of the cloudiest regions on Earth), and Union Glacier (79°S, a snow-covered glacier in the vast interior of Western Antarctica). The sites were strategically selected for studying the influence of urban aerosols, frequent and thick clouds, and extremely high albedo on the sky-diffuse radiance distribution. Our results show that, due to changing site-specific atmospheric conditions, the characterization of the weather-driven sky radiance distribution may require ground-based measurements.

Panoramic reconstruction of quasi-cylindrical objects with digital holography and a conical mirror.

In this work, we present a panoramic digital holographic system for the first time capable of obtaining 3D information of a quasi-cylindrical object by using a conical mirror. The proposed panoramic digital holographic system is able to scan the entire surface of the object to determine the amplitude and phase simultaneously. This paper demonstrates the feasibility of analyzing quasi-cylindrical objects in a short time (0.5 s) with a single camera and a minimum number of optical components. In addition, it can be applied to determine not only topographic measurement of the cylindrical surface but also measurements of radial deformations. Experimental results are presented at different magnifications, thus illustrating its capabilities and versatility.

Dynamic parallel phase-shifting electronic speckle pattern interferometer.

Methods for measuring variations in diffuse surfaces using electronic speckle pattern interferometry (ESPI) are widely used and well known. In this research, we present an out-of-plane ESPI system coupled to a Michelson configuration to generate simultaneous parallel interferograms with different phase shifts. The system uses circular polarization states to generate parallel phase shifted interferograms. Due to the polarization states, the fringes do not experience a contrast reduction, thus avoiding measurement errors that affect spatial or temporal phase-shifting in interferometry. The basic operating principle of polarization modulation is described, and results that represent the temporal evolution of an aluminum plate are presented. The generation of two simultaneous patterns allows one to track the dynamic performance of the plate.

Application of speckle interferometry to the mechanical characterization of a biopolymer sample.

In this work, we present an optical and mechanical characterization of the behavior of an inhomogeneous biopolymer sample through the use of an in-plane electronic speckle pattern interferometer with a pulling system along the $y$y direction. The characterization of the sample subjected to stress comprised the acquisition of speckle patterns for 1360 states. Displacement maps and their corresponding strain maps were computed for every state. Since the information of the maps changes with size due to the sample being pulled at the upper end while it is clamped at the lower end, a scaling method to relate the maps to each other, point-to-point, is presented. The method allows the correct evaluation of sequential strain maps, which depicts the mechanical evolution of the material. Upon managing to relate the strain maps, it is possible to extract strain values for zones of interest from every map in order to build the respective stress-strain curves. Three stress-strain curves associated with three zones in the sample (upper, middle, and bottom) are constructed. When sequential displacement and deformation maps are optically obtained by the interferometer, we present a full-field characterization, along with the obtention stress-strain curves associated with the three zones of strain maps. The curves represent the inhomogeneous performance of the sample. Three different elastic moduli (${E_u} = 2.59\;{\rm MPa}$Eu=2.59MPa, ${E_m} = 1.97\;{\rm MPa}$Em=1.97MPa, and ${E_{b}} = 1.67\;{\rm MPa}$Eb=1.67MPa), associated with three respective zones, were obtained. The experimental results for a biopolymer sample here presented show that the technique, in conjunction with the scaling method, is a novel proposal to characterize inhomogeneous materials.

Relative error in out-of-plane measurement due to the object illumination.

In this work the sensitivity vector is analyzed for collimated and divergent oblique illumination for an out-of-plane arrangement. In the case of collimated illumination, the variation of the sensitivity vector components was considered and a geometric model was proposed for its evaluation. The geometry of the optical setup used allowed us to find sensitivity mostly along the pulling direction; the other two components of the sensitivity vector were relatively small. We measured the displacement induced only along the pulling direction. Errors in the displacement measurement associated with divergent and collimated illumination can be predicted when the sensitivity vector is considered constant. Experimental and theoretical results are presented for the out-of-plane electronic speckle pattern interferometer. The analyzed object was an aluminum plate.

Dual-plane slightly off-axis digital holography based on a single cube beam splitter.

In order to recover the holographic object information, a method based on the recording of two digital holograms, not only at different planes but also in a slightly off-axis scheme, is presented. By introducing a π-phase shift in the reference wave, the zero-order diffracted term and the twin image are removed in the frequency domain during the processing of the recorded holograms. We show that the zero-order elimination by the phase-shifted holograms is better than working with weak-order beam and average intensity removal methods. For recording experimentally two π-shifted holograms at different planes slightly off-axis, a single cube beam splitter is used. Computer simulations and experimental results, carried out to validate our proposal, show a high accuracy of π/14 that can be comparable with phase-shifting digital holography. For high fringe spacing, our proposal could be applied in electron holography, avoiding high voltage in a biprism.

The world's highest levels of surface UV.

Chile's northern Atacama Desert has been pointed out as one of the places on earth where the world's highest surface ultraviolet (UV) may occur. This area is characterized by its high altitude, prevalent cloudless conditions and relatively low total ozone column. Aimed at detecting those peak UV levels, we carried out in January 2013 ground-based spectral measurements on the Chajnantor Plateau (5100 m altitude, 23°00'S, 67°45'W) and at the Paranal Observatory (2635 m altitude, 24°37'S, 70°24'W). The UV index computed from our spectral measurements peaked at 20 on the Chajnantor Plateau (under broken cloud conditions) and at 16 at the Paranal Observatory (under cloudless conditions). Spectral measurements carried out in June 2005 at the Izaña Observatory (2367 m altitude, 28°18'N, 16°30'W) were used for further comparisons. Due to the differences in sun-earth separation, total ozone column, altitude, albedo, aerosols and clouds, peak UV levels are expected to be significantly higher at southern hemisphere sites than at their northern hemisphere counterparts.

Downwelling and upwelling radiance distributions sampled under cloudless conditions in Antarctica.

We have sampled both the downwelling and upwelling radiance distributions at a camp located in the southern Ellsworth Mountains on the broad expanse of Union Glacier (700 m altitude, 79° 46' S, 82° 52' W). The measurements (at 320-440 nm wavelength range) were carried out under cloudless conditions by using a sky scanner system, during a campaign (in December, 2012) meant to assess the effects of the high albedo on the radiance distribution. The angular variations observed in both the downwelling and upwelling radiance distributions increase with the wavelength. However, these variations were considerably greater in the case of the downwelling radiance than in the case of the upwelling radiance. Indeed, we found that downwelling radiance tends to be less isotropic than the corresponding upwelling radiance. Regardless of the solar zenith angle and the wavelength, the minima of the downwelling and the upwelling radiance distributions were measured close to the zenith and to the nadir, respectively. The downwelling (upwelling) radiance increased nearly monotonically toward the horizon and peaked at zenith (nadir) angles that ranged from 75° to 90°. Comparisons with the UVSPEC radiative transfer model were used to weight up the response of the downwelling radiance distribution to changes in the albedo.

High topographical accuracy by optical shot noise reduction in digital holographic microscopy.

In this work, we present a new method to reduce the shot noise in phase imaging of digital holograms. A spatial averaging process of phase images reconstructed at different reconstruction distances is performed, with the reconstruction distance range being specified by the numerical focus depth of the optical system. An improved phase image is attained with a 50% shot noise reduction. We use the integral of the angular spectrum as a reconstruction method to obtain a single-object complex amplitude that is needed to perform our proposal. We also show the corresponding simulations and experimental results. The topography of a homemade TiO2 stepwise of 100 nm high was measured and compared with the atomic force microscope results.

Half-quadratic cost function for computing arbitrary phase shifts and phase: Adaptive out of step phase shifting.

We present a phase shifting robust method for irregular and unknown phase steps. The method is formulated as the minimization of a half-quadratic (robust) regularized cost function for simultaneously computing phase maps and arbitrary phase shifts. The convergence to, at least, a local minimum is guaranteed. The algorithm can be understood as a phase refinement strategy that uses as initial guess a coarsely computed phase and coarsely estimated phase shifts. Such a coarse phase is assumed to be corrupted with artifacts produced by the use of a phase shifting algorithm but with imprecise phase steps. The refinement is achieved by iterating alternated minimization of the cost function for computing the phase map correction, an outliers rejection map and the phase shifts correction, respectively. The method performance is demonstrated by comparison with standard filtering and arbitrary phase steps detecting algorithms.

Uncertainty analysis of displacements measured by in-plane electronic speckle-pattern interferometry with spherical wave fronts.

Displacement measurements by optical interferometry depend on the induced phase difference and on the interferometer's sensitivity vector; the latter depends in turn on the illuminating sources and on the geometry of the optical arrangement. We have performed an uncertainty analysis of the in-plane displacements measured by electronic speckle-pattern interferometry with spherical incident wave fronts. We induced the displacements by applying a uniaxial tensile load on a nominally flat elastic sample. We approached the displacement uncertainty by propagating the uncertainties that we considered reasonable to assign to the measured phase difference and to the characteristic parameters of the interferometer's sensitivity vector. Special attention was paid to evaluating contributions to the displacement uncertainty. Moreover, we observed that the uncertainty decreases if the angles of incidence and the source-target distances are increased.

Three-dimensional deformation measurement from the combination of in-plane and out-of-plane electronic speckle pattern interferometers.

An optical setup that can be switched to produce in-plane and out-of-plane sensitivity interferometers was designed for three-dimensional deformation measuring by electronic speckle pattern interferometry. Divergent illumination is considered in the evaluation of sensitivity vectors to measure both in-plane and out-of-plane displacement components. The combination of these interferometers presents the advantage of greater sensitivity in directions u, v, and w than a typical interferometer with three illumination beams provides. The system and its basic operation are described, and results with an elastic target that is exposed to a mechanical load are reported.

Development in situ for gratings recorded in photoresist.

A technique for developing recording gratings in situ in Photoresist Shipley S-1822 is described. The developing process is accomplished by use of a spray without removing the sample from the optical setup. The results for the diffraction efficiency show that there is not a large difference between gratings achieved with the traditional wet development process and those obtained with the in situ developing technique. The potential of this in situ developing technique is shown with a moiré interferometric experimental setup used for displacement showing.