Effective Dynamic Range Research Articles

The new RFX Thomson scattering system

The present Thomson scattering system at the reversed field experiment routinely determines electron temperature and density profiles in various modes of operation but only provides a single profile of each discharge. The new system will use a Nd:YLF laser providing measurements every 20 ms. At the same time we wish to improve the accuracy of the measurement: to this end a new detection system based on filter spectrometers and avalanche photodiodes has been built. The higher throughput of the spectrometers and the higher quantum efficiency of the detectors give an improvement of signal to noise levels of almost two orders. To improve dynamic range the output is ac coupled. Noise figure of detectors has been investigated in detail. The system is novel in the way data are recorded using transient recorders with multiplexing of two channels on each input. The effective accuracy and dynamic range of this system is compared to charge integrating systems. With a look to the future we have also investigated the possible use of multi-element avalanche photodiodes.

Effect of Focal Plane Array Cold Shield Aperture Size on Fourier Transform Infrared Micro-Imaging Spectrometer Performance

Micro-imaging spectrometers incorporating focal plane array (FPA) detection require careful demarcation of cold shield aperture size for both optimal performance and prevention of errors. This study examines the effects of changing the diameter of the cold shield aperture on the intensity and spatial homogeneity of the incident radiation. A uniform polystyrene film was repeatedly imaged by using cold shields of varying aperture sizes. It is shown that a smaller than optimal aperture size leads to image edge clipping, resulting in an inefficient use of the array, lower overall signal, spectral distortions, and higher noise characteristics. Use of an aperture size larger than required causes a decrease in the effective dynamic range of measurements, resulting in higher noise levels. The advantages and necessity of optimizing imaging spectrometer performance by employing a cold shield with an appropriately sized aperture are discussed.

THE DYNAMICS OF HOLOGRAPHIC STORAGE IN DOUBLY DOPED LiNbO3:Fe:Mn

Considering the three transport mechanisms of diffusion, drift and the photovoltaic effect, we set up a set of coupling differential equations that describe the dynamics of the two-color holographic storage in doubly doped LiNbO3:Fe:Mn in the case of small signal and modulation, and solve the equations to explain the time dynamic development process of holographic storage by using the numerical calculation method. On the basis of these results, we analyze the effect of oxidation-reduction on the process of holographic storage, the non-volatile holographic storage is realized only if the sum Na of number density of the acceptors (Fe3+ and Mn3+) is larger than the number density of iron ions N2. Diffraction efficiency of fixed grating increases with oxidation, photorefractive sensitivity decreases with oxidation, higher diffraction efficiency is at the cost of lower photorefractive sensitivity. When the concentration of doped iron is constant, the higher the concentration of doped manganese, the larger the effective dynamic range of state of oxidation-reduction is, in which holograph can be saved permanently.

Overview of the Multi-Spectral Imager on the NEAR spacecraft

The Multi-Spectral Imager on the Near Earth Asteroid Rendezvous (NEAR) spacecraft is a 1 Hz frame rate CCD camera sensitive in the visible and near infrared bands (~400−1100 nm). MSI is the primary instrument on the spacecraft to determine morphology and composition of the surface of asteroid 433 Eros. In addition, the camera will be used to assist in navigation to the asteroid. The instrument uses refractive optics and has an eight position spectral filter wheel to select different wavelength bands. The MSI optical focal length of 168 mm gives a 2.9 ° × 2.25 ° field of view. The CCD is passively cooled and the 537×244 pixel array output is digitized to 12 bits. Electronic shuttering increases the effective dynamic range of the instrument by more than a factor of 100. A one-time deployable cover protects the instrument during ground testing operations and launch. A reduced aperture viewport permits full field of view imaging while the cover is in place. A Data Processing Unit (DPU) provides the digital interface between the spacecraft and the Camera Head and uses an RTX2010 processor. The DPU provides an eight frame image buffer, lossy and lossless data compression routines, and automatic exposure control. An overview of the instrument is presented and design parameters and trade-offs are discussed.

Digital imaging technique for optical emission spectroscopy of a hydrogen arcjet plume

A digital imaging technique has been developed for optical emission spectroscopy measurements of a 1.6-kW hydrogen arcjet plume. Emissions from the Balmer α and β transitions of excited atomic hydrogen were measured with a computer-controlled red-green-blue color CCD detector with and without line-centered bandpass interference filters. A method for extending the effective dynamic range of the detector was developed, whereby images obtained with a wide range of exposure times are combined to form a single composite nonsaturated map of the plume emission structure. The line-of-sight measurements were deconvoluted to obtain the true radial intensity distribution with an inverse Abel transformation. Analysis of the inverted measurements indicates that the upper levels of the Balmer α and β transitions are not thermalized with the electrons in the plasma. The local thermodynamic equilibrium assumption fails for this plasma, and the electron temperature is not equivalent to the apparent excitation temperature obtained when a Boltzmann energy distribution is assumed for the atomic hydrogen excited states.

The ZEUS calorimeter first level trigger

An overview of the ZEUS calorimeter first level trigger is presented. The CFLT uses a pipelined architecture to accept and analyze calorimeter data for every 96 ns beam crossing interval. PMT signals are combined by analog electronics into electromagnetic and hadronic sums for 896 trigger towers. The analog sums are then digitized and analyzed. The CFLT determines the total, transverse, and missing transverse energy, identifies isolated electrons and muons, and sums energies in programmable subregions. Calculations are performed in 96 ns steps, and new data are accepted for every beam crossing. Trigger data are forwarded to the global first level trigger (GFLT) after 2 μs, allowing a GFLT accept to be issued 5 μs after the beam crossing which produced the event. Important features of the CFLT include a 12-bit effective dynamic range, extensive use of memory lookup tables for trigger calculations, fast pattern searches for isolated leptons, and low electronics noise. During the 1993 HERA run, the CFLT reduced a 50 kHz background rate to around 100 Hz.

Performance model for high spatial resolution CsI scintillator screens coupled to CCD detectors

A performance model for an x-ray imaging camera that is currently being developed at York University for the recording of crystallographic diffraction patterns produced by synchrotron radiation is presented. The camera is based on charge coupled device (CCD) sensors which are coupled to a CsI(Tl) x-ray scintillator screen using 3:1 reducing fiber optic tapers. The model predicts the accuracy to which diffraction spot intensities can be measured over a range of incident x-ray flux. The effect of the point spread function of the scintillator and optics and the typical expected diffraction spot geometry is included in the model and shown to have the most significant effect on the system performance for low spot intensities, limiting the camera's effective dynamic range. However, it is shown that quantum limited performance for incident dose fluxes as low as 100 photons per spot can be readily achieved in this design. Finally, the camera performance is predicted for x-ray energies above the scintillator K absorption edges (>33 keV). The effects of energy loss through scintillation K shell fluorescent escape photons reduces the camera's detective quantum efficiency.

Performance Model for High Spatial Resolution Csl Scintillator Screens Coupled to CCD Detectors

A performance model for an x-ray imaging camera that is currently being developed at York University for the recording of crystallographic diffraction patterns produced by synchrotron radiation is presented. The camera is based on charge coupled device (CCD) sensors which are coupled to a CsI(Tl) x-ray scintillator screen using 3:1 reducing fiber optic tapers. The model predicts the accuracy to which diffraction spot intensities can be measured over a range of incident x-ray flux. The effect of the point spread function of the scintillator and optics and the typical expected diffraction spot geometry is included in the model and shown to have the most significant effect on the system performance for low spot intensities, limiting the camera's effective dynamic range. However, it is shown that quantum limited performance for incident dose fluxes as low as 100 photons per spot can be readily achieved in this design. Finally, the camera performance is predicted for x-ray energies above the scintillator K absorption edges (>33 keV). The effects of energy loss through scintillation K shell fluorescent escape photons reduces the camera's detective quantum efficiency.

Improved visualization of simulated nodules by adaptive enhancement of digital chest radiography

Although digital radiography of the chest has a wide dynamic range, difficulties still remain in visualizing the acquired images faithfully on gray-scale displays (cathode-ray tube [CRT] displays), which have a much lower level of luminance than film view boxes. We propose an adaptive-enhancement algorithm for digital chest radiography that provides faithful visualization of the chest on the CRT. We investigated the contrast sensitivity of a CRT monitor and developed an image processing algorithm that compresses the dynamic range and enhances image contrast selectively in the mediastinal area and that transforms the gray scale to visualize the image by use of the full effective dynamic range of the CRT. We performed a receiver-operating characteristic (ROC) study by using simulated nodules to evaluate the clinical value of the proposed algorithm. The processed images provided improved visualization of both mediastinal and lung regions. The area under the ROC curve for retrocardiac or subdiaphragmatic nodule detection increased significantly (from 0.69 to 0.79; P < 0.05). The area under the ROC curve for lung nodule detection also increased (from 0.64 to 0.75; P < 0.1), although not to the level of statistical significance. The proposed algorithm allows improved visualization of nodules on digital chest radiographs with the CRT display.

Performance advances in ion mobility spectrometry through combination with high speed vapor sampling, preconcentration and separation techniques

Rugged, low weight, hand-held ion mobility spectrometry devices, initially developed for chemical warfare detection purposes, possess attractive characteristics as field-portable instruments for paramilitary (treaty verification, chemical demilitarization, drug interdiction, counterterrorism operations) and civilian (environmental monitoring, forensic characterization, process control) applications. Generally, however, such devices tend to exhibit limited resolution, narrow dynamic range, nonlinear response and long clearance times which severely limit their usefulness for qualitative and quantitative analysis of mixtures. To overcome these restrictions a prototype combined gas chromatography-ion mobility spectrometry (GC-IMS) unit was constructed by replacing the membrane inlet of a military IMS device known as the CAM (chemical agent monitor) with suitable front-end modules. These modules enable high speed automated vapor sampling (AVS), microvolume preconcentration/thermal desorption, and isothermal GC preseparation of analytes using a short capillary column while operating the IMS source and cell at subambient pressures as low as 0.5 atm. The AVS-GC-IMS methodology sharply reduces competitive ionization and facilitates identification of mixture components, thereby enabling quantitation of volatile and semivolatile compounds over a broad range of concentration in air. At higher concentration levels (e.g. > 1 ppm) using the AVS inlet in automatic attenuation control (AAC) mode maintains excellent linear response. At ultralow concentration levels, e.g. < 10 ppb, a microvolume, trap-and-desorb type preconcentration module, maintains adequate signal to noise levels, thereby expanding the effective dynamic range of the method to approx. 6 orders of magnitude (100 ppt-100 ppm). The resulting “hyphenated” GC-IMS technique has the potential of evolving into the first hand-portable, combined chromatography-spectroscopy instruments for field screening applications.

A high-quality analog oscillator using oversampling D/A conversion techniques

This paper describes a high-quality analog oscillator for low-frequency applications, which uses a combination of over-sampling and delta-sigma modulation. With the exception of a lowpass filter and a 1-bit D/A, the proposed circuit is entirely digital, providing accurate control over the oscillation frequency and amplitude. At the core of the oscillator is a digital simulation of an LC-tank circuit consisting of two cascaded integrators. This arrangement guarantees oscillation by constraining the poles of the resonator to locations on the z-plane unit-circle, even in a finite-precision implementation. To minimize circuit complexity, the entire oscillator is operated at the oversampled rate, thereby eliminating the associated interpolation filter. Furthermore, the incorporation of a delta-sigma modulator inside the resonator loop leads to a very efficient implementation requiring only 4 multi-bit adders and a 2-input multiplexor. The desired analog signal may be recovered by lowpass filtering the 1-bit output of the delta-sigma modulator. Experiments performed thus far have indicated an effective dynamic range exceeding 80 dB. >

Large dynamic range computations over small finite rings

Presents a new multivariate mapping strategy for the recently introduced Modulus Replication Residue Number System (MRRNS). This mapping allows computation over a large dynamic range using replications of extremely small rings. The technique maintains the useful features of the MRRNS, namely: ease of input coding; absence of a Chinese Remainder Theorem inverse mapping across the full dynamic range; replication of identical rings; and natural integration of complex data processing. The concepts are illustrated by a specific example of complex inner product processing associated with a radix-4 decimation in time fast Fourier transform algorithm. A complete quantization analysis is performed and an efficient scaling strategy chosen based on the analysis. The example processor uses replications of three rings: modulo-3, -5, and -7; the effective dynamic range is in excess of 32 b. The paper also includes very-large-scale-integration implementation strategies for the processor architecture that consists of arrays of massively parallel linear bit-level pipelines. >

Mammographic scanning equalization radiography.

It is well recognized that variations in breast thickness and parenchymal composition can produce a range of exposure which exceeds the latitude of high contrast mammographic film/screen combinations. Optimal imaging of the dense breast is desired since 30%-60% of women present with dense breasts, and they are believed to be at the highest relative risk of developing breast cancer. The application of scanning equalization radiography to mammography has been investigated through the construction and characterization of a prototype mammographic scanning equalization radiography (MSER) system, designed to image mammographic phantoms. The MSER system exposes a Min-R/MRH cassette by raster scanning a 2.0 x 1.6 cm beam of pulsed x-rays across the cassette. A scanning detector behind the cassette measures the local x-ray transmission of the breast. Feedback of the transmission information is used to modulate the duration of each x-ray pulse, to equalize the film exposure. The effective dynamic range of the MSER system is 25 times greater than that of conventional mammography. Artifact-free images of mammographic phantoms show that MSER effectively overcomes the latitude limitations of film/screen mammography, enabling high contrast imaging over a wide range of object x-ray transmission. Anthropomorphic phantom images show that MSER offers up to a sixfold increase in film contrast in the normally underexposed regions of conventional mammograms. Characterization of the entrance exposure shows that there is not a significant difference in exposure between MSER and conventional mammographic techniques, suggesting that both would pose comparable risk to the patient. Calculations show that the construction of a clinical multiple beam MSER system is feasible with minor changes to existing technology.

An Approach to Estimate the Areal Rain-Rate Distribution from Spaceborne Radar by the Use of Multiple Thresholds

Estimates of rain rate derived from a spaceborne weather radar will be most reliable over an intermediate range of values. In forming an area average of the rain rate, an alternative to the averaging of the high-resolution estimates, irrespective of their individual accuracies, is a multiple threshold approach. The method is based on the fact that the fractional area above a particular rain-rate threshold R(j) is related to the cumulative distribution of rain rates evaluated at R(j). Varying the threshold over the effective dynamic range of the radar yields the cumulative distribution function over this range. To obtain the distribution at all rain rates, a lognormal or gamma test function is selected such that the mean-square error between the test function and the measured values is minimized. Once the unknown parameters are determined, the first-order statistics of the areawide rain-rate distribution can be found. Tests of the method with data from the SPANDAR radar provide comparisons between it and the single threshold and the direct averaging approaches.

Phase-scrambled RF excitation for 3D volume-selective multislice NMR imaging.

An RF excitation technique with which one can improve the effective dynamic range of the receiver and reduce the interference between slices for 3D volume-selective multislice MRI is described. The basic idea of the technique is to use phase scrambling in conjunction with slice encoding through the use of RF pulses. The spins in each slice are encoded by RF pulses which have the scrambled as well as slice-encoded phase components along the slice-selection direction. The scrambled, or randomly distributed, phase reduces the peak signal intensity, thereby reducing the dynamic range of the signal. Since the proposed technique utilizes RF slice encoding together with phase scrambling, interslice image interference is greatly reduced and the dynamic range is improved. In addition, the method has several advantages such as a reduction in the power requirement for the RF pulses and the elimination of the necessity for hardware such as nonlinear gradient coils.

A Study of Rain Estimation Methods from Space Using Dual-Wavelength Radar Measurements at Near-Nadir Incidence over Ocean

A question arising from the recent interest in spaceborne weather radar is what methods can be used to estimate precipitation parameters from space. In this paper, dual-wavelength airborne radar data obtained from flights conducted during 1988 and 1989 are used to compare rain rates derived from backscattering and attenuation methods. We begin with a survey of path-averaged rain rates estimated from six methods over four flights. The fairly large number of high rain-rate cases encountered during these experiments allows for the first tests of the surface-reference method applied to the low-frequency (10-GHz) data. To help interpret the results the surface reference methods are studied by means of scatterplots of the surface cross sections at the two frequencies under rain and no-rain conditions. Approximate criteria are given on combining attenuation and backscattering methods to increase the effective dynamic range of the radar. The dual-wavelength capability of the radar is also used to examine the vertical structure of the precipitation: critical to the success of most methods is the ability to distinguish rain from mixed-phase precipitation. Another factor affecting the accuracy of the methods is the drop-size distribution. In the final section of the paper a procedure to estimate the profiled drop-size distribution is applied to the measured radar data.

Range profiling of the rain rate by an airborne weather radar

One of the quantities of interest in a spaceborne weather radar is the rain rate or liquid water content as a function of height. In the presence of attenuation, however, the reconstruction of the profile is possible with usual methods only if the rain rate is uniform or if the relationships between the radar measurables and the rain characteristics are well known. Even with a dual-wavelength radar, estimates of attenuation between adjacent range gates tend to be noisy. In this paper we investigate a class of methods that are based on a measure of path attenuation which is used to constrain the Hitschfeld-Bordan solution. Methods of this type have been investigated for lidar, radar, and combined radar-radiometer applications; their function can be interpreted as allocating the attenuation in proportion to the strength of the measured reflectivity. Four estimates of rain rate are described and tested using data from a dual-wavelength airborne radar at 10 GHz and 35 GHz. When attenuation is significant, the estimates are generally more accurate than those without attenuation correction. This suggests that such methods can be used to extend the effective dynamic range of the radar to higher rain rates. Comparisons between the estimates also provide some diagnostic capability for the detection of radar calibration errors. Instabilities in the estimates can occur, however, in cases of low signal to noise ratios and in situations where partially melted hydrometeors contribute to the attenuation.

Integral transforms, data compression, and automatic analysis of seismic sections

It is shown that seismic sections contain 10/sup 5/ times more data than is needed to fully describe a completely interpreted section. Three ways of reducing the size of a data set are suggested: mapping into subsets of smaller dimension; normalization providing a reduction factor of four or more; and conversion into zero crossings, further reducing the data by a factor of 20. If combined, these methods can reduce the data set by a factor of 100 or more and may also improve the S/N (signal/noise) of the data and their effective dynamic range. In implementing these methods, two families of transforms that are designed to extract the information from the data (integral and morphological transforms) are introduced. Interpretation by visual inspection is more of a pattern classification rather than quantified analysis of the data. Ways to identify such patterns using clustering algorithms are suggested. >

Correction of phase wrapping in magnetic resonance imaging.

In phase reconstruction MR imaging, e.g., for velocity measurement, phase shifts beyond +/- pi radians will "wrap around" to smaller apparent phases. Such large phase shifts could arise either due to large background (non-flow-related) phase variations or due to large velocity-induced phase shifts. For sufficiently smooth phase variation, such discontinuities can be automatically recognized and corrected, thus restoring the correct phase values and extending the effective dynamic range of such phase imaging techniques.

Characterization Of A Charge-Injection-Device Camera System As A Multichannel Spectroscopic Detector

A solid-state camera system designed for use as a multichannel detection system for spectroscopic applications is evaluated. The camera is based on a charge-injection-device sensor that permits very high dynamic range operation by virtue of its unique nondestructive readout capability. Methods of determining gain and readout noise are discussed. Performance of the camera system is evaluated with respect to sensitivity, linearity, readout noise, and dynamic range. The use of nondestructive readout to reduce effective readout noise and increase dynamic range is described. Spectral responses in the visible and ultraviolet of coronene-coated and uncoated devices are reported. Various time-dependent anomalies caused by interface trapped charge are discussed.