Time Delay Integration Research Articles

Present status of the 4-m ILMT data reduction pipeline: application to space debris detection and characterization

The 4-m International Liquid Mirror Telescope (ILMT) located at the ARIES Observatory (Devasthal, India) has been designed to scan at a latitude of +29° 22’ 26” a band of sky having a width of about half a degree in the Time Delayed Integration (TDI) mode. Therefore, a special data-reduction and analysis pipeline to process online the large amount of optical data being produced has been dedicated to it. This requirement has led to the development of the 4-m ILMT data reduction pipeline, a new software package built with Python in order to simplify a large number of tasks aimed at the reduction of the acquired TDI images. This software provides astronomers with specially designed data reduction functions, astrometry and photometry calibration tools. In this paper we discuss the various reduction and calibration steps followed to reduce TDI images obtained in May 2015 with the Devasthal 1.3m telescope. We report here the detection and characterization of nine space debris present in the TDI frames.

The 4-m International Liquid Mirror Telescope

The 4-m International Liquid Mirror Telescope (ILMT) is presently (March-June 2017) being erected on the ARIES site in Devasthal (Uttarakhand). We describe and illustrate in the present paper its different components. The ILMT will be used in the Time Delayed Integration (TDI) mode to carry out a deep survey and high S/N photometric and astrometric observations of solar system, galactic and extra-galactic objects within a narrow (24’) strip of sky. In principle, the ILMT should detect and regularly monitor more than 50 multiply imaged quasars. It will also detect numerous supernovae (see Kumar et al., these proceedings) as well as space debris (see Pradhan et al., also in these proceedings).

Analysis and reduction of the TDI CCD charge transfer image shift.

Based on the description of the charge transfer process of a time delay integration charge coupled device (TDI CCD), it is pointed out that the relative displacement of the target image and the transferred charge introduces the charge transfer image shift problem in a line transfer period, which leads to the decrease of the modulation transfer function (MTF) in the scanning direction. Based on the basic definition of MTF, the charge transfer image shift model of the TDI CCD is established. According to the quantitative calculation of this model, the MTF curves of the three-phase TDI CCD are obtained under the condition of different pulse widths of computer interface (CI) signals. The MTF changes with the pulse width of CI signals, and the maximum value at the spatial Nyquist frequency is obtained when the CI signals are equally spaced, which is difficult to achieve in the actual system because of the limitation of computed radiography readout signals. In this paper, we present the improved TDI CCD driving timing, which makes it possible to realize the equal interval distribution of CI signals by adjusting the technology compatibility kit signal. Finally, the experimental platform is built, and the MTF curves are calculated from the obtained target images. The result is consistent with the theoretical model.

A High-Dynamic-Range Optical Remote Sensing Imaging Method for Digital TDI CMOS

The digital time delay integration (digital TDI) technology of the complementary metal-oxide-semiconductor (CMOS) image sensor has been widely adopted and developed in the optical remote sensing field. However, the details of targets that have low illumination or low contrast in scenarios of high contrast are often drowned out because of the superposition of multi-stage images in digital domain multiplies the read noise and the dark noise, thus limiting the imaging dynamic range. Through an in-depth analysis of the information transfer model of digital TDI, this paper attempts to explore effective ways to overcome this issue. Based on the evaluation and analysis of multi-stage images, the entropy-maximized adaptive histogram equalization (EMAHE) algorithm is proposed to improve the ability of images to express the details of dark or low-contrast targets. Furthermore, in this paper, an image fusion method is utilized based on gradient pyramid decomposition and entropy weighting of different TDI stage images, which can improve the detection ability of the digital TDI CMOS for complex scenes with high contrast, and obtain images that are suitable for recognition by the human eye. The experimental results show that the proposed methods can effectively improve the high-dynamic-range imaging (HDRI) capability of the digital TDI CMOS. The obtained images have greater entropy and average gradients.

Analysis of MTF in TDI-CCD Subpixel Dynamic Super-Resolution Imaging by Beam Splitter

The subpixel dynamic imaging technique of a beam splitter is one of the most effective super-resolution imaging methods. Aiming to create a linear time delay integration charge coupled device (TDI–CCD) subpixel imaging system based on the optical assembly method, its modulation transfer function (MTF) is analyzed based on the spatial over-sampling theory. Firstly, Fourier transformation of the sampling point is used to describe the frequency domain characteristics of TDI–CCD, which transform a unit cell of the spatial sampling lattice into a bandwidth cell in the spatial–frequency domain. Considering the effects of velocity mismatch and misalignment, the best subpixel staggering position of the linear TDI–CCD pair is given. Moreover, according to the analysis of the MTF of super-resolution reconstruction results from multiple subpixel images with random spatial offsets, the condition of sampling in the limitation of the enhancement of MTF is obtained. The numerical simulation and real experimental analysis reveal results that are consistent with the theoretical model.

Research on the shortwave infrared hyperspectral imaging technology based on Integrated Stepwise filter

Space-borne hyperspectral imagery is an important tool for earth sciences and industrial applications. Higher spatial and spectral resolutions have been sought persistently, although this results in more power, larger volume and weight during a space-borne spectral imager design. For miniaturization of hyperspectral imager and optimization of spectral splitting methods, several methods are compared in this paper. Spectral time delay integration (TDI) method with high transmittance Integrated Stepwise Filter (ISF) is proposed.With the method, an ISF imaging spectrometer with TDI could achieve higher system sensitivity than the traditional prism/grating imaging spectrometer. In addition, the ISF imaging spectrometer performs well in suppressing infrared background radiation produced by instrument. A compact shortwave infrared (SWIR) hyperspectral imager prototype based on HgCdTe covering the spectral range of 2.0–2.5μm with 6 TDI stages was designed and integrated. To investigate the performance of ISF spectrometer, a method to derive the optimal blocking band curve of the ISF is introduced, along with known error characteristics. To assess spectral performance of the ISF system, a new spectral calibration based on blackbody radiation with temperature scanning is proposed. The results of the imaging experiment showed the merits of ISF. ISF has great application prospects in the field of high sensitivity and high resolution space-borne hyperspectral imagery.

Time-Delay Integration Imaging with ICON's Far-Ultraviolet Imager.

A Time-Delay Integration (TDI) image processing system has been developed to capture ICON's Far Ultraviolet (FUV) Spectrographic Imager data. The TDI system is designed to provide variable-range motion-compensated imaging of Earth's nightside ionospheric limb and sub-limb scenes viewed from Low Earth Orbit in the 135.6 nm emission of oxygen with an integration time of 12 seconds. As a pre-requisite of the motion compensation the TDI system is also designed to provide corrections for optical distortions generated by the FUV Imager's optical assembly. On the dayside the TDI system is used to process 135.6 nm and 157.0 nm wavelength altitude profiles simultaneously. We present the TDI system's design methodology and implementation as an FPGA module with an emphasis on minimization of on-board data throughput and telemetry. We also present the methods and results of testing the TDI system in simulation and with Engineering Ground Support Equipment (EGSE) to validate its performance.

Joint Chroma Subsampling and Distortion-Minimization-Based Luma Modification for RGB Color Images With Application.

In this paper, we propose a novel and effective hybrid method, which joins the conventional chroma subsampling and the distortion-minimization-based luma modification together, to improve the quality of the reconstructed RGB full-color image. Assume the input RGB full-color image has been transformed to a YUV image, prior to compression. For each 2×2 UV block, one 4:2:0 subsampling is applied to determine the one subsampled U and V components, Us and Vs . Based on Us , Vs , and the corresponding 2×2 original RGB block, a main theorem is provided to determine the ideally modified 2×2 luma block in constant time such that the color peak signal-to-noise ratio (CPSNR) quality distortion between the original 2×2 RGB block and the reconstructed 2×2 RGB block can be minimized in a globally optimal sense. Furthermore, the proposed hybrid method and the delivered theorem are adjusted to tackle the digital time delay integration images and the Bayer mosaic images whose Bayer CFA structure has been widely used in modern commercial digital cameras. Based on the IMAX, Kodak, and screen content test image sets, the experimental results demonstrate that in high efficiency video coding, the proposed hybrid method has substantial quality improvement, in terms of the CPSNR quality, visual effect, CPSNR-bitrate trade-off, and Bjøntegaard delta PSNR performance, of the reconstructed RGB images when compared with existing chroma subsampling schemes.

Noise Model of a Multispectral TDI CCD Imaging System and Its Parameter Estimation of Piecewise Weighted Least Square Fitting

The time-delayed integration (TDI) charge-coupled device (CCD) imaging technology is widely used in remote sensing field and accurate estimation of noise level is essential to assure good performance of denoising. Therefore, in order to achieve images with high signal-to-noise ratio by the TDI CCD sensor, the noise model of the imaging system is investigated. First, the design of the imaging circuit system is presented, and the main noises of the system are analyzed, such as the TDI CCD sensor, amplifiers, and so on. Then a theoretical noise model of the imaging system is established as a priori information. An improved least square fitting method of parameter estimation is proposed. To validate the theoretical model and the proposed method, extensive tests are carried out to obtain multiple uniform images on a developed TDI CCD imaging system. The experiment results confirm that a precise noise model can be achieved by this method and it verifies the theoretical model, meanwhile. It can be used for noise removal and aided design of imaging systems to acquire high-quality images. It is meaningful for remote sensing to obtain more targets of interest.

GaiaNIR – A future all-sky astrometry mission

AbstractWith the launch of Gaia in December 2013, Europe entered a new era of space astrometry following in the footsteps of the very successful Hipparcos mission. A weakness of Gaia is that it only operates at optical wavelengths. However, much of the Galactic centre and the spiral arm regions are obscured by interstellar extinction. An obvious improvement on Gaia is to include the Near-Infra-Red (NIR) which requires the use of new types of detectors. Additionally, to scan the entire sky and measure global absolute parallaxes the spacecraft must have a constant rotation resulting in a moving image that must be compensated for by, for example, operating the detectors in Time Delayed Integration (TDI) mode. If these technical issues can be solved a new Gaia-like mission separated by a 20 year interval would give; 1) NIR all-sky astrometry and photometry to penetrate the obscured regions and to observe intrinsically red objects with almost diffraction limited resolution; 2) improved proper motions with fourteen times smaller errors than from Gaia alone opening up new science cases, such as long period exoplanets and accurate halo measurements; 3) allow the slowly degrading accuracy of the Gaia reference frame, which will be the basis for future astronomical measurements, to be reset.

A 32-Stage 15-b Digital Time-Delay Integration Linear CMOS Image Sensor With Data Prediction Switching Technique

This paper presents a 512-column linear CMOS image sensor (CIS) with 32-stage digital time-delay integration (TDI) operation. A signal processing architecture consists of analog-front-ends, analog-to-digital converters (ADCs), and digital accumulators (DAs) are designed with optimization of timing, area, and power efficiency. An eight-column-shared 10-b successive approximation register ADC with data prediction switching technique and 11-b DA are proposed to achieve a data depth of 15 b after 32-stage TDI. The achieved signal-to-noise ratio boost is 14.84 dB after 32-stage TDI operation. The proposed linear TDI sensor is implemented in 0.11- $\mu \text{m}$ TSMC backside illumination CIS technology with a line time of $104~\mu \text{s}$ , a pixel pitch of $7.5~\mu \text{m}$ , and a power consumption of $153.2~\mu \text{W}$ /column.

A Tileable CMOS X-Ray Line Detector Using Time-Delay-Integration With Pseudomultisampling for Large-Sized Dental X-Ray Imaging Systems

This paper presents a low-power and low-noise tileable CMOS X-ray line detector with time-delay-integration (TDI) for large-sized dental X-ray imaging systems. The proposed X-ray line detector with TDI adopts pseudomultisampling (PMS) using shifted clocks for the single-slope analog-to-digital converter (ADC) to reduce the required clock cycles for the ADC. The reduced clock cycle decreases the dynamic power and power fluctuation to ensure power stability for large-sized X-ray detectors. The TDI X-ray line detector with PMS was fabricated using 0.18- $\mu \text{m}$ CMOS image sensor process technology with a stitching process, which has an image format of 1010 $\times $ 86 and a pixel size of $75\,\,\mu \text{m}\,\,\times 75\,\,\mu \text{m}$ . The X-ray line detectors are tiled with $1 \times 2$ and $1\,\,\times3$ arrays to enlarge the active areas for dental panoramic and cephalometric X-ray imaging systems, respectively. The measurement results show the random noise of 0.43 LSB and a dynamic range of 78.5 dB, when the number of TDI stages is 86 and the number of PMS steps is 4. The dynamic current and power consumption per ADC are reduced from 157.4 $\mu \text{A}$ and $283.32~\mu \text{W}$ to $41.2~\mu \text{A}$ and 74.16 $\mu \text{W}$ , respectively, when PMS is adopted for TDI.

Simple and effective method to improve the signal-to-noise ratio of compressive imaging

This Letter presents a simple and effective method to improve the signal-to-noise ratio (SNR) of compressing imaging. The main principles of the proposed method are the correlation of the image signals and the randomness of the noise. Multiple low SNR images are reconstructed firstly by the compressed sensing reconstruction algorithm, and then two-dimensional time delay integration technology is adopted to improve the SNR. Results show that the proposed method can improve the SNR performance efficiently and it is easy to apply the algorithm to the real project.

Precise alignment method of time-delayed integration charge-coupled device charge shifting direction in aerial panoramic camera

Precise alignment method of time-delayed integration charge-coupled device charge shifting direction in aerial panoramic camera

A Low Power Digital Accumulation Technique for Digital-Domain CMOS TDI Image Sensor.

In this paper, an accumulation technique suitable for digital domain CMOS time delay integration (TDI) image sensors is proposed to reduce power consumption without degrading the rate of imaging. In terms of the slight variations of quantization codes among different pixel exposures towards the same object, the pixel array is divided into two groups: one is for coarse quantization of high bits only, and the other one is for fine quantization of low bits. Then, the complete quantization codes are composed of both results from the coarse-and-fine quantization. The equivalent operation comparably reduces the total required bit numbers of the quantization. In the 0.18 µm CMOS process, two versions of 16-stage digital domain CMOS TDI image sensor chains based on a 10-bit successive approximate register (SAR) analog-to-digital converter (ADC), with and without the proposed technique, are designed. The simulation results show that the average power consumption of slices of the two versions are J/line and J/line, respectively. Meanwhile, the linearity of the two versions are 99.74% and 99.99%, respectively.

An Antivibration Time-Delay Integration CMOS Image Sensor With Online Deblurring Algorithm

This paper presents an antivibration time-delay integration (TDI) CMOS image sensor (CIS) for small remote imaging systems, introducing a hardware-implemented online deblurring (ODB) algorithm to address the image blur problems caused by vibrations. The proposed sensor has eight TDI stages, column-parallel TDI accumulating and ODB circuits. A $256\times 8$ -pixel prototype chip was fabricated using a 0.18- $\mu \text{m}$ CIS technology with a pixel footprint of $6.5~\mu {\mathrm{ m}}\times 6.5~\mu \text{m}$ and a fill factor of 28%. Measurement results show that the sensor can achieve dynamic ranges of 45.1 and 51.8 dB, respectively, with and without enabling the ODB algorithm. Compared with a single-stage line scanner imager, it offers an improvement in signal-to-noise ratios of 1.9 and 8.6 dB, respectively, with and without the ODB algorithm.

A solution to RPCs of satellite imagery with variant integration time

To get a sufficient swath width and the higher resolution, most spaceborne cameras acquire the ground information by multiple time delay integration charge-coupled devices. Owing to the special imaging mechanism, TDI charge-coupled devices (CCD) is required to adjust the integration time dynamically. If the offset between two adjacent chips in the along-track direction is large enough, integration time adjustment will generate innegligible pixel offsets in different integration segments. Under this circumstance, if the PRCs are solved by traditional terrain-independent strategy, which uses the rigorous sensor model to generate evenly distributed object-image points, system errors are inevitably introduced and thus the rational polynomial coefficients (RPCs) cannot obtain the high fitting accuracy. In this paper, we derived the quantitative relationship between integration time and pixel offsets. The RPCs fitting technology based on integration time equalisation for improving the fitting accuracy was then discussed. The potential for this method to yield high fitting accuracy and the ability to keep raw object-image relations are illustrated with the experimental results obtained with four high-resolution imageries of Mapping Satellite-1 (TH-1).

A 128-Stage CMOS TDI Image Sensor With On-Chip Digital Accumulator

A 128-stage CMOS time delay integration (TDI) image sensor with on-chip digital accumulator was presented in this paper. By using two sets of sampling capacitors and an optimized timing, the on-chip column parallel cyclic analog-to-digital converter (ADC) in the sensor conducts the correlated double sampling, digital program gain amplifying, and A/D conversion all altogether in one ADC’s conversion time simultaneously. An on-chip digital accumulator suitable for this sensor is also proposed. A prototype $1024 \times 128$ CMOS TDI image sensor is fabricated in the 0.18- $\mu \text{m}$ CMOS technology, and the measurement results proof that the pixels and readout circuits in the sensor work properly. With a line rate of 3875 lines/s, the sensor costs a power consumption of 290 mW and a silicon area of $18.2~\textrm {mm}\times 18.9$ mm. The estimated maximum sensitivity of the fabricated sensor is 2010 $\textrm {V}/\textrm {lux}\cdot \textrm {sec}$ . This paper is suitable for applications in low-illumination, high scanning speed, and remote sensing systems.

A Two-Step Analog Accumulator for CMOS TDI Image Sensor With Temporal Undersampling Exposure Method

A two-step analog accumulator structure, fit for CMOS time-delay integration image sensor, is proposed to combat the impacts of parasitic phenomenon of the traditional one-step analog accumulator. To implement the two-step accumulation, the exposure signals are divided into groups. The synchronous signals in each group are integrated in step1 integrators, which are designed based on the switched-capacitor amplifier. Then, the integral signals from step1 are integrated in step2 integrators. The temporal undersampling exposure method is adopted in the sensor to reduce the number of integrators. Two versions of a prototype 64-stage two-step accumulator, with and without the decoupling capacitance $C_{d}$ , are designed and fabricated in 0.18- $\mu \text{m}$ one-poly four-metal 1.8 V/3.3 V CMOS technology. With an $8 \times 8$ stepping pattern, the signal-to-noise ratio improvements of the two versions are 17.278 and 17.192 dB at 64 stages, respectively, while the ideal value is 18.062 dB. The experimental results have proved the effectiveness of the two-step structure in combating the parasitic phenomenon and made the analog accumulator with higher stage realizable.

Quantitative assessment of image motion blur in diffraction images of moving biological cells

Motion blur (MB) presents a significant challenge for obtaining high-contrast image data from biological cells with a polarization diffraction imaging flow cytometry (p-DIFC) method. A new p-DIFC experimental system has been developed to evaluate the MB and its effect on image analysis using a time-delay-integration (TDI) CCD camera. Diffraction images of MCF-7 and K562 cells have been acquired with different speed-mismatch ratios and compared to characterize MB quantitatively. Frequency analysis of the diffraction images shows that the degree of MB can be quantified by bandwidth variations of the diffraction images along the motion direction. The analytical results were confirmed by the p-DIFC image data acquired at different speed-mismatch ratios and used to validate a method of numerical simulation of MB on blur-free diffraction images, which provides a useful tool to examine the blurring effect on diffraction images acquired from the same cell. These results provide insights on the dependence of diffraction image on MB and allow significant improvement on rapid biological cell assay with the p-DIFC method.