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Atmosphere Monitoring Using Methods of Absorption of Electromagnetic Radiation—Fourier Transform Infrared Spectroscopy
Published in Ghenadii Korotcenkov, Handbook of Humidity Measurement, 2018
As regards general errors related to FTIR measurements, the systematic errors (and random noise) in the interferogram can be grouped into three broad categories: additive errors, intensity errors, and phase errors. The reasons for their appearance and approaches used to reduce them considered in detail in Mertz (1965, 1967), Forman et al. (1966), Griffiths and de Haseth (1986), Remedios (1990), and Davis et al. (2001) and taken into account in the software of FTIR spectrometers. Therefore, a number of steps are involved in calculating the frequency domain spectrum. Instrumental imperfections and basic scan limitations need to be accommodated by performing a phase correction and apodization steps. These electronic and optical imperfections can cause erroneous readings due to a different time or phase delays of various spectral components. Apodization is used to correct spectral leakage, artificial creation of spectral features due to the truncation of the scan at its limits (a FT of sudden transition will have a very broad spectral content).
Performance comparison of commonly used photoacoustic tomography reconstruction algorithms under various blurring conditions
Published in Journal of Modern Optics, 2022
Note that is the pressure measured by a point detector positioned at at time t. However, a realistic detector has a finite size. The impinging pressure waves become spatially averaged due to the finite aperture effect and hence, the collective signal (over the surface area S) can be expressed as [23] where is the sensitivity profile of the detector. It can be taken as [23] where σ is the standard deviation for the Gaussian apodization function and denotes the centre of the detector. One can use the resultant signals [i.e. Equation (8)] detected by the sensors with finite apertures for image reconstruction via the BP algorithm. The spatial averaging of the PA signals for finite detectors introduces blurring in the reconstructed images. The amount of blurring can be reduced by decreasing σ of the Gaussian apodization function [23]. In this work, a finite detector (a line element in two dimensions) has been broken into many points and the resultant signal (recorded by that detector) has been distributed in the same Gaussian manner over those points [34]. After that, the conventional BP algorithm has been applied for image reconstruction. This is termed as the MBP algorithm, which has been found to remove blurring effectively.
Effect of gradual widening of d/w-ratio on optical performance of photon sieves produced by two maskless lithography techniques: lift-off and chemical etching
Published in Journal of Modern Optics, 2020
Diffractive optical elements, especially the Fresnel zone plate (FZP) and the photon sieves (PS), have taken the attention of many researchers for more than a decade [1–3]. A wide spectral range of optical devices has been studied, including x-ray focusing, deep UV-lithography and remote sensing applications [1,2,4–8]. Also, there are devices produced for enhanced resolution and intensities in visible and near-infrared regions [2,4,5,7,9]. The photon sieve intensity is strictly related to both the position and size of the pinholes. The ratio of the pinhole diameter (d) to the width (w) of the related Fresnel zone, i.e. the d/w ratio, is one of the main parameters to affect the focusing performance of PS devices, directly [10–13]. Some researchers reported improvement in intensity and resolution by combining two diffraction optics, FZP and PS [6,7], and also by using apodization [14]. There are design and production studies, too. Multi-wavelength photon sieves were produced, to increase the performance by defining multiple discrete regions for selected wavelengths [15], by distributing the pinholes regarding a special algorithm, named wavefront coding [16] and by dividing the zones in Fresnel element into multiple levels [17]. Since depending on the light intensity passing through the pinholes, the light intensity transmitted may become as low as 0.07 of the incident [12].
The analysis of the Coriolis interactions in the v10 = 1, v7 = 1 and v4 = 1 triad rovibrational states of 13C2D4 by high-resolution FTIR spectroscopy
Published in Molecular Physics, 2020
The 13C2D4 gas sample used in the experiment was procured from Cambridge Isotope Laboratories in Massachusetts, USA and has a chemical purity of better than 98%. Four spectra of samples at gas pressures of 0.20, 0.61, 1.02 and 5.53 mbar were recorded in the 515 cm−1–825 cm−1 wavenumber region at an unapodised resolution of 0.0019 cm−1 using the Bruker IFS 125 HR spectrometer located in National Institute of Education, Nanyang Technological University of Singapore. The spectrometer uses a globar infrared source, a high-sensitivity MCT detector cooled with liquid-nitrogen, and a KBr beamsplitter. Spectra collection was typically carried out at an ambient temperature of 296 K. A total absorption path length of 8.0 m was used by adjusting for 40 passes in the multiple-pass gas cell with 0.20 m base length. The gas pressures as measured by the capacitance pressure gauge, numbers of co-added scans and the total scanning times of the respective spectra are listed in Table 1. The interferograms were processed with a 4-point apodization function and a post-zero fill factor of 8. The full-width at half maximum (FWHM) of the absorption lines were measured to have an average value of 0.003 cm−1 due to a combination of pressure and Doppler broadenings.