Defective Pixels Research Articles

Removing sparse noise from hyperspectral images with sparse and low-rank penalties

In diffraction grating, at times, there are defective pixels on the focal plane array; this results in horizontal lines of corrupted pixels in some channels. Since only a few such pixels exist, the corruption/noise is sparse. Studies on sparse noise removal from hyperspectral noise are parsimonious. To remove such sparse noise, a prior work exploited the interband spectral correlation along with intraband spatial redundancy to yield a sparse representation in transform domains. We improve upon the prior technique. The intraband spatial redundancy is modeled as a sparse set of transform coefficients and the interband spectral correlation is modeled as a rank deficient matrix. The resulting optimization problem is solved using the split Bregman technique. Comparative experimental results show that our proposed approach is better than the previous one.

(Invited) Pixel-Parallel CMOS Image Sensors with 16-Bit A/D Converters Developed by 3-D Integration of SOI Layers with Au/SiO2 Hybrid Bonding

To meet the increasing demands for higher performance of video imaging, several 3D integrated CMOS image sensors have been recently studied by using through-silicon vias (TSVs) and micro-bumps [1-2]. Although these sensors improved some of their performances by 3D integration, in-pixel interconnection was not realized because the TSV or bump size was larger than the imaging pixel size. We have studied on the pixel-parallel 3D integrated CMOS image sensors by using embedded Au electrodes [3-5] suitable for high density integration instead of TSVs or bumps. Fig. 1(a) shows a schematic of the developed CMOS image sensor. Pixels including 128 × 96 photodiodes (PDs) and A/D convertors (ADCs) in the upper layer were connected to 16-bit counters in the lower layer for each pixel. The two functional layers were formed on silicon-on-insulator (SOI) wafers and directly bonded, mediated by Au electrodes embedded in the intermediate SiO2 layer by electroplating and chemical mechanical polishing (CMP). Each pixel size was 80 μm square with a Au electrode of 10 μm in diameter. No voids were observed at the bonded interface. By measuring the input–output response of the developed sensor, we obtained a linear output to the light illumination intensity. The sensor also had a wide dynamic range of 96 dB, which corresponds to the 16-bit output. Signals were detected even under low illumination of 0.1 lx. Fig. 1(b) demonstrates an example of video images captured by the developed sensor. The 16-bit digital output contrast was adjusted to the standard display by compressed to 8-bit data. We confirmed the pixel-parallel operation without any pixel defects. In summary, a 128 × 96 pixel-parallel 3D integrated CMOS image sensor with 16-bit A/D converters was successfully demonstrated by applying Au/SiO2 hybrid bonding of SOI layers. The developed process is promising for CMOS image sensors of higher performance and More-than-Moore type devices for the next generation electronics.

Increases in Hot Pixel Development Rates for Small Digital Pixel Sizes

The rising demand for digital imagers has resulted in the push to reduce pixel size while increasing imager sensitivity, which in turn, results in an increasing rate of defects that develop in the field. Research has shown that “Hot Pixels” are the most common type of defects in modern digital imagers, with their number in a given imager increasing over time. In our previous studies we had developed an empirical formula to project the growth rate of hot pixel defects in terms of defects/year/mm2. We discovered that hot pixel densities tend to grow via a power law of the inverse of the pixel size raised to the power of about 3. This paper explores the effects on defect growth rate, of reducing pixel sizes even more, specifically in cell phone imagers. Due to lack of noise suppression algorithms in these imagers, we have developed specialized procedures for analyzing the collected dark frame data. We also ensure that hot pixel detections in cell phone cameras are statistically significant within the error margins. Our current results confirm the accelerated growth rate for this small pixel range, emphasizing the need for caution by designers and further study in this area of defect development.

Modeling of high-precision wavefront sensing with new generation of CMT avalanche photodiode infrared detectors.

Near-infrared wavefront sensing allows for the enhancement of sky coverage with adaptive optics. The recently developed HgCdTe avalanche photodiode arrays are promising due to their very low detector noise, but still present an imperfect cosmetic that may directly impact real-time wavefront measurements for adaptive optics and thus degrade performance in astronomical applications. We propose here a model of a Shack-Hartmann wavefront measurement in the presence of residual fixed pattern noise and defective pixels. To adjust our models, a fine characterization of such an HgCdTe array, the RAPID sensor, is proposed. The impact of the cosmetic defects on the Shack-Hartmann measurement is assessed through numerical simulations. This study provides both a new insight on the applicability of cadmium mercury telluride (CMT) avalanche photodiodes detectors for astronomical applications and criteria to specify the cosmetic qualities of future arrays.

Pixel-Parallel 3-D Integrated CMOS Image Sensors With Pulse Frequency Modulation A/D Converters Developed by Direct Bonding of SOI Layers

We have developed for the first time a 3-D integrated CMOS image sensor with pixel-parallel analog-to-digital converters (ADCs). Photodiode (PD) and inverter layers are prepared on separate silicon-on-insulator layers and directly bonded with damascened Au electrodes. The handle layer is then removed by grinding and XeF2 vapor phase etching to expose the PD surface. The developed process is suitable for pixelwise interconnection because it allows the damascened Au electrodes to be 1 $\mu \text{m}$ in diameter or less. An ADC circuit is designed based on pulse frequency modulation where pulses are generated proportional to the illumination intensity, and contains a PD, inverters, a reset transistor, and counters. A prototype 3-D integrated CMOS image sensor is also developed with 64 pixels, which acquires video images without pixel defects. A wide dynamic range of >80 dB is confirmed for the incident light intensity. The experimental results demonstrate the feasibility of pixel-level 3-D integration for high-performance CMOS image sensors.

Effects of defect pixel correction algorithms for x-ray detectors on image quality in planar projection and volumetric CT data sets

In this study we compared various defect pixel correction methods for reducing artifact appearance within projection images used for computed tomography (CT) reconstructions.Defect pixel correction algorithms were examined with respect to their artifact behaviour within planar projection images as well as in volumetric CT reconstructions. We investigated four algorithms: nearest neighbour, linear and adaptive linear interpolation, and a frequency-selective spectral-domain approach.To characterise the quality of each algorithm in planar image data, we inserted line defects of varying widths and orientations into images. The structure preservation of each algorithm was analysed by corrupting and correcting the image of a slit phantom pattern and by evaluating its line spread function (LSF). The noise preservation was assessed by interpolating corrupted flat images and estimating the noise power spectrum (NPS) of the interpolated region.For the volumetric investigations, we examined the structure and noise preservation within a structured aluminium foam, a mid-contrast cone-beam phantom and a homogeneous Polyurethane (PUR) cylinder.The frequency-selective algorithm showed the best structure and noise preservation for planar data of the correction methods tested. For volumetric data it still showed the best noise preservation, whereas the structure preservation was outperformed by the linear interpolation.The frequency-selective spectral-domain approach in the correction of line defects is recommended for planar image data, but its abilities within high-contrast volumes are restricted. In that case, the application of a simple linear interpolation might be the better choice to correct line defects within projection images used for CT.

Failure mechanisms of microbolometer thermal imager sensors using chip-scale packaging

This paper analyzes relevant failure mechanisms for microbolometer thermal imager sensors that are assembled with a small size and low cost chip scale package. The analyses focus on device specific elements like the bolometer sensor structures, the longtime stability of the sensor and its performance, and the stability of the hermetic chip scale package. Executed reliability tests showed a high reliability of the sensor and the package without hard failures. The package survived harsh environmental accelerated stress tests and showed only a slight reduction of the shear strength through void formation and small cracks within the lead frame that could be verified through FEM simulations. The stress on the bolometers is investigated by thermomechanical FEM simulations. Executed reliability tests showed no enlargement in the number of defect pixel. The sensor performance showed a longtime drift and temperature dependence through outgassing processes inside the package leading to a significant performance reduction. Thus this effect is investigated more closely and possible countermeasures are proposed.

Calibration of uncooled thermal infrared cameras

Abstract. The calibration of uncooled thermal infrared (IR) cameras to absolute temperature measurement is a time-consuming, complicated process that significantly influences the cost of an IR camera. Temperature-measuring IR cameras display a temperature value for each pixel in the thermal image. Calibration is used to calculate a temperature-proportional output signal (IR or thermal image) from the measurement signal (raw image) taking into account all technical and physical properties of the IR camera. The paper will discuss the mathematical and physical principles of calibration, which are based on radiometric camera models. The individual stages of calibration will be presented. After start-up of the IR camera, the non-uniformity of the pixels is first corrected. This is done with a simple two-point correction. If the microbolometer array is not temperature-stabilized, then, in the next step the temperature dependence of the sensor parameters must be corrected. Ambient temperature changes are compensated for by the shutter correction. The final stage involves radiometric calibration, which establishes the relationship between pixel signal and target object temperature. Not all pixels of a microbolometer array are functional. There are also a number of defective, so-called "dead" pixels. The discovery of defective pixels is a multistep process that is carried out after each stage of the calibration process.

73.1: A Novel Pixel Structure for High Transmittance and High Image Quality LCDs

We propose a new 2P2D pixel design which has a trapezoidal shape. for high transmittance and image quality.The pixel design facilitates a reduction of disclination area in the pixel and image defects. As a result, we have achieved 14% higher transmittance than the conventional AH5‐IPS and also a better image characteristic than the general 2P2D structure in 5inch HD LCD panels.

SU‐C‐304‐05: Use of Local Noise Power Spectrum and Wavelets in Comprehensive EPID Quality Assurance

Purpose:As EPIDs are increasingly used for IMRT QA and real‐time treatment verification, comprehensive quality assurance (QA) of EPIDs becomes critical. Current QA with phantoms such as the Las Vegas and PIPSpro™ can fail in the early detection of EPID artifacts. Beyond image quality assessment, we propose a quantitative methodology using local noise power spectrum (NPS) to characterize image noise and wavelet transform to identify bad pixels and inter‐subpanel flat‐fielding artifacts.Methods:A total of 93 image sets including bar‐pattern images and open exposure images were collected from four iViewGT a‐Si EPID systems over three years. Quantitative metrics such as modulation transform function (MTF), NPS and detective quantum efficiency (DQE) were computed for each image set. Local 2D NPS was calculated for each subpanel. A 1D NPS was obtained by radial averaging the 2D NPS and fitted to a power‐law function. R‐square and slope of the linear regression analysis were used for panel performance assessment. Haar wavelet transformation was employed to identify pixel defects and non‐uniform gain correction across subpanels.Results:Overall image quality was assessed with DQE based on empirically derived area under curve (AUC) thresholds. Using linear regression analysis of 1D NPS, panels with acceptable flat fielding were indicated by r‐square between 0.8 and 1, and slopes of −0.4 to −0.7. However, for panels requiring flat fielding recalibration, r‐square values less than 0.8 and slopes from +0.2 to −0.4 were observed. The wavelet transform successfully identified pixel defects and inter‐subpanel flat fielding artifacts. Standard QA with the Las Vegas and PIPSpro phantoms failed to detect these artifacts.Conclusion:The proposed QA methodology is promising for the early detection of imaging and dosimetric artifacts of EPIDs. Local NPS can accurately characterize the noise level within each subpanel, while the wavelet transforms can detect bad pixels and inter‐subpanel flat fielding artifacts.

Reliability of Microbolometer Thermal Imager Sensors Using Chip-scale Packaging

This paper analyses relevant failure mechanisms for microbolometer thermal imager sensors that are assembled with a small size and low cost chip scale package. The analyses focus at device specific elements like the bolometer sensor structures, the longtime stability of the sensor and its performance, and the stability of the hermetic chip scale package. Executed reliability tests showed a high reliability of the sensor and the package without hard failures. The package survived harsh environmental accelerated stress tests and showed only a slight reduction of the shear strength through void formation and small cracks within the lead frame that could be verified through FEM simulations. The stress on the bolometers is investigated by thermomechanical FEM simulations. Executed reliability tests showed no enlargement in the number of defect pixel. The sensor performance showed a longtime drift and temperature dependence through outgassing processes inside the package leading to a significant performance reduction. Thus this effect is investigated closer and possible countermeasures are proposed.

Flaw detection and measurement for mobile display

With advanced small-pixel and high-resolution mobile displays, the display are obtaining smaller pixel size and higher resolution on thin-film-transistor (TFT) array process. This study proposed the flaw detection and measurement in small pixel design mobile displays based on oxide-semiconductor-based TFT liquid crystal displays. The measurement is with respect to electrical-physic characterisation metrics and observer studies. Studies resulted in small-sized pixel TFT array between the flaw detection performance and maker preference approaches. Detection performance factors provided information on the differences among small pixel design for advanced mobile display technologies. The results of the detection performance and preference study showed that the application for advanced mobile display panels. It also shown that critical pixel defect played an important role than previously seen in non-small pixel TFT array panels for managing process yield. The method of voltage imaging for detection, developed in this study provides an initial insight into the high resolution of small-sized pixel designs for advanced mobile displays with precision and diagnostic images in portable devices.

Radiation damage of the PCO Pixelfly VGA CCD camera of the BES system on KSTAR tokamak

A PCO Pixelfly VGA CCD camera which is part a of the Beam Emission Spectroscopy (BES) diagnostic system of the Korea Superconducting Tokamak Advanced Research (KSTAR) used for spatial calibrations, suffered from serious radiation damage, white pixel defects have been generated in it. The main goal of this work was to identify the origin of the radiation damage and to give solutions to avoid it. Monte Carlo N-Particle eXtended (MCNPX) model was built using Monte Carlo Modeling Interface Program (MCAM) and calculations were carried out to predict the neutron and gamma-ray fields in the camera position. Besides the MCNPX calculations pure gamma-ray irradiations of the CCD camera were carried out in the Training Reactor of BME. Before, during and after the irradiations numerous frames were taken with the camera with 5s long exposure times. The evaluation of these frames showed that with the applied high gamma-ray dose (1.7Gy) and dose rate levels (up to 2Gy/h) the number of the white pixels did not increase. We have found that the origin of the white pixel generation was the neutron-induced thermal hopping of the electrons which means that in the future only neutron shielding is necessary around the CCD camera. Another solution could be to replace the CCD camera with a more radiation tolerant one for example with a suitable CMOS camera or apply both solutions simultaneously.

Infrared modeling and imaging simulation of midcourse ballistic targets based on strap-down platform

An infrared (IR) imaging simulation framework based on the strap-down platform is proposed for midcourse ballistic targets. It overcomes the shortcoming of the existing algorithms, which cannot simulate IR imaging from the entire midcourse process. The proposed framework includes three steps, target characteristic modeling, motion modeling, and imaging modeling. In imaging modeling, the staring focal plane is taken into account due to its wide employment. In order to obtain IR images of high fidelity, especially that the fluctuation of the target signal-to-noise ratio (SNR) is reasonably similar to the actual one, this paper proposes an improved IR imaging simulation method. The proposed method considers two critical factors of the pixel plane, occupy-empty ratio and defect elements, which affect the imaging of targets markedly but are neglected in previous work. Finally, the IR image sequence of high fidelity is obtained. And the correlative parameters of simulation can be set according to the given scene. Thus the generated images can satisfy the needs of algorithms validation for tracking and recognition.

Hardware-Software Complex for Laser Scanning in Color Sorting Machines

The paper concerns the problem of grain mixture analysis based on processing images synthesized during line-by-line scanning of each object in color sorting systems. The paper presents a hardware-software complex for sorting objects in the real time. The hardware part of the complex consists of two blocks: light source and device for reception and processing of images. The light source is a laser, passed through optical fiber and linearly expanded across the entire width of the photoseparator’s chute. Linear laser scan produces significant intensity of illumination. It is sufficient for working on the transmission of radiation through objects. The software part of the complex also consists of two blocks: the thresholding algorithm and an automated program for finding the optimal parameters of sorting on the basis of that algorithm. The algorithm calculates the number of connected defective pixels with arbitrary shape. Automated program works on the basis of pre-formed images of the objects of two classes: good and defective. As a result, the program displays in tabular form the most optimal sorting parameters. The program shows the dependence of the loss of good product from the missouts of the defective objects. The customer gets a clear choice of the most suitable sorting results. This complex was tested for sorting of unshelled rice seeds via transmission. It is shown that the complex allows to effectively detect hidden seed defects: red pigmentation, immaturity, fungal diseases, and others.

A study on full color organic light emitting diodes with blue common layer under the patterned emission layer

Color patterning steps for red, green, and blue emission layers (EMLs) are crucial for the production of full color organic light-emitting diodes (OLEDs). The most common method to form individually patterned EMLs is to use a shadow mask as the key component for patterning. However, most pixel defects are caused by such kinds of patterning steps. Therefore, skipping certain color patterning steps could significantly improve the production yield during the fine metal masking process in the OLED fabrication. A representative example of such approach is the top blue common layer (TBCL) structure with a non-patterned BCL on top of both green and red EMLs. However, this structure could cause blue color mixing in green or red devices. To prevent this effect, we propose a newly devised bottom BCL (BBCL) structure with the BCL that is totally separated from both green and red EMLs. In particular, we utilized 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile interlayer (7nm) between the underlying BCL and the hole injection layer to completely extinguish blue emission. As a result, both green and red devices with the BBCL structure showed relatively better efficiencies compared to those with the TBCL structure without any color mixing.

Improvements of MCT MBE Growth on GaAs

In recent years, continuous progress has been published in the development of HgCdTe (MCT) infrared (IR) focal plane arrays (FPAs) fabricated by molecular beam epitaxy on GaAs substrates. In this publication, further characterization of the state-of-the art 1280 × 1024 pixel, 15-μm pitch detector fabricated from this material in both the mid-wavelength (MWIR) and long-wavelength (LWIR) IR region will be presented. For MWIR FPAs, the percentage of defective pixel remains below 0.5% up to an operating temperature (TOP) of around 100 K. For the LWIR FPA, an operability of 99.25% was achieved for a TOP of 76 K. Additionally, the beneficial effect of the inclusion of MCT layers with a graded composition region was investigated and demonstrated on current–voltage (IV) characteristics on test diodes in a MWIR FPA.

Free software for performing physical analysis of systems for digital radiography and mammography.

In this paper, the authors present a free software for assisting users in achieving the physical characterization of x-ray digital systems and image quality checks. The program was developed as a plugin of a well-known public-domain suite ImageJ. The software can assist users in calculating various physical parameters such as the response curve (also termed signal transfer property), modulation transfer function (MTF), noise power spectra (NPS), and detective quantum efficiency (DQE). It also includes the computation of some image quality checks: defective pixel analysis, uniformity, dark analysis, and lag. The software was made available in 2009 and has been used during the last couple of years by many users who gave us valuable feedback for improving its usability. It was tested for achieving the physical characterization of several clinical systems for digital radiography and mammography. Various published papers made use of the outcomes of the plugin. This software is potentially beneficial to a variety of users: physicists working in hospitals, staff working in radiological departments, such as medical physicists, physicians, engineers. The plugin, together with a brief user manual, are freely available and can be found online (www.medphys.it/downloads.htm). With our plugin users can estimate all three most important parameters used for physical characterization (MTF, NPS, and also DQE). The plugin can run on any operating system equipped with ImageJ suite. The authors validated the software by comparing MTF and NPS curves on a common set of images with those obtained with other dedicated programs, achieving a very good agreement.

星地激光通信ATP系统探测相机的坏点校正

星地激光通信ATP系统探测相机的坏点校正

Linear array of InAs APDs operating at 2 µm

A linear array of avalanche photodiodes (APDs) comprising of 128 pixels was fabricated from InAs. The uniformity of the dark currents and avalanche gain was investigated at 77, 200 K and room temperature. The array shows highly uniform results apart from some defective pixels at the edge of the arrays. At 200 K and at a wavelength of 2.04 µm, we obtained an unmultiplied responsivity of 0.61 A/W at 0 V, along with a gain of 8.5 at a bias of 10 V.