China
Africa
Explore chapters and articles related to this topic
Physical Basis of X-ray Breast Imaging
Published in Paolo Russo, Handbook of X-ray Imaging, 2017
Chai Hong Yeong, Kwan Hoong Ng, Noriah Jamal
In screen–film mammography, one of the important sources of noise is the random fluctuation contributed by the granularity of the film itself. The film emulsion comprises grains of silver halide and their random structure contributes to the total noise. It is difficult to eliminate the effect of structural noise from the image since each sheet of film has a different pattern of structural granularity. In digital mammography, although film granularity is eliminated, there are some structural variations across the detector which are usually associated with the spatial variations in detector sensitivity. Unlike the conventional noise characteristic of being random in space and time, these variations remain constant over time, hence they are also known as the “fixed pattern noise.” This type of structural noise can be removed from the image using an image correction method utilizing the digital data from the detector.
Basics of Image Sensors
Published in Junichi Nakamura, Image Sensors and Signal Processing for Digital Still Cameras, 2017
An image sensor for still pictures reproduces two-dimensional image (spatial) information. Noise appearing in a reproduced image, which is “fixed” at certain spatial positions, is referred to as fixed-pattern noise (FPN). Because it is fixed in space, FPN at dark can be removed, in principle, by signal processing. Noise fluctuating over time is referred to as “random” or “temporal” noise. In this book, we use “temporal” when referring to noise fluctuating over time because “random” can also be associated with FPN; for example, “pixel-random” FPN is seen randomly in two-dimensional space.
CMOS Imager Array Design, Operation, and Trends
Published in Krzysztof Iniewski, Circuits at the Nanoscale, 2018
Mark Jaffe, John Ellis-Monaghan, Jim Adkisson
Reducing the manufacturing variability of parameters such as transistor thresholds will certainly help reduce fixed pattern noise, but there will be an inevitable distribution in the gains and nominal bias of the read out circuits for each pixel. Double sampling was the obvious tool to use to subtract this effect as much as possible. Virtually all CMOS imagers employ a double sample technique with most imagers sampling a reference level and an image level for each pixel in sequential read operations and subtracting the two readouts with a differential amplifier in the column circuit.
A spectroscopic bandwidth correction method based on multi-bandwidth functions
Published in Journal of Modern Optics, 2022
Researches show that the noise [39–41] of the spectrograph can be classified into four main parts: the readout noise , dark noise , fixed pattern noise , and photoelectron noise . The readout noise is independent of temperature and essentially depends on the design of the circuit of spectrographs. It is generated when an AD chip converts analog signals to digital signals and when internal amplifiers amplify the voltage. The dark noise is generated because of the spurious accumulation of dark electrons in CCD pixels and increases with the increase of integration time and ambient temperature. Both the readout noise and dark noise are Gaussian white noise. The fixed pattern noise is induced by the differences in the mean dark current of each pixel and is determined by the manufacturing process of the CCD. For a fixed integration time and temperature, the fixed pattern noise is constant and unique to each CCD spectrograph. The photoelectron noise is related to the spectral intensity and gain of the spectrograph, which increases with the increase of spectral intensity. The four kinds of noise are independent of each other, so the total noise can be expressed as
Investigation of random telegraph signal in CMOS image sensors irradiated by protons.
Published in Journal of Nuclear Science and Technology, 2021
Bingkai Liu, Yudong Li, Lin Wen, Dong Zhou, Jie Feng, Xiang Zhang, Yulong Cai, Jing Fu, Qi Guo
The studied devices, CIS1 and CIS2, are the backside-illuminated (BSI) CISs with a high quantum efficiency and sensitivity, and they are different chips in the same lot. The BSI CISs, constituted of 2048 × 2048–11 μm-pitch-4 T pixels using the PPD, are manufactured in a 0.18 μm CMOS process dedicated to imaging. The chip area is 24.3 mm×25.3 mm and the active image size is 22.528 mm×22.528 mm. The sensor works on electronic rolling shutter mode and uses a correlated double sampling circuit to remove noise, such as fixed-pattern noise and reset noise. The cross-section of a pixel for the BSI CISs is depicted in Figure 1. It consists of a PPD used for the collection of photoelectrons and four control transistors. They are a transfer transistor which controls the transfer of signal charge from PPD to floating diffusion, a reset transistor used to reset PPD to a fixed potential, a source follower transistor which forms pixel buffer amplifier together with load devices, and a row select transistor which works as a switching transistor, respectively. The pixel is electrically and optically isolated by shallow trench isolation (STI) structures.
CMOS Implementation of Time Delay Integration (TDI) for Imaging Applications: A Brief Review
Published in IETE Technical Review, 2020
Sushil Kumar Semwal, Raghvendra Sahai Saxena
Ji et al. proposed current-mode sensor to achieve TDI operation in analog domain [35]. Also, a current mode accumulation unit is introduced to add current effectively after correlated double sampling (CDS), which further reduces the fixed pattern noise. As shown in Figure 22(a) transistor M2 converts the voltage generated across diode/detector to current. M1 is used to reset the detector for fresh signal detection. For linear conversion of detector voltage it is required that M2 operate in triode region. Figure 22(b) shows the current mode accumulator schematic. It works in two modes. In first mode CP2 is ON and accumulator is connected with bus. The current Iin is thus added to capacitor C2. In second mode CP1 is ON and accumulator is isolated from bus. In this mode charge on C2 is added to C1. Thus each time accumulator change the mode, the voltage on capacitor continuously increased thus achieving the function of accumulation.