Background Intensity Distributions Research Articles

Back Cover: Quantum Imaging Exploiting Twisted Photon Pairs (Adv. Quantum Technol. 5/2023)

Quantum correlation of two-photon states has been utilized to suppress the environmental noise in imaging down to the single-photon level. However, the size of the coherence area of photon pairs limits the applications of quantum imaging based on spatial correlations. Here, we propose a quantum imaging scheme exploiting twisted photon pairs with tunable spatial-correlation regions to circumvent this limitation. We employ a bulk-density coincidence to enhance the imaging signal. Specifically, we introduce a re-scaled image signal, which is immune to the background intensity distribution profile of the photon pulse. We reveal a destructive interference between the anti-bunched photon pair and bunched photon pair in the imaging process. Our work could pave a way for twisted-photon-based quantum holography and quantum microscopy.

Quantum Imaging Exploiting Twisted Photon Pairs

AbstractQuantum correlation of two‐photon states is utilized to suppress the environmental noise in imaging down to the single‐photon level. However, the size of the coherence area of the photon pairs limits the applications of quantum imaging based on spatial correlations. Here, a quantum imaging scheme exploiting twisted photon pairs with tunable spatial‐correlation regions to circumvent this limitation is proposed. A bulk‐density coincidence is employed to enhance the imaging signal. Specifically, a re‐scaled image signal is introduced, which is immune to the background intensity distribution profile of the photon pulse. A destructive interference between the anti‐bunched photon pair and bunched photon pair in the imaging process is revealed. This work can pave a way for twisted‐photon‐based quantum holography and quantum microscopy.

Visibility enhancement of underwater images based on polarization common-mode rejection of a highly polarized target signal.

Underwater active polarization imaging is promising due to its effect of significantly descattering. Polarization-difference is commonly used to filter out backscattered noise. However, the polarization common-mode rejection of target signal has rarely been utilized. In this paper, via taking full advantage of this feature of Stokes vectors S2 which ably avoids interference from target light, the spatial variation of the degree of polarization of backscattered light is accurately estimated, and the whole scene intensity distribution of background is reconstructed by Gaussian surface fitting based on least square. Meanwhile, the underwater image quality measure is applied as optimization feedback, through iterative computations, not only sufficiently suppresses backscattered noise but also better highlights the details of the target. Experimental results demonstrate the effectiveness of the proposed method for highly polarized target in strongly scattering water.

Phase analysis error reduction in the Fourier transform method using a virtual interferogram

We propose a method for reducing artifactual phase errors inherent to the Fourier transform method (FTM) for fringe analysis. The phase obtained by the FTM is subject to ripple errors at the boundary edges of the fringe pattern where fringes become discontinuous. We note that these artifactual phase errors are found to have certain systematic relations to the form of the phase, amplitude, and background intensity distributions, which can be modeled by low-order polynomials, such as Zernike polynomials, in many cases of practical interest. Based on this observation, we estimate the systematic ripple errors by analyzing a virtual interferogram that is numerically created for a fringe model with known phase, amplitude, and background intensity distributions. Starting from a rough initial guess, the virtual interferogram is sequentially improved by an iterative algorithm, and the estimated errors are finally subtracted from the experimental data. We present the results of simulations and experiments that demonstrate the validity of the proposed method.

Theoretical analysis of the background intensity distribution in X-ray Birefringence Imaging using synchrotron bending-magnet radiation

In the recently developed technique of X-ray Birefringence Imaging, molecular orientational order in anisotropic materials is studied by exploiting the birefringence of linearly polarized X-rays with energy close to an absorption edge of an element in the material. In the experimental setup, a vertically deflecting high-resolution double-crystal monochromator is used upstream from the sample to select the appropriate photon energy, and a horizontally deflecting X-ray polarization analyzer, consisting of a perfect single crystal with a Bragg reflection at Bragg angle of approximately 45°, is placed downstream from the sample to measure the resulting rotation of the X-ray polarization. However, if the experiment is performed on a synchrotron bending-magnet beamline, then the elliptical polarization of the X-rays out of the electron orbit plane affects the shape of the output beam. Also, because the monochromator introduces a correlation between vertical position and photon energy to the X-ray beam, the polarization analyzer does not select the entire beam, but instead selects a diagonal stripe, the slope of which depends on the Bragg angles of the monochromator and the polarization analyzer. In the present work, the final background intensity distribution is calculated analytically because the phase space sampling methods normally used in ray traces are too inefficient for this setup. X-ray Birefringence Imaging data measured at the Diamond Light Source beamline B16 agree well with the theory developed here.

Method for Assessing Modulation Transfer Function of Trifocal Intraocular Lens

The article investigates the quality of images formed by multifocal optical elements. The method for calculating of multifocal diffractive lens modulation transfer function (MTF) at presence of background is proposed. The method is based on a geometric calculation of the intensity of background images and the diffraction efficiency of the main image in each focal plane is considerated. The article has studied the contrast of main images formed by trifocal intraocular lens (IOL). Based on the proposed models of uniform and non-uniform background intensity distributions MTF of given IOL are calculated in each focal plane. It is shown that the main image diffraction efficiency determines a decrease in its contrast at low spatial frequencies with uniform and non-uniform backgrounds. The MTF dependence on the type and background position is analyzed and found that in the case of non-uniform background the main image contrast is slightly higher than for a uniform.

Spacecraft observations of electromagnetic perturbations connected with seismic activity

Results of a statistical study of intensity of VLF electromagnetic waves observed in the vicinity of earthquakes are presented. A unique set of data obtained by the micro‐satellite DEMETER (altitude of about 700 km, nearly Sun‐synchronous orbit) and a robust two‐step data processing has been used. In the first step, all the measured data are used to construct a map of electromagnetic emissions containing a statistical description of wave intensity at a given point of the satellite orbit under given conditions. In the second step, the intensity measured close to earthquakes is analyzed using the statistical distribution of background intensity obtained in the first step. The changes of wave intensity caused by seismic activity are investigated and their statistical significance is evaluated. Altogether, more than 2.5 years of satellite data have been analyzed and about 9000 earthquakes with magnitudes larger than or equal to 4.8 that occurred all over the world during the analyzed period have been included in the study. It is shown that, during the night, there is a statistically significant decrease by 4 – 6 dB of the measured wave intensity shortly (0–4 hours) before an intense surface (depth less than or equal to 40 km) earthquake.

Characteristics of oligonucleotide tiling arrays measured by hybridizing full-length cDNA clones: Causes of signal variation and false positive signals

An assessment of the hybridization characteristics of oligonucleotide tiling arrays was carried out using 162 full-length sequenced cDNA clones in spike-in experiments. The properties of array probes that influence signal intensity were investigated, and their capability in the detection of the cDNA exons was evaluated. The signal intensities detected in exonic and nonexonic genomic regions were examined by focusing on the features of probe sequences that raise or lower the level of intensity and on the causes of false positive signals found in nonexonic regions. The effectiveness of measures used in published protocols to improve the separation between signal and background intensity distributions, including the use of replicates and threshold parameterization of signal intensity, was assessed. Sensitivity and specificity in the detection of exons were measured using various sets of threshold parameters, and the effects of each parameter on the detection efficiency and the rate of false positives were evaluated. It was also demonstrated that hybridization of full-length cDNA clones is an excellent method to investigate the characteristics of oligonucleotide tiling arrays.

Blind phase shift estimation in phase-shifting interferometry

A blind phase shift estimation algorithm that allows simultaneous calculation of phases and phase shifts from three or more interferograms is presented. In phase-shifting interferometry, the phase shift errors introduce specific correlations between the calculated background intensity distribution and the fringe component. These correlations can be measured with a cross-power spectrum. By minimization of an objective function based on this cross-power spectrum, the actual phase shifts are estimated and used for phase recovery. The validity of this algorithm is verified by both the numerical simulation and the experiment results.

Algorithm for near-field reconstruction based on radial-shearing interferometry

A new iterative algorithm to be used to precisely reconstruct near-field distribution from an interferogram of a laser output generated by a cyclic radial-shearing interferometer is proposed. First, by use of a window function around the zero-frequency part of the Fourier transform of the interferogram and calculation of the inverse Fourier transform of the zero-frequency part, we obtain the background intensity distribution of the interferogram. Then, according to the iterative algorithm, the near-field distribution of the laser output is precisely reconstructed from the background intensity distribution obtained in the first step. A numerical simulation and an actual experiment of the near-field reconstruction of the laser output with arbitrary amplitude distribution are implemented successfully.

Contextual clustering for analysis of functional MRI data.

We present a contextual clustering procedure for statistical parametric maps (SPM) calculated from time varying three-dimensional images. The algorithm can be used for the detection of neural activations from functional magnetic resonance images (fMRI). An important characteristic of SPM is that the intensity distribution of background (nonactive area) is known whereas the distributions of activation areas are not. The developed contextual clustering algorithm divides an SPM into background and activation areas so that the probability of detecting false activations by chance is controlled, i.e., hypothesis testing is performed. Unlike the much used voxel-by-voxel testing, neighborhood information is utilized, an important difference. This is achieved by using a Markov random field prior and iterated conditional modes (ICM) algorithm. However, unlike in the conventional use of ICM algorithm, the classification is based only on the distribution of background. The results from our simulations and human fMRI experiments using visual stimulation demonstrate that a better sensitivity is achieved with a given specificity in comparison to the voxel-by-voxel thresholding technique. The algorithm is computationally efficient and can be used to detect and delineate objects from a noisy background in other applications.

The Radiation Transfer of Soft X-rays

AbstractWe discuss the link between the halo plasma temperature and the power-law spectral index of the extragalactic background radiation. This link is of strong influence for the derivation of the Galactic halo intensity distribution. In principal, we can distinguish between two combinations of Galactic halo plasma temperature and power-law slope. The first combination consists of a halo plasma of Thalo = 106 K and an E−2 approximation of the extragalactic background radiation. The second combination is Thalo = 106.2 K and an E−1.5. Both combinations are in agreement with recent observational results, thus it is not feasible to discriminate between both models on the basis of X-ray data available. But, the soft X-ray background intensity distribution in the ¼ keV and ¾ keV ROSAT energy bands differs significantly. The Thalo = 106 K and an E−2 allows a patchy ¼ keV intensity distribution while the Thalo = 106.2 K and an E−1.5 predicts a much smoother intensity variation since the hotter halo plasma accounts for a significant fraction of the ¾ keV background radiation.

A critical investigation of energy‐dispersive microprobe analysis of steel using the Reed/Fiori method of background calculation

AbstractPublished investigations of the energy‐dispersive microprobe analysis of stainless steel repeated using the Reed/Fiori method of calculating the background intensity distribution. It is shown that the ‘interference effect’ in Kβ/Kα intensity ratios for elements in steels, which arose from studies using the Fourier transformation and frequency filtered background subtraction method, is due to deficiencies in this method. The systematically negative errors in analyses of Ni in steel are studied and can be corrected by using the Reed/Fiori method.

On non-uniform illumination in a lens polariscope and remedies

Non-uniform illumination originates from (1) non-uniform transmission of light by model and polariscope elements, (2) diffraction and obstruction of light by the edges of elements and camera shutter, and (3) internal reflections of light from lenses and covering glasses. Effects appear as (1) a locally non-uniform background intensity distribution on the viewing screen or camera film, (2) a peripheral zone of generally reduced intensity, and (3) a bright central spot. This paper shows how local non-uniformities can be detected by photography. The peripheral zone may be avoided by use of a reduced size of field. Remedies for the deleterious bright spot may include a suitably chosen distance between first field lens and model, a small camera aperture and, in extereme cases, a translucent screen between light source and first field lens.