Pixel Crosstalk Research Articles

Model-based compensation of pixel crosstalk in liquid crystal spatial light modulators.

Spatial light modulators (SLMs) based on liquid crystals are widely used for wavefront shaping. Their large number of pixels allows one to create complex wavefronts. The crosstalk between neighboring pixels, also known as fringing field effect, however, can lead to strong deviations. The realized wavefront may deviate significantly from the prediction based on the idealized assumption that the response across a pixel is uniform and independent of its neighbors. Detailed numerical simulations of the SLM response based on a full 3D physical model accurately match the measured response and properly model the pixel crosstalk. The full model is then used to validate a simplified model that enables much faster crosstalk evaluation and pattern optimization beyond standard performance. General conclusions on how to minimize crosstalk in liquid crystal on silicon (LCoS) SLM systems are derived, as well as a readily accessible estimation of the amount of fringing in a given SLM.

Optical 3D object security and reconstruction using pixel-evaluated integral imaging algorithm.

Under the framework of computational integral imaging, an optical 3D objects security and high-quality reconstruction method based on pixel-evaluating mapping (PEM) algorithm is proposed. In this method, the pixel crosstalk caused by noneffective pixel overlap is effectively reduced by a pixel-evaluated mask, which can improve the image quality of the reconstructed 3D objects. Meanwhile, compared with the other computational integral imaging reconstruction methods, our proposed PEM algorithm can obtain more accurate pixel mapping weight parameters, thereby the reconstructed 3D objects provide higher quality. In addition, the nonlinear feedback shift register cellular automata algorithm is proposed to increase the security of the proposed method. We have experimentally verified the proposed 3D objects encryption and reconstruction algorithm. The experimental results show that the proposed method is superior to the other computational reconstruction methods.

Longitudinal mode seeding in modulated InGaN laser diodes

The modulation of InGaN laser diodes is important for applications such as laser projection in cinemas as well as in virtual and augmented reality applications. Here, a modulation frequency in the 100 MHz to 1 GHz range is necessary. On this timescale, pixel crosstalk is a major issue that affects the intensity and spectral properties. We investigate the spectral and temporal dynamics of InGaN laser diodes driven with different pulse patterns. We identify longitudinal mode seeding as a mechanism between interacting pulses which are spaced by several nanoseconds: photons, remaining in the cavity after the trailing edge of the electric pulse, “seed” consequent pulses and promoting their own longitudinal mode. This leads to a changed spectral–temporal mode pattern of the consequent pulse. The long cavity ring-down time of these photons is a consequence of relatively low losses. Further investigations were performed by streak camera measurements of differently biased laser diodes. For a bias below, but close to, the threshold, the optical gain nearly compensates internal and mirror losses. Additional simulations are performed to confirm the experimental results.

Technical Note: Noise models for virtual clinical trials of digital breast tomosynthesis.

To investigate the use of an affine-variance noise model, with correlated quantum noise and spatially dependent quantum gain, for the simulation of noise in virtual clinical trials (VCT) of digital breast tomosynthesis (DBT). Two distinct technologies were considered: an amorphous-selenium (a-Se) detector with direct conversion and a thallium-doped cesium iodide (CsI(Tl)) detector with indirect conversion. A VCT framework was used to generate noise-free projections of a uniform three-dimensional simulated phantom, whose geometry and absorption match those of a polymethyl methacrylate (PMMA) uniform physical phantom. The noise model was then used to generate noisy observations from the simulated noise-free data, while two clinically available DBT units were used to acquire projections of the PMMA physical phantom. Real and simulated projections were then compared using the signal-to-noise ratio (SNR) and normalized noise power spectrum (NNPS). Simulated images reported errors smaller than 4.4% and 7.0% in terms of SNR and NNPS, respectively. These errors are within the expected variation between two clinical units of the same model. The errors increase to 65.8% if uncorrelated models are adopted for the simulation of systems featuring indirect detection. The assumption of spatially independent quantum gain generates errors of 11.2%. The investigated noise model can be used to accurately reproduce the noise found in clinical DBT. The assumption of uncorrelated noise may be adopted if the system features a direct detector with minimal pixel crosstalk.

Positioning and Data Broadcasting Using Illumination Pattern Sequences Displayed by LED Arrays

Illumination of a scene with a time sequence of spatial light patterns enables clients within the scene to navigate, receive broadcast wireless data, or make subsequent space-division multiple access connections to a high bandwidth wireless system. We have developed dedicated binary pattern sequences, for use with arrays of light-emitting diodes (LEDs), which are projected on the area of interest. The LED arrays can be in either active matrix or matrix-addressable format. The properties of the different sequences are compared theoretically and experimentally, highlighting a tradeoff between position update rate and resilience against pixel cross-talk and interference.

Front and Back Illuminated Dynamic and Active Pixel Vision Sensors Comparison

Back side illumination has become standard image sensor technology owing to its superior quantum efficiency and fill factor. A direct comparison of front and back side illumination (FSI and BSI) used in event-based dynamic and active pixel vision sensors (DAVIS) is interesting because of the potential of BSI to greatly increase the small 20% fill factor of these complex pixels. This brief compares identically designed front and back illuminated DAVIS silicon retina vision sensors. They are compared in term of quantum efficiency (QE), leak activity and modulation transfer function (MTF). The BSI DAVIS achieves a peak QE of 93% compared with the FSI DAVIS, peak QE of 24%, but reduced MTF, due to pixel crosstalk and parasitic photocurrent. Significant “leak events” in the BSI DAVIS limit its use to controlled illumination scenarios without very bright light sources. Effects of parasitic photocurrent and modulation transfer functions with and without IR cut filters are also reported.

Parametric numerical study of the modulation transfer function in small-pitch InGaAs/InP infrared arrays with refractive microlenses.

We report on the modulation transfer function (MTF) in short-wave infrared indium gallium arsenide (InGaAs) on indium phosphide (InP) planar photodetector arrays. Our two-dimensional numerical method consists of optical simulations using the finite-difference time domain method and drift-diffusion simulations using the finite-element method. This parametric study investigates MTF dependence on pitch, the addition of refractive microlenses, the thickness of the InGaAs absorber, and the doping concentration of the InGaAs absorber. A focus is placed on the connection between the lateral diffusion of photogenerated holes in InGaAs and the MTF. It is found that the MTF of small-pitch arrays exhibit sub-ideal behavior due to pixel cross-talk resulting from a long minority carrier diffusion length. By incorporating monolithic microlenses with the InP substrate, the MTF response is improved for all pitches investigated, particularly for spatial frequencies near the respective cutoff frequencies. We also find a strong dependence of the MTF on the thickness and doping concentration in the absorbing region. Trends in dark current and quantum efficiency are reported.

Quantitative analysis of pixel crosstalk in AMOLED displays

ABSTRACTThe resolution of organic light-emitting diode (OLED) displays is increasing steadily as these displays are adopted for mobile and virtual reality (VR) devices. This leads to a stronger pixel crosstalk effect, where the neighbors of active pixels unintentionally emit light due to a lateral electric current between the pixels. Recently, the crosstalk was quantified by measuring the current flowing through the common hole transport layer between the neighboring pixels and comparing it to the current through the active pixel diode [S.-K. Kwon, K.-S. Kim, H.-C. Choi and J. H. Kwon, presented at the International Meeting on Information Display, Jeju, South Korea, 2016]. The measurements showed that the crosstalk is more crucial for low light levels. In such cases, the intended and parasitic currents are similar. The simulations performed in this study validated these measurement results. By simulations, we quantify the crosstalk current through the diode. The luminous intensity can be calculated with the measured current efficiency of the diodes. For low light levels, the unintended luminance can reach up to 40% of the intended luminance. The luminance due to pixel crosstalk is perceivable by humans. This effect should be considered for OLED displays with resolutions higher than 300 PPI.

基于光线追迹的LED裸眼三维显示技术准直性研究

In the integrated imaging naked eye 3D display based on LED, the divergence angle of the pixel point of LED large screen was very large. Flat convex and double convex lens are often employed. The reconstructed light of the image diverges and the pixel crosstalk between adjacent lens units was large, resulting in unclear images during the reproduction of the three-dimensional scene with the reduced imaging quality. The imaging process of integrated imaging based on LED was analyzed according to the principle of ray tracing. The relationship of crosstalk between different shapes of the lens array and adjacent lens units of LED integrated imaging was investigated. A single meniscus lens was used to effectively control the divergence of the reconstructed light. The reconstructed light passing through the lens was more convergent and collimated. The emergent rays through the lens becomes more parallel and the crosstalk between adjacent lens units was reduced effectively. By comparing the simulation result of the letter model, the meniscus lens was more suitable for LED integrated imaging. The imaging effect of the meniscus lens is better than that of the other lens.

Method for Simulating Dose Reduction in Digital Breast Tomosynthesis

This paper proposes a new method of simulating dose reduction in digital breast tomosynthesis, starting from a clinical image acquired with a standard radiation dose. It considers both signal-dependent quantum and signal-independent electronic noise. Furthermore, the method accounts for pixel crosstalk, which causes the noise to be frequency-dependent, thus increasing the simulation accuracy. For an objective assessment, simulated and real images were compared in terms of noise standard deviation, signal-to-noise ratio (SNR) and normalized noise power spectrum (NNPS). A two-alternative forced-choice (2-AFC) study investigated the similarity between the noise strength of low-dose simulated and real images. Six experienced medical physics specialists participated on the study, with a total of 2 160 readings. Objective assessment showed no relevant trends with the simulated noise. The relative error in the standard deviation of the simulated noise was less than 2% for every projection angle. The relative error of the SNR was less than 1.5%, and the NNPS of the simulated images had errors less than 2.5%. The 2-AFC human observer experiment yielded no statistically significant difference ( =0.84) in the perceived noise strength between simulated and real images. Furthermore, the observer study also allowed the estimation of a dose difference at which the observer perceived a just-noticeable difference (JND) in noise levels. The estimated JND value indicated that a change of 17% in the current-time product was sufficient to cause a noticeable difference in noise levels. The observed high accuracy, along with the flexible calibration, make this method an attractive tool for clinical image-based simulations of dose reduction.

29-2: Pixel Crosstalk: A New Metric to Characterize DOI Loss Due to AG Treatments of Display Glasses

A method is proposed that characterizes the loss of image clarity caused by anti-glare coatings. The method uses high resolution imaging, giving a distribution of scattered light that can be related easily to the clarity impression of the human eye. Results of different AG treated cover glasses are shown and a new parameter is proposed to characterize the loss of image clarity. This can be applied to the specification and characterization of the cover glasses as well as the characterisation of the final product for quality assurance.

29-4: Image Blurring Induced by Scattering Anti-Glare Layers

We introduce the scattering-induced spreading of a narrow linear light source in transmission as an alternative method for characterization of image blurring induced by anti-glare layers. This novel method is compared to the recently presented concept of pixel crosstalk and to the transmissive scatter distribution function of anti-glare layers measured in two different ways. The reduction of transmittance modulation is presented as an additional method for characterization of image blur. The ranking of the results is compared to a generalized haze level of the AG samples as obtained from the transmittance distribution functions measured with two different methods.

Impact of detector-element active-area shape and fill factor on super-resolution

In many undersampled imaging systems, spatial integration from the individual detector elements is the dominant component of the system point spread function (PSF). Conventional focal plane arrays (FPAs) utilize square detector elements with a nearly 100% fill factor, where fill factor is defined as the fraction of the detector element area that is active in light detection. A large fill factor is generally considered to be desirable because more photons are collected for a given pitch, and this leads to a higher signal-to-noise-ratio (SNR). However, the large active area works against super-resolution (SR) image restoration by acting as an additional low pass filter in the overall PSF when modeled on the SR sampling grid. A high fill factor also tends to increase blurring from pixel cross-talk. In this paper, we study the impact of FPA detector-element shape and fill factor on SR. A detailed modulation transfer function analysis is provided along with a number of experimental results with both simulated data and real data acquired with a midwave infrared (MWIR) imaging system. We demonstrate the potential advantage of low fill factor detector elements when combined with SR image restoration. Our results suggest that low fill factor circular detector elements may be the best choice. New video results are presented using robust adaptive Wiener filter SR processing applied to data from a commercial MWIR imaging system with both high and low detector element fill factors.

Use of high dynamic range imaging for quantitative combustion diagnostics.

High dynamic range (HDR) imaging is applied to quantitative combustion diagnostics in coflow laminar diffusion flames as a way to improve the signal-to-noise ratio (SNR) and measurement sensitivity. The technique relies on the combination of partially saturated frames into a single unsaturated image; in this work, the effectiveness of the HDR approach is demonstrated when applied to two-color ratio pyrometry. Specifically, it is shown than an increase in SNR results in more precise temperature measurements for both soot and thin filament pyrometry. Linearity and reciprocity analysis under partially saturated conditions were performed on three selected detectors, and the camera response functions, which are required for HDR image reconstruction, were determined. The linearity/reciprocity of the detectors allowed the use of a simplified algorithm that was implemented to compute the HDR images; soot and flame temperature were calculated from those images by employing color-ratio pyrometry. The reciprocity analysis revealed that pixel cross talk can be a limiting factor in a detector's HDR capabilities. The comparison with low dynamic range results showed the advantage of the HDR approach. Due to the higher SNR, the measured temperature exhibits a smoother distribution, and the range is extended to lower temperature regions, where the pyrometry technique starts to lose sensitivity due to detector limitations.

Calibration method of microgrid polarimeters with image interpolation.

Microgrid polarimeters have large advantages over conventional polarimeters because of the snapshot nature and because they have no moving parts. However, they also suffer from several error sources, such as fixed pattern noise (FPN), photon response nonuniformity (PRNU), pixel cross talk, and instantaneous field-of-view (IFOV) error. A characterization method is proposed to improve the measurement accuracy in visible waveband. We first calibrate the camera with uniform illumination so that the response of the sensor is uniform over the entire field of view without IFOV error. Then a spline interpolation method is implemented to minimize IFOV error. Experimental results show the proposed method can effectively minimize the FPN and PRNU.

Adaptive Pixel Crosstalk Compensation for CMOS Image Sensor

A new algorithm for removing pixel crosstalk in CMOS image sensors with Bayer color filter array was proposed. The algorithm can estimate low/high frequency image and provide adaptive threshold based on the human visual characteristics. By simulation experimentation on Macbeth color checker, resolution checker and real image, the experimental results revealed that this algorithm can remove the pixel crosstalk effectively.

Numerical Simulation of Refractive-Microlensed HgCdTe Infrared Focal Plane Arrays Operating in Optical Systems

The optoelectronic performance of the mid-wavelength HgCdTe infrared focal plane array (IRFPA) with refractive microlenses integrated on its CdZnTe substrate has been numerically simulated. A reduced light-distribution model based on scalar Kirchhoff diffraction theory was adopted to reveal the true behavior of IRFPAs operating in an optical system under imaging conditions. The pixel crosstalk obtained and the energy-gathering characteristics demonstrated that the microlenses can delay the rise in crosstalk when the image point shifts toward pixel boundaries, and can restrict the major optical absorption process in any case within a narrow region around the pixel center. The dependence of the microlenses’ effects on the system's properties was also analyzed; this showed that intermediate relative aperture and small microlens radius are required for optimized device performance. Simulation results also indicated that for detectors farther from the center of the field of view, the efficacy of microlenses in crosstalk suppression and energy gathering is still maintained, except for a negligible difference in the lateral magnification from an ordinary array without microlenses.

Challenges in CMOS‐based images

AbstractHigh resolution CMOS image sensor demand is driven by consumer applications like mobile phone or digital still camera. To maintain the image quality, we have to save the signal to noise ratio (SNR) despite the number of photons reduction gathered by the pixel. In this paper, we present how back‐side illumination technology could help to recover signal and how to optimize the process and device in order to reduce the noise. A quantum efficiency larger than 70% is targeted to justify efforts done on BSI process development. We also have to pay attention to pixel to pixel crosstalk coming from free carrier diffusion. To overcome this parasitic phenomenon, deep trench isolation (DTI) should be considered in order to significantly reduce crosstalk between two neighboring pixels. The second main noise contributor related to BSI process is the dark current signal. Dark current less than 1 aA per pixel at room temperature is mandatory to save the image quality at low light level. Therefore, all the Si‐SiO2 interfaces at the pixel side walls have to be processed without any process damage to reduce as much as possible the surface defects. On top of that, side wall implantation and side wall dielectric fabrication have been optimized to reduce this noise source. The pixel layout has to be fully optimized also to collect and store charges properly. With respect to the 3D pixel shape, a new fully depleted pinned photodiode with vertical charge storage and no lag has been developed. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of PEDOT–PSS resistivity and work function on PLED performance

The effect of a commonly used hole injection layer, poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT–PSS), on polymer light-emitting diode (PLED) performance has been investigated. A series of four different types of commercial PEDOT–PSS, with varying resistivity and work function were examined in devices with the structure Indium Tin Oxide (ITO)/PEDOT–PSS/High Molecular Weight Poly(n-vinylcarbazole) (PVKH): 30% N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD)/Low molecular Weight Poly(n-vinylcarbazole) (PVKL): 40% 2-(4-Biphenyl)-5-(4-tert-butylphenyl)-1,2,4-oxadiazole (PBD): 8% Ir(ppy)3. It was found that the PEDOT–PSS with the highest work function and resistivity produced the devices with the highest efficiencies; this is due to the improved hole injection effect, the decrease in electron leakage current and the prevention of pixel crosstalk. A maximum device current efficiency of 33.4cdA−1 has been achieved for the most resistive PEDOT; this corresponded to an external quantum efficiency (E.Q.E.) of 11%. Increasing the work function of the PEDOT used resulted in a 60% increase in E.Q.E. and device efficiency for PEDOTs in the same resistivity range. Drift–diffusion simulations, carried out using SEmiconducting Thin Film Optics Simulation software (SETFOS) 3.2, produced J–V curves in good agreement with the experimentally observed results; this allowed us to extract qualitative values for the effective device mobility along with the PEDOT work function and resistivity.

A new current mode APS in 0.18 μm standard CMOS process for smart image sensors

In this paper we present a new current mode structure for active pixel sensor (APS) which is an essential part in fast parallel processing Smart CMOS Image Sensors such as wireless capsule endoscopy. Using two diodes (N+/P-Well and P-Well/Deep-N-Well) in parallel like a pinned photo-diode (PD) improves sensing of optical signal and thus leads to higher sensitivity than a conventional PD. Also integrated signal amplification inside the collection area of the pixel increases the sensitivity of the device due to the amplification in the pixel. The proposed structure with regards to using Deep-N-Well/P-Substrate junction as a guard ring, suppresses the pixel cross-talk highly. In pixel delta reset sampling helps to make feasible on-chip parallel processing. A test structure, consist of 8 × 8 pixels of the proposed current mode APS has been simulated by standard 0.18 μm RF-CMOS technology of TSMC with a 21 × 23 μm2 pixel size. Fill factor of the pixel is 24 %.