RESUMO
Sampling rate and frequency content determination for optical quantities related to light propagation through turbulence are paramount experimental topics. Some papers about estimating properties of the optical turbulence seem to use ad hoc assumptions to set the sampling frequency used; this chosen sampling rate is assumed good enough to perform a proper measurement. On the other hand, other authors estimate the optimal sampling rate via fast Fourier transform of data series associated to the experiment. When possible, with the help of analytical models, cut-off frequencies, or frequency content, can be determined; yet, these approaches require prior knowledge of the optical turbulence. The aim of this paper is to propose an alternative, practical, experimental method to estimate a proper sampling rate. By means of the discrete wavelet transform, this approach can prevent any loss of information and, at the same time, avoid oversampling. Moreover, it is independent of the statistical model imposed on the turbulence.
RESUMO
Under the Geometrics Optics approximation is possible to estimate the covariance between the displacements of two thin beams after they have propagated through a turbulent medium. Previous works have concentrated in long propagation distances to provide models for the wandering statistics. These models are useful when the separation between beams is smaller than the propagation path-regardless of the characteristics scales of the turbulence. In this work we give a complete model for these covariances, behavior introducing absolute limits to the validity of former approximations. Moreover, these generalizations are established for non-Kolmogorov atmospheric models.
Assuntos
Atmosfera , Lasers , Modelos Teóricos , Nefelometria e Turbidimetria/instrumentação , Refratometria/instrumentaçãoRESUMO
Maize presents a substantial contamination by fumonisins (FB) throughout the world. In particular, Argentina presents a high frequency of positive samples and levels of up to 10,000 µg/kg of total FB, depending on the harvest campaign studied. Corn puff, an extruded product of high consumption among children and adolescent populations, was analyzed with the aim of determining the presence and levels of fumonisins B1, B2 and B3. In the present study, 20 samples of different brands were collected from commercial stores. Two solvent systems, two types of agitation, and the number of extractions were tested in order to find the most suitable method to establish the level of fumonisins in corn puff. Extracts were analyzed by high performance liquid chromatography (HPLC) with fluorescence detection. A C18 column and precolumn derivatization with orthophtalaldialdehyde were utilized. The average of fumonisins found in 20 samples was 257.5 µg/kg (n = 19), 70.4 µg/kg (n = 14) and 73.3 µg/kg (n = 6) for FB1, FB2 and FB3, on positive samples, respectively. These are the first analyses in Argentina of this type of product, highlighting the need to continue the studies in the processing industries because, despite the extrusion process that the raw material suffers, a sample was found with a contamination of 1,649 µg/kg total fumonisins.