Infinite Dynamic Range Research Articles

Charge sharing simulations and measurements for digital algorithms aiming at subpixel resolution in photon counting pixel detectors

Hybrid pixel detectors are segmented devices used for X-ray detection that consist of a sensor attached to the readout electronics. Detectors working in single-photon counting mode process each incoming photon individually, have essentially infinite dynamic range, and by applying energy discrimination they provide noiseless imaging. To improve the resolution of the detector and allow operation with high-intensity photon fluxes, the pixel size is reduced. However, with decreasing pixel size, a charge sharing effect is more prominent. This leads to false event registration or omitting the event, and degradation of the energy resolution of the detector. Algorithms aiming at reducing the influence of charge sharing have already been implemented on-chip. However, the spatial resolution of the detector can be increased beyond the physical size of the pixel if the charge proportions collected by neighboring pixels are analyzed.The simulations show that charge cloud size referred to pixel size and noise are the key parameters that determine the accuracy of the subpixel algorithm. The article shows the concept of subpixel algorithm and the simulations for different detector parameters and approximation algorithms. The slanted-edge method was implemented to quantify the resolution of detectors consisting standard readout architecture. The chips were simulated and tested to verify the influence of different bias voltages, sensor materials, and thicknesses on charge sharing and as a consequence on the detector resolution. The simulated and measured edge spread functions were compared. The results show that the simulator can be used to describe the spatial resolution of the detectors, and can be used for further studies of the resolution of detectors with subpixel algorithm implemented.

On the Identifiability of Sparse Vectors From Modulo Compressed Sensing Measurements

Compressed sensing deals with recovery of sparse signals from low dimensional projections, but under the assumption that the measurement setup has infinite dynamic range. In this letter, we consider a system with finite dynamic range, and to counter the clipping effect, the measurements crossing the range are folded back into the dynamic range of the system through modulo arithmetic. For this setup, we derive theoretical results on the minimum number of measurements required for unique recovery of sparse vectors. We also show that recovery using the minimum number of measurements is achievable by using a measurement matrix whose entries are independently drawn from a continuous distribution. Finally, we present an algorithm based on convex relaxation and develop a mixed integer linear program (MILP) for recovering sparse signals from the modulo measurements. Our empirical results demonstrate that the minimum number of measurements required for recovery using the MILP algorithm is close to the theoretical result for signals with low variance.

Adaptive High Dynamic Range for Time-of-Flight Cameras

The limited dynamic range of conventional digital cameras is a well-known problem. A common solution is to apply high-dynamic-range (HDR) techniques over several images acquired using different exposure times. Time-of-flight (ToF) cameras using a photonic mixer device (PMD) are not an exception, since the dynamic range of its dual pixels is also limited. Furthermore, in this case, the saturation of the pixel channels leads to wrong depth measurements. An appropriate solution is the suppression of background illumination (SBI) system designed by the PMD. This system actually extends the dynamic range of the camera by hardware, but it also introduces noise when activated. In this paper, we present an adaptive HDR (AHDR) solution to the problem for the ToF case that overcomes the limited dynamic range of the system, allowing sensing along a theoretically infinite dynamic range with the only limitations of the power of the illumination system and the decay of the SNR with higher distances or lower illumination intensities. Our method is able to detect and segment relevant scene regions responsible for unexpected saturation, i.e., close foreground objects, from the rest of the scene and adjust the exposure times of the acquired images considering them. The results show a reduction in detail losses and a higher SNR in the AHDR raw images, with respect to single acquisitions. This results in a dramatic depth error reduction and effective axial resolution improvement in critical areas, while keeping a high frame rate. In addition, the SBI-related noise is eliminated.

Which uses the most spectrum: transmitters or receivers?

When the author first got involved in spectrum management more than 30 years ago he was surprised to hear from colleagues at FCC that "transmitters don't use up spectrum, receivers do". This at first seems to contradict what one learns in basic electrical engineering courses where it is taught how to calculate the bandwidth of a signal as a function of its modulation. However, we also learn that radio signals that transmit real information (in the Shannon sense) also must have infinite bandwidth. Finite bandwidth channels must of necessity distort the signal somewhat. Realizable receivers can not have a finite bandwidth with a "brick wall" frontend that excludes all power outside the intended bandwidth. And finally practical receivers cannot have infinite dynamic range and exhibit nonlinear characteristics if signal limits are exceeded. When all these issues are combined it turns out that that achievable minimum separation in space and frequency between multiple transmitters and their receivers depends critically on receiver characteristics in most cases.

Vector Perturbation Precoding Revisited

We consider the downlink of a multiuser system with multiple antennas at the base station. Vector perturbation (VP) precoding is a promising variant of transmit-side channel inversion allowing the users to detect their data in a simple, noncooperative manner. VP precoding has so far been developed and analyzed under the assumptions that the transmitter has perfect channel state information (CSI) and that the receivers know perfectly a channel-dependent transmit power normalization factor and have infinite dynamic range. We demonstrate that the violation of any of these idealizing assumptions degrades the performance of VP significantly and almost always results in an error floor. Motivated by this observation, we propose a novel scheme which we term transmit outage precoding (TOP). With TOP, the transmitter uses a prearranged power scaling known by the receivers and refrains from transmitting when channel conditions are poor. We further show how to augment TOP and conventional VP to deal with a finite dynamic range at the receiver. The performance of the proposed schemes under various levels of transmit CSI is studied in terms of a theoretical diversity analysis and illustrated by numerical results.

Dynamic-Range Widening in a CMOS Image Sensor Through Exposure Control Over a Dual-Photodiode Pixel

In this paper, we propose a technique for automatic-exposure control and synthesis for a wide-dynamic-range sensor based on dual-exposure method. Using the proposed technique, a sensor can adaptively adjust its exposure to widely varying illumination conditions and consequently accomplish an infinite dynamic range in theory. An artificial test bench consisting of a virtual illumination source and its imaging system is introduced to verify the performance of the proposed technique. Simulations show how a wider dynamic range can be achieved by the proposed technique. It is also shown that a VGA-resolution CMOS image sensor developed based on the proposed technique can support up to a 119-dB dynamic range.

Medipix 2 detector applied to low energy electron microscopy

Low energy electron microscopy (LEEM) and photo-emission electron microscopy (PEEM) traditionally use microchannel plates (MCPs), a phosphor screen and a CCD-camera to record images and diffraction patterns. In recent years, however, MCPs have become a limiting factor for these types of microscopy. Here, we report on a successful test series using a solid state hybrid pixel detector, Medipix 2, in LEEM and PEEM. Medipix 2 is a background-free detector with an infinite dynamic range, making it very promising for both real-space imaging and spectroscopy. We demonstrate a significant enhancement of both image contrast and resolution, as compared to MCPs. Since aging of the Medipix 2 detector is negligible for the electron energies used in LEEM/PEEM, we expect Medipix to become the detector of choice for a new generation of systems.

High-Order Band-Limited Coronagraphs in Adaptive Optics Systems

In principle, band-limited coronagraphic image masks are capable of removing all on-axis starlight providing infinite dynamic range for the direct imaging of exoplanets. In practice, optical aberrations left uncorrected by the adaptive optics system and practical limitations on mask alignment prevent band-limited masks from reaching contrast levels necessary for exoplanet imaging. So-called, “high-order” band-limited masks reduce the coronagraph's sensitivity to low order aberrations, and have been measured to reach contrast levels of without the aid of a deformable mirror. In this work, we simulate the performance of high-order band-limited coronagraphic image masks in a variety of adaptive optics systems. We compare their performance with that of hard-edge stops, soft-edge Gaussian masks, and traditional (4th-order) band-limited coronagraphs.

Characterization of digital waveforms using thermodynamic analogs: applications to detection of materials defects

We describe characterization of digital signals using analogs of thermodynamic quantities: the topological entropy, Shannon entropy, thermodynamic energy, partition function, specific heat at constant volume, and an idealized version of Shannon entropy in the limit of digitizing with infinite dynamic range and sampling rate. We show that analysis based on these quantities is capable of detecting differences between digital signals that are undetectable by conventional methods of characterization based on peak-to-peak amplitude or signal energy. We report the results of applying thermodynamic quantities to a problem from nondestructive materials evaluation: detection of foreign objects (FO) embedded near the surface of thin graphite/epoxy laminates using backscattered waveforms obtained by C-scanning the laminate. The characterization problem was to distinguish waveforms acquired from the region containing the FO from those acquired outside. In all cases the thermodynamic analogs exhibit significant increases (up to 20-fold) in contrast and for certain types of FO materials permit detection when energy or amplitude methods fail altogether.

Using Notch‐Filter Masks for High‐Contrast Imaging of Extrasolar Planets

We present the first laboratory experiments using a notch-filter mask, a coronagraphic image mask that can produce infinite dynamic range in an ideal Lyot coronagraph, according to scalar diffraction theory. We fabricated the first notch-filter mask prototype with 0.25 μm precision using an electron-beam lithography machine. Our initial optical tests show that the prototype masks generate contrast levels of 10-5 at 3λ/D and 10-6 at ~8λ/D, with a throughput of 27%. We speculate on the "as-is" performance of such a mask in the Hubble Space Telescope.

A phase digitizer for digital radio frequency memory

Abstract This paper presents a general method for designing a phase digitizer incorporating a D/A converter. Systematic derivation of design method as well as performance analysis are given for N‐bit phase digitizer, and implementations of the 3‐bit and 4‐bit phase digitizers are provided. In addition, the design method includes two different design models. The phase digitizer quantizes the phase of the incoming signal rather than the amplitude, thus it offers the advantage of being invariant of amplitude fluctuation to a theoretically infinite dynamic range.

Restored Impulse Response of Finite-Range Image Deblurring Filter

In the absence of noise, a linear spatially invariant blur is completely removed by the application of a transfer function proportional to the inverse of the blur transfer function. However, most blur transfer functions of interest either contain zeros or approach zero; and, consequently, the inverse should cover an infinite dynamic range. In these cases, the inverse can be approximated by a holographic spatial filter that employs a finite range of amplitude transmittance. The effect of finite range on the restored impulse response or point spread function of the combined blurring and deblurring operations are studied. A computer simulation is compared with experimental results for the case of severe defocusing through a circular aperture. (auth)

Versatile Spectrophotofluorometer-Polarization Fluorometer

A highly versatile spectrophotofluorometer with unique optical as well as electronic features is described. In addition to corrected excitation, emission, and polarization spectra, absorption and transmission spectra can also be produced. Spectra may be recorded as a function of wavelength or wavenumber. The sample area is designed to permit temperature and pH control and use of an automatic titration assembly. A three quartz plate beam splitter produces two identical beams (within 0.5%) over a range of 280 to 600 mμ, regardless of the polarization of the incident beam, thus allowing the measurement of accurate corrected excitation spectra. Total stray light present at exciting wavelengths from 210 to 240 mμ was measured: it amounted to 7% at 240 mμ. Polarization spectra (excitation as well as emission) can be measured continuously without altering the apparatus; a discriminator was built which separates two normally polarized components. Degree of polarization can be obtained within 1%, and this figure can be further improved by correcting for imperfect discrimination of the two polarized components. Corrections for light source fluctuations and nonlinearities as a function of wavelength and continuous polarization measurements are effected by a digital ratio system which allows unlimited time integration and has an infinite dynamic range. The long range drift introduced by this system and the photomultiplier current preamplifiers is below 0.03%. The limit on overall stability is determined by the photomultipliers, and amounts to 0.5% over the long range (due to gain variations).