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We put forward an enantioselective method for chiral nanoparticles using optical tweezers. We demonstrate that the optical trapping force in a typical, realistic optical tweezing setup with circularly-polarized trapping beams is sensitive to the chirality of core-shell nanoparticles, allowing for efficient enantioselection. It turns out that the handedness of the trapped particles can be selected by choosing the appropriate circular polarization of the trapping beam. The chirality of each individual trapped nanoparticle can be characterized by measuring the rotation of the equilibrium position under the effect of a transverse Stokes drag force. We show that the chirality of the shell gives rise to an additional twist, leading to a strong enhancement of the optical torque driving the rotation. Both methods are shown to be robust against variations of size and material parameters, demonstrating that they are particularly useful in (but not restricted to) several situations of practical interest in chiral plasmonics, where enantioselection and characterization of single chiral nanoparticles, each and every one with its unique handedness and optical properties, are in order. In particular, our method could be employed to unveil the chiral response arising from disorder in individual plasmonic raspberries, synthesized by close-packing a large number of metallic nanospheres around a dielectric core.

EXEMPLIFYING THE USE OF MACRO ELEMENTAL MAPPING BY XRF (MA-XRF) SCANNING IN FORENSIC INVESTIGATION OF ARTWORKS. This work exemplifies the potential of employing macro elemental mapping by XRF (MA-XRF) scanning in the forensic investigation of artworks. The results of the investigation of two artworks of different styles and periods are presented and discussed. The images provided by MA-XRF scanning have information about the elemental distribution, which is related to pigments used in the artwork. These images bring different information such as: if there are underlying paintings; if a given pigment was used for the purpose of restoration or in the creation. Therefore, through the images, it is possible to obtain information, which refers to the process of creating the artwork and the history of conservation and restoration. Through the results, it was possible to verify that the two artworks are probably false because they present anachronistic pigments with the supposed period of creation. The analyzes were performed using a portable MA-XRF scanning system, and in addition to presenting the utility of the instrument in these investigations, the work also presents possibilities to explore the MA-XRF data through multivariate statistical analysis and image correlation.

This paper performs a semi-quantitative and low-cost experiment, combining video analysis technique and mechanical modelling, to discuss the rotation movement of an electric whirl on the top of a well-known and popular electrostatic generator. To this end, we perform video analysis of the circular trajectory described by a given sharp point of the electric whirl arm. We propose a simple model to describe the inertia moment of the electric whirl in terms of simple geometries, like: cylinders, truncated cone, and one-dimensional linear arms. The analytical results for the inertia moment are validated using SolidWorks simulations and the electronic microscopy experimental technique. When considering a simple electrostatic model for the electric field around sharp points of the electric whirl arms, kinematic and dynamic analyses are used to estimate an upper bound for the magnitude of the electric charge pulled from the air, which results in the electric wind phenomenon.

This work aims to characterize the elemental concentrations of two banana types gold and silver. In both were analyze in the pulp and peel by X-Ray Fluorescence technique. The results showed that the elements in both types are more concentrated in the peel than in the pulp. In addition, it was also observed that the normalization of the peel and pulp concentrations for the elements K, Fe, Zn, Br, Rb, and Sr is higher in the silver banana compared to the gold banana. The results indicate that banana peels can be used to supplement nutritional deficiencies. The work also demonstrates the potential that X-Ray Fluorescence has in investigation of elemental composition of foods. As the technique has easy instrumentation and data interpretation, it can be implemented as a routine in the investigation of the elemental composition of foods. In addition, the technique has the advantage of carrying out in situ analyzes by portable instruments. These in situ investigations can even be applied to make quality control of bananas at the time of harvest. Este trabajo tiene como objetivo caracterizar las concentraciones elementales de dos tipos de banano, oro y plata. Ambos tipos fueron analizados, en la pulpa y en la piel, mediante la técnica de Fluorescencia de Rayos X. Los resultados mostraron que los elementos en ambos tipos de banano están más concentrados en la piel que en la pulpa. Además, también se observó que la normalización de las concentraciones de cáscara y pulpa para los elementos K, Fe, Zn, Br, Rb y Sr es mayor en el banano plateado que en el banano dorado. Los resultados indican que las cáscaras de plátano se pueden utilizar para suplir las deficiencias nutricionales. El trabajo también demuestra el potencial que tiene la fluorescencia de rayos X en la investigación de la composición elemental de los alimentos. Debido a su fácil instrumentación e interpretación de datos, la técnica se puede implementar de forma rutinaria en la investigación de la composición elemental de los alimentos. Además, la técnica tiene la ventaja de realizar análisis in situ utilizando instrumentos portátiles. Estas investigaciones in situ pueden incluso aplicarse para controlar la calidad de los bananos en el momento de la cosecha. Este trabalho tem como objetivo caracterizar as concentrações elementares de dois tipos de banana, ouro e prata. Ambos os tipos foram analisados, na polpa e na casca, pela técnica de Fluorescência de Raios-X. Os resultados mostraram que os elementos de ambos os tipos de banana estão mais concentrados na casca do que na polpa. Além disso, também foi observado que a normalização das concentrações de casca e polpa para os elementos K, Fe, Zn, Br, Rb e Sr é maior na banana prata em relação à banana ouro. Os resultados indicam que as cascas de banana podem ser utilizadas para suprir deficiências nutricionais. O trabalho também demonstra o potencial que a Fluorescência de Raios-X tem na investigação da composição elementar de alimentos. Por possuir fácil instrumentação e interpretação dos dados, a técnica pode ser implementada rotineiramente na investigação da composição elementar dos alimentos. Além disso, a técnica tem a vantagem de realizar análises in situ por instrumentos portáteis. Essas investigações in situ podem até ser aplicadas para fazer o controle de qualidade da banana na época da colheita.

We measure the colloidal interaction between two silica microspheres in aqueous solution in the distance range from $0.2\,\mu$m to $0.5\,\mu$m with the help of optical tweezers. When employing a sample with a low salt concentration, the resulting interaction is dominated by the repulsive double-layer interaction which is fully characterized. The double-layer interaction is suppressed when adding $0.22\,$M of salt to our sample, thus leading to a purely attractive Casimir signal. When analyzing the experimental data for the potential energy and force, we find good agreement with theoretical results based on the scattering approach. At the distance range probed experimentally, the interaction arises mainly from the unscreened transverse magnetic contribution in the zero-frequency limit, with nonzero Matsubara frequencies providing a negligible contribution. In contrast, such unscreened contribution is not included by the standard theoretical model of the Casimir interaction in electrolyte solutions, in which the zero-frequency term is treated separately as an electrostatic fluctuational effect. As a consequence, the resulting attraction is too weak in this standard model, by approximately one order of magnitude, to explain the experimental data. Overall, our experimental results shed light on the nature of the thermal zero-frequency contribution and indicate that the Casimir attraction across polar liquids has a longer range than previously predicted., Comment: 19 pages, 9 figures; updated references; added a detailed discussion of the subtraction procedure leading to the interaction potential

We derive an explicit partial-wave (Mie) series for the image of a dielectric microsphere collected by a typical infinity-corrected microscope. We model the propagation of the illumination and scattered vector fields through the optical components of the microscope by using the angular-spectrum theorem with the help of Wigner rotation matrix elements, allowing us to identify the contribution from spin-orbit helicity reversal. We consider a high numerical aperture objective well beyond the validity range of the paraxial approximation. The spherical aberration introduced by refraction at the planar interface between the sample and the glass slide is fully taken into account. By comparing our theoretical model with images of colloidal particles placed at different positions with respect to the objective focal plane, we characterize their radii and refractive index. We employ polystyrene microspheres with a known refractive index in order to fit the transverse attenuation length describing the transmission loss of the scattered field. As an application, we measure the radius and refractive index of individual silica beads. We compare the result for the radius with an independent measurement using high-resolution scanning electron microscopy. To validate the result for the refractive index, we develop a second method, independent of the theoretical model, based on the image contrast in glycerin-water solutions. In all cases we find very good agreement between our method and the validation procedures. In addition, the nonparaxial theory provides a reliable description of the images found for all focal-plane positions and for both polystyrene and silica microspheres. Our approach allows a common optical microscope to be used to measure the refractive index and radius of spherical particles covering the entire size range from the Rayleigh regime to the ray optics one.

Neste trabalho apresentamos uma atividade de baixo custo que visa a investigação da força de arrasto, que atua na geometria de um disco, através das oscilações amortecidas de um sistema massa-mola. Utilizando discos de papelão de diferentes raios e controlando a massa do sistema oscilatório, mantendo-a fixa, estudamos a influência da variação exclusiva da magnitude da força de arrasto na dinâmica desse sistema, por meio da variação do raio do disco, mantendo todos os outros parâmetros acessíveis do sistema fixos. A técnica de vídeo-análise é utilizada para rastrear o movimento do sistema, levando o estudante a investigar e a extrair conclusões acerca da dependência funcional da força de atrito viscosa com o raio do disco, bem como a obter a constante elástica da mola e uma estimativa para a espessura dos discos. In this work we present a low-cost activity that aims to investigate the drag force, which acts on the geometry of a disc, through the damped oscillations of a mass-spring system. Using cardboard disks of different radii and controlling the mass of the oscillatory system, keeping it fixed, we studied the influence of the exclusive variation of the magnitude of the drag force on the dynamics of this system, by varying the radius of the disk, keeping all the others fixed system accessible parameters. The video analysis technique is used to track the movement of the system, leading the student to investigate and draw conclusions about the functional dependence of the viscous frictional force with the radius of the disk, as well as to obtain the spring elastic constant and an estimate for the thickness of the disks.

We have theoretically investigated optical tweezing of gain-functionalized microspheres using a highly focused single beam in the nonparaxial regime. We employ the Mie-Debye theory of optical tweezers to calculate the optical force acting on homogeneous and core-shell Mie microspheres with gain. We demonstrate that the optical gain plays a crucial role in optical manipulation, especially to optimize the restoring force and thus allowing for trapping of large refractive index and plasmonic particles. Indeed we demonstrate that one can trap such particles, which is usually not possible in the case of passive media, by functionalizing them with an optical gain material. We show that by varying the value of the gain, which can be realized by changing the pump power, one can not only achieve trapping but also manipulate the equilibrium position of the tweezed particle. Altogether our findings open new venues for gain-assisted optomechanics, where gain functionalized systems could facilitate optical trapping and manipulation of plasmonic nanoparticles in particular, with potential applications in self-assembling of nanoparticle suspensions and on a chip., Comment: This paper is submitted to Journal of Optics

We report for the first time the theory of optical tweezers of spherical dielectric particles embedded in a chiral medium. We develop a partial-wave (Mie) expansion to calculate the optical force acting on a dielectric microsphere illuminated by a circularly-polarized, highly focused laser beam. When choosing a polarization with the same handedness of the medium, the axial trap stability is improved, thus allowing for tweezing of high-refractive-index particles. When the particle is displaced off-axis by an external force, its equilibrium position is rotated around the optical axis by the mechanical effect of an optical torque. Both the optical torque and the angle of rotation are greatly enhanced in the presence of a chiral host medium when considering radii a few times larger than the wavelength. In this range, the angle of rotation depends strongly on the microsphere radius and the chirality parameter of the host medium, opening the way for a quantitative characterization of both parameters. Measurable angles are predicted even in the case of naturally occurring chiral solutes, allowing for a novel all-optical method to locally probe the chiral response at the nanoscale.

Optical pulling forces or tractor beams can pull particles against light propagation by redirecting the incident photons forward. This is typically achieved using Bessel beams with very small half-cone angles, which considerably limits its applicability. One can circumvent such issue by using a superposition of plane waves. In order to investigate optical pulling forces exerted by a pair of non-colinear plane waves, we develop a theoretical framework based on Mie theory, Debye potentials and Wigner rotation matrices. We apply this framework to calculate the optical pulling force on metallo-dielectric composite particles, which we put forward as an alternative material platform to optimize and tailor tractor beams. Indeed we demonstrate that by adding a few plasmonic inclusions to low-refractive index dielectric particles of arbitrary sizes, we are able to produce polarization dependent optical pulling forces that cannot occur in the corresponding homogeneous particles. Altogether our findings not only provide innovative theoretical methods to compute optical pulling forces, but also provide new strategies to tailor and optimize them, paving the way to increase their applicability., Comment: 8 pages 4 figures

Resumo Neste trabalho realizamos uma abordagem didática de aspectos relacionados ao geomagnetismo e como esses aspectos podem ser influenciados pela ação do vento solar. A carência de textos didáticos que promovam explicações em termos de conceitos de física básica foi a principal motivação para esse trabalho. Iniciamos com uma discussão histórica sobre o estudo do magnetismo que culminou na conhecida teoria do dínamo para explicar a origem do campo magnético terrestre, na qual mostramos a possiblidade de abordar este assunto em cursos de eletricidade e magnetismo. Apresentamos também dois cenários para a magnetosfera terrestre, com o intuito de modelar a interação do vento solar com a magnetosfera, em que representamos tal interação por meio da superposição dos campos de dois dipolos magnéticos. A seguir mostramos a possiblidade de discutir o movimento das cargas provenientes do vento solar ao redor da Terra, na região da magnetosfera, utilizando a segunda lei de Newton e a força de Lorentz, discutindo assim a formação de anéis de cargas em movimento e dos cinturões de Van Allen. Finalmente apresentamos um simples argumento, baseado no modelo apresentado, para a deformação da magnetosfera, para explicar a limitação na quantidade de anéis e cinturões que envolvem o nosso planeta. Abstract In this work we perform a pedagogic approach to aspects related to geomagnetism and how they can be influenced by the action of the solar wind. The lack of didactic textbooks that promote explanations in terms of basic physics concepts is the main motivation for this work. We begin with a historical discussion about the study of the magnetism that culminated in the well-known dynamo theory which explains the origin of the Earth's magnetic field, where we show the possibility to treat this subject in classes of electricity and magnetism. We also present two scenarios for the Earth's magnetosphere, in order to model the interaction of the solar wind with the magnetosphere, where we represent such action by superposing the fields of two magnetic dipoles. After we show the possibility of discussing the movement of charges coming from the solar wind around the Earth, in the region of the magnetosphere, using Newton's second law and the Lorentz force, where we discuss the formation of moving charged rings and the well known Van Allen belts. Finally, we present a simple argument based on the presented model, for the deformation of the magnetosphere, to explain the limitation in the amount of rings and belts that surround our planet.

Summary Background The sepsis burden on acute care services in middle-income countries is a cause for concern. We estimated incidence, prevalence, and mortality of sepsis in adult Brazilian intensive care units (ICUs) and association of ICU organisational factors with outcome. Methods We did a 1-day point prevalence study with follow-up of patients in ICU with sepsis in a nationally representative pseudo-random sample. We produced a sampling frame initially stratified by geographical region. Each stratum was then stratified by hospitals' main source of income (serving general public vs privately insured individuals) and ICU size (ten or fewer beds vs more than ten beds), finally generating 40 strata. In each stratum we selected a random sample of ICUs so as to enrol the total required beds in 1690 Brazilian adult ICUs. We followed up patients until hospital discharge censored at 60 days, estimated incidence from prevalence and length of stay, and generated national estimates. We assessed mortality prognostic factors using random-effects logistic regression models. Findings On Feb 27, 2014, 227 (72%) of 317 ICUs that were randomly selected provided data on 2632 patients, of whom 794 had sepsis (30·2 septic patients per 100 ICU beds, 95% CI 28·4–31·9). The ICU sepsis incidence was 36·3 per 1000 patient-days (95% CI 29·8–44·0) and mortality was observed in 439 (55·7%) of 788 patients (95% CI 52·2–59·2). Low availability of resources (odds ratio [OR] 1·67, 95% CI 1·02–2·75, p=0·045) and adequacy of treatment (OR 0·56, 0·37–0·84, p=0·006) were independently associated with mortality. The projected incidence rate is 290 per 100 000 population (95% CI 237·9–351·2) of adult cases of ICU-treated sepsis per year, which yields about 420 000 cases annually, of whom 230 000 die in hospital. Interpretation The incidence, prevalence, and mortality of ICU-treated sepsis is high in Brazil. Outcome varies considerably, and is associated with access to adequate resources and treatment. Our results show the burden of sepsis in resource-limited settings, highlighting the need to establish programmes aiming for sepsis prevention, early diagnosis, and adequate treatment. Funding Fundacao de Apoio a Pesquisa do Estado de Sao Paulo (FAPESP).

We show that the optical force field in optical tweezers with elliptically polarized beams has the opposite handedness for a wide range of particle sizes and for the most common configurations. Our method is based on the direct observation of the particle equilibrium position under the effect of a transverse Stokes drag force, and its rotation around the optical axis by the mechanical effect of the optical torque. We find overall agreement with theory, with no fitting, provided that astigmatism, which is characterized separately, is included in the theoretical description. Our work opens the way for characterization of the trapping parameters, such as the microsphere complex refractive index and the astigmatism of the optical system, from measurements of the microsphere rotation angle., 10 pages, 5 figures

We put forward a novel, twofold scheme that enables, at the same time, all-optical enantioselection and sorting of single multipolar chiral microspheres based on optical pulling forces exerted by two non-collinear, non-structured, circularly polarized light sources. Our chiral resolution method can be externally controlled by varying the angle between their incident wavevectors, allowing for fine-tuning of the range of chiral indices for enantioselection. Enantioselectivity is achieved by choosing angles such that only particles with the same handedness of the light sources are pulled. This proposal allows one to achieve all-optical sorting of chiral microspheres with arbitrarily small chiral parameters, thus outperforming current optical methods.

In this paper, we propose a simple experiment to discuss some fundamental concepts of electricity and thermal physics, based on a very popular device within the current education scenario, the Arduino. In this proposal, we use the Arduino microcontroller as a data acquisition device to obtain the temperature of a nichrome resistor, that heats up in a circuit by Joule Effect over time. Using the data obtained, we developed a protocol to experimentally estimate the nichrome resistor heat capacity.

We propose an enantioselective scheme to sort homogeneous chiral particles using optical tweezers. For a certain range of material parameters, we show that a highly focused circularly-polarized laser beam traps particles of a specific chirality selected by the handedness of the trapping beam. Furthermore, by applying a transverse Stokes drag force that displaces the trapped particle off-axis, we allow for the rotation of the particle center-of-mass around the trapping beam axis. The rotation angle is highly dependent on the handedness of the trapped particle and is easily measurable with standard video-microscopy techniques, allowing for an alternative mechanism for chiral resolution. Our platform not only allows for enantioselection of particles dispersed in solution but also paves the way to the characterization of the chiral parameter of individual, homogeneous chiral microspheres using optical tweezing., 9 pages, 5 figures

This work presents a study of the influence of the field size of an epithermal neutron beam on the dose distribution in treatments with boron neutron capture therapy using an anthropomorphic voxel-based simulator. To calculate the doses in the tissues and organs of the head, the Monte Carlo N-Particle radiation transport code is used. The results suggest the possibility of using a beam of 6 cm in diameter for the treatment of tumours in the frontal lobe and parietal regions. With a beam of 10 cm in diameter, we could treat tumours located in the frontal lobe, parietal lobe and thalamus. The work also shows that the contribution of the secondary components (gamma rays, fast and thermal neutrons) in the calculation of the total dose can result in up to 15% of the dose in the tumour tissue, 68% of the dose in the healthy brain tissue and 87% of the dose in the non-cephalic regions.

Resumo Neste trabalho o problema clássico de uma partícula que descreve um arco de círculo ao deslizar pela superfície polida de um iglu esférico é estendido para o caso de um iglu elipsoidal. Neste caso, a altura na qual a partícula perde contato com a superfície do iglu passa a obedecer a uma equação cúbica com coeficientes que dependem dos parâmetros que determinam a trajetória elíptica. A solução analítica dessa equação é então obtida a partir do método de Cardano-Tartaglia.

Trapping of microspheres with a single focused laser beam is usually limited to materials with relative refractive indexes slightly larger than one. We show that directional light scattering can be employed to optically trap high-index materials. For this purpose, we propose a material platform to achieve zero backward scattering (ZBS), also known as the first Kerker condition, in a composite media containing spherical inclusions of silica embedded in a SiC microsphere. By tuning the volume filling fraction of inclusions and the microsphere radius, stable trapping can be achieved, provided that ZBS is combined with the condition for destructive interference between the fields reflected at the external and internal interfaces of the microsphere when located at the focal point. We show that our proposal also holds even in the presence of a significant amount of spherical aberration, which is a common condition in most optical tweezers setups. In this case, achieving ZBS is essential for trapping high-index materials., Comment: 8 pages,6 figures

Optical tweezers have become a powerful tool for basic and applied research in cell biology. Here, we describe an experimentally verified theory for the trapping forces generated by optical tweezers based on first principles that allows absolute calibration. For pedagogical reasons, the steps that led to the development of the theory over the past 15 years are outlined. The results are applicable to a broad range of microsphere radii, from the Rayleigh regime to the ray optics one, for different polarizations and trapping heights, including all commonly employed parameter domains. Protocols for implementing absolute calibration are given, explaining how to measure all required experimental parameters, and including a link to an applet for stiffness calculations.

In this paper will be reported the principal results about absolute calibration of optical tweezers that we have published at reference.1

The automation of surveillance systems is important to allow real-time analysis of critical events, crime investigation and prevention. A crucial step in the surveillance systems is the person re-identification (Re-ID) which aims at maintaining the identity of agents in non-overlapping camera networks. Most of the works in literature compare a test sample against the entire gallery, restricting the scalability. We address this problem employing multiple indexing lists obtained by color name descriptors extracted from part-based models using our proposed Predominant Color Name (PCN) indexing structure. PCN is a flexible indexing structure that relates features to gallery images without the need of labelled training images and can be integrated with existing supervised and unsupervised person Re-ID frameworks. Experimental results demonstrate that the proposed approach outperforms indexation based on unsupervised clustering methods such as k-means and c-means. Furthermore, PCN reduces the computational efforts with a minimum performance degradation. For instance, when indexing 50% and 75% of the gallery images, we observed a reduction in AUC curve of 0.01 and 0.08, respectively, when compared to indexing the entire gallery.

Optical-tweezers experiments in molecular and cell biology often take place near the surface of the microscope slide that defines the bottom of the sample chamber. There, as elsewhere, force measurements require force-calibrated tweezers. In bulk, one can calculate the tweezers force from first principles, as recently demonstrated. Near the surface of the microscope slide, this absolute calibration method fails because it does not account for reverberations from the slide of the laser beam scattered by the trapped microsphere. Nor does it account for evanescent waves arising from total internal reflection of wide-angle components of the strongly focused beam. In the present work we account for both of these phenomena. We employ Weyl's angular spectrum representation of spherical waves in terms of real and complex rays and derive a fast-converging recursive series of multiple reflections that describes the reverberations, including also evanescent waves. Numerical simulations for typical setup parameters evaluate these effects on the optical force and trap stiffness, with emphasis on axial trapping. Results are in good agreement with available experimental data. Thus, absolute calibration now applies to all situations encountered in practice.

We extend the MDSA (Mie–Debye spherical aberration) theory of trapping forces in optical tweezers, previously developed for circularly polarized trapping beams, to linear polarization. Although it does not significantly affect the trap stiffness, linear polarization may introduce a strong axial asymmetry of the optical forces near the edge of a trapped microsphere, arising from Mie resonance effects.

The study on possibilities for commercialization of mangoes cv. Tommy Atkins fruit in the form of canned food is an important step to augment new agro-industrial activities in the regions of production and exportation of this fruit. Several products such as jam, jelly, chutney and crystallized mango, using as raw material the mango fruits, which do not classify for export quality but are in good conditions for processing, were elaborated by varying their formulations and processing conditions. All products were submitted to sensory analysis, which was performed at a supermarket in the capital of Paraiba state. The taste evaluation of the jam and jelly resulted in an acceptance of more than 93 and 80% by the panel members. However, despite a good acceptance of the chutney and of crystallized mango, there were some suggestions for changing the optimization conditions of these products.

The absolute calibration of optical tweezers has been demonstrated by including the aberration effects of the optical system into the theoretical framework, thereby opening the door to an improved instrument design, reliable force measurements, and a better control over experiments.

Surveillance cameras are present almost everywhere, indicating an increasing interest regarding people safety. The automation of surveillance systems is important to allow real time analysis of critical events, crime investigation and prevention. A crucial step in the surveillance systems is the person re-identification which aims at maintaining the identity of agents that pass through the monitored environment, despite the occurrence of significant gaps in time and space. Many approaches have been proposed to person re-identification. However, there are still problems to be solved, such as illumination changes, pose variation, occlusions, appearance modeling and the management of the large number of people being monitored. This work approaches the last problem with the employment of multiple indexing structures associated with a weighting strategy to maintain the scalability and improve the accuracy. Experimental results demonstrate that the proposed approach is able to improve results based only on a single indexing structure.

The role of person re-identification has increased in the recent years due to the large camera networks employed in surveillance systems. The goal in this case is to identify individuals that have been previously identified in a different camera. Even though several approaches have been proposed, there are still challenges to be addressed, such as illumination changes, pose variation, low acquisition quality, appearance modeling and the management of the large number of subjects being monitored by the surveillance system. The present work tackles the last problem by developing an indexing structure based on inverted lists and a predominance filter descriptor with the aim of ranking candidates with more probability of being the target search person. With this initial ranking, a more strong classification is done by means of a mean Riemann covariance method, which is based on a appearance strategy. Experimental results show that the proposed indexing structure returns an accurate short-list containing the most likely candidates, and that manifold appearance model is able to set the correct candidate among the initial ranks in the identification process. The proposed method is comparable to other state-of-the-art approaches.

We extend a previous proposal for absolute calibration of optical tweezers by including optical setup aberrations into the first-principles theory, with no fitting parameters. Astigmatism, the dominant term, is determined from images of the focused laser spot. Correcting it can substantially increase stiffness. Comparison with experimental results yields agreement within error bars for a broad range of bead sizes and trap heights, as well as different polarizations. Absolute calibration is established as a reliable and practical method for applications and design of optical tweezers systems.

We extend a previous proposal for absolute calibration of optical tweezers by including optical setup aberrations into the first-principles theory. Astigmatism, the dominant term, is determined from images of the focused laser spot.

Resumo Neste trabalho exploramos a técnica da transformada de Laplace inversa, conhecida como transformada de Fourier-Mellin, para solucionar, de forma direta e rigorosa, o problema de um sistema que oscila sob a ação de uma força externa periódica. Também propomos um modelo em que essa força externa é descrita por uma sucessão de deltas de Dirac, uma estrutura conhecida na literatura como pente de Dirac. Esse modelo é adequado para descrever o problema clássico de uma criança sendo impulsionada em um balanço, usualmente descrito em termos de uma força externa senoidal. Indicamos nosso modelo de oscilador forçado como mais realista na descrição desse tipo de problema por considerar a atuação da força externa apenas no intervalo que corresponde ao tempo de contato entre a criança e o agente externo que realiza a força, intervalo esse que tende a zero. O principal resultado deste trabalho foi obtido no regime de ressonância, no qual a potência média transferida ao sistema apresentou uma série de picos, associados aos múltiplos inteiros da frequência natural de oscilação, diferente do que ocorre no caso ordinário, em que a força externa é descrita por uma função trigonométrica. Abstract In this work we explore the technique of the inverse of the Laplace transform, which is known as the Fourier-Mellin transform, to solve, in a direct and rigorous way, the problem of a system that oscillates under the action of a periodic external force. We also present a model in which the external force is described in terms of a sequence of Dirac deltas. This model is pertinent to describe the classical problem of a child boosted in a park swing, usually described in terms of a sinusoidal function. We indicate our forced oscillator model as more realistic to the description of this kind of problem as it considers the action of the external force only during the very small time interval of contact between the child and the agent that applies the force. The principal result of this paper were obtained in the resonance regime, in which the average power transferred to the system presented a series of peaks, corresponding to the entire multiples of the natural frequency of oscillation, in contrast to what is obtained in the ordinary case in which the external force is described by a trigonometric function.