Detection of partially coherent optical emission sources

Detection of airborne optical emission sources in Infrared Search and Track (IRST) systems is usually carried out using the blackbody temperature or emissivity difference between the emission source and the background. Recent countermeasure techniques include emissivity tailoring and temperature distribution tailoring across the emission source area to avoid the formation of "hot spots" which effectively embeds the emission source in its background. A technique relying on coherence rather than irradiance is presented, allowing detection with poor signal-to-clutter ratios.The technique has some similarity with Fourier Transform Spectroscopy (FTS), and its key components are a Michelson interferometer, which measures the coherence profile of the scene in the field of view, and an interference filter, which uses the background illumination to create amplitude minima in the interferogram envelope. Unlike FTS, the interferometer moving mirror scans only a tiny portion of the interferogram, this being the region surrounding the first minimum of its envelope. It is shown that this envelope is a sine function, and that at this minimum the phase of the interferogram undergoes a pi phase step, which is used to define the position accurately, When a partially coherent emission source comes into the field of view, the modulus of the net complex degree of coherence of the scene increases, and the phase step position changes; this latter optical feature is used to declare detection.We present a simple theoretical model and compare it with experimental results for highly emissive sources having various coherence lengths in the presence of incoherent background illumination. Agreement between the experimental results and the theory is discussed

High-sensitivity detection of narrowband light in a more intense broadband background using coherence interferogram phase

This paper describes an optical interferometric detection technique,. known as the interferogram phase step shift, which detects narrowband, coherent, and partially coherent light in more intense broadband incoherent background light using changes in the phase gradient with the optical path difference of the coherence interferograin to distinguish the bandwidth or coherence of the signal from that of the background. The detection sensitivity is assessed experimentally by measuring the smallest signal-to-background ratio or signal-to-clutter ratio (SCR), which causes a detectable change in the self-coherence interferograin phase. This minimum detectable SCR (MDSCR) is measured for the multimode He-Ne laser, resonant-cavity light-emitting diode (LED), narrowband-filtered white light, and LED signal sources in a more intense tungsten-halogen-lamp white-light background. The highest MDSCRs to date, to the authors' knowledge, are -46.42 dB for coherent light and -31.96 dB for partially coherent light, which exceed those of existing automatic single-domain techniques by 18.97 and 4.51 dB with system input dynamic ranges of 19.24 and 11.39 dB, respectively. The sensitivity dependence on the signal-to-system bandwidth ratio and on the relative offset of their central wavelengths is also assessed, and optimum values are identified

Detection of coherent light in an incoherent background

The change in position of the self-coherence function minimum is used to detect the presence of a coherent source, rather than the change in strength of the self-coherence function at the reference path difference. The system uses both optical and digital signal processing with MATLAB algorithm. An experimental system was built in the visible band, employing a Michelson interferometer, an interference filter centered in the red, and a silicon photodetector. The results were averaged over up to 50 scans, depending on the relative visibility of the white light and laser fringes, to reduce the scan to scan variability. Amplifier gain was introduced to reduce quantization noise

Variable numerical-aperture temporal-coherence measurement of resonant-cavity LEDs

The first interferometric measurements of temporal-coherence length variation with numerical aperture (NA) are described for 650 nm, resonant-cavity light-emitting diodes (LEDs) agreeing with spectrally derived results. The interferometrically measured coherence length (22 mum to 32 mum) reduced by 37% for a 0.42 increase in NA. For a larger range of NA (0-1), this would give coherence lengths (10 mum-40 mum) lying in the gap between that of conventional LEDs (similar to5 mum) and superluminescent diodes (similar to60 mum)

Microkinetic analysis of ethanol to 1,3-butadiene reactions over MgO-SiO 2 catalysts based on characterization of experimental fluctuations

Microkinetic analysis of ethanol to 1,3-butadiene reactions over MgO-SiO2 catalysts was performed based on the detailed characterization of experimental fluctuations, taking into account the influence of the reaction temperature and catalyst properties on ethanol conversion and product selectivities. The obtained results show that both reaction temperature and catalysts properties affected experimental fluctuations significantly. The local microkinetic information contained in the covariance matrix of experimental fluctuations indicated the change of the rate-limiting step as reaction temperature increased: from 300 to 400 °C, the rate-limiting step was identified as the acetaldehyde condensation, while at 450 °C, ethanol dehydrogenation step limits the 1,3-butadiene production

Measures and models for causal inference in cross-sectional studies: arguments for the appropriateness of the prevalence odds ratio and related logistic regression

<p>Abstract</p> <p>Background</p> <p>Several papers have discussed which effect measures are appropriate to capture the contrast between exposure groups in cross-sectional studies, and which related multivariate models are suitable. Although some have favored the Prevalence Ratio over the Prevalence Odds Ratio -- thus suggesting the use of log-binomial or robust Poisson instead of the logistic regression models -- this debate is still far from settled and requires close scrutiny.</p> <p>Discussion</p> <p>In order to evaluate how accurately true causal parameters such as Incidence Density Ratio (IDR) or the Cumulative Incidence Ratio (CIR) are effectively estimated, this paper presents a series of scenarios in which a researcher happens to find a preset ratio of prevalences in a given cross-sectional study. Results show that, provided essential and non-waivable conditions for causal inference are met, the CIR is most often inestimable whether through the Prevalence Ratio or the Prevalence Odds Ratio, and that the latter is the measure that consistently yields an appropriate measure of the Incidence Density Ratio.</p> <p>Summary</p> <p>Multivariate regression models should be avoided when assumptions for causal inference from cross-sectional data do not hold. Nevertheless, if these assumptions are met, it is the logistic regression model that is best suited for this task as it provides a suitable estimate of the Incidence Density Ratio.</p

Carbon related defects in irradiated silicon revisited

Electronic structure calculations employing hybrid functionals are used to gain insight into the interaction of carbon (C) atoms, oxygen (O) interstitials, and self-interstitials in silicon (Si). We calculate the formation energies of the C related defects C(i)(Si(I)), C(i)O(i), C(i)C(s), and C(i)O(i)(Si(I)) with respect to the Fermi energy for all possible charge states. The C(i)(Si(I))(2+) state dominates in almost the whole Fermi energy range. The unpaired electron in the C(i)O(i)(+) state is mainly localized on the C interstitial so that spin polarization is able to lower the total energy. The three known atomic configurations of the C(i)C(s) pair are reproduced and it is demonstrated that hybrid functionals yield an improved energetic order for both the A and B-types as compared to previous theoretical studies. Different structures of the C(i)O(i)(Si(I)) cluster result for positive charge states in dramatically distinct electronic states around the Fermi energy and formation energies