KEYWORDS: Signal detection, Cadmium sulfide, Signal to noise ratio, Image sensors, High dynamic range imaging, Capacitors, Sensors, Logic, Photodiodes, Photons
This paper presents a high dynamic range imaging sensor for detection of low light level signals. The sensor
utilises a 12x12 array of large 150μm x 150μm pixels. The readout circuitry allows for multiple readout options
including; multiple sampling (which allows for techniques such as Correlated Double Sampling (CDS)) and Time
to Digital Conversion (TDC) techniques, operated both independently and under the same integration period.
Scope for test patterns is also present in the design. All samples taken from the pixels before during and after
exposure are converted digitally through the use of a single slope ADC utilising a 10 bit DAC and a comparator.
No sample and hold capacitor is present. 4x10 bit SRAMs (Static Random Access Memory) per pixel are utilised
to record multiple samples, or act as a counter for the TDC mode of operation. The large dynamic range of the
system is attributable to both the novel timing system implemented within the multiple sampling mode of
operation and the TDC mode of operation (operated independently or intermittently within the same
integration time), which combines the use of 4x10 bit SRAMs with the 10 bit DAC to produce a counter capable
of monitoring the pixel signal over extremely long integration times; in this case up to 30 seconds.
This paper presents an object detection system based upon the use of multiple single photon avalanche diode (SPAD)
proximity sensors operating upon the time-of-flight (ToF) principle, whereby the co-ordinates of a target object in a coordinate
system relative to the assembly are calculated. The system is similar to a touch screen system in form and
operation except that the lack of requirement of a physical sensing surface provides a novel advantage over most existing
touch screen technologies. The sensors are controlled by FPGA-based firmware and each proximity sensor in the system
measures the range from the sensor to the target object. A software algorithm is implemented to calculate the x-y coordinates
of the target object based on the distance measurements from at least two separate sensors and the known
relative positions of these sensors. Existing proximity sensors were capable of determining the distance to an object with
centimetric accuracy and were modified to obtain a wide field of view in the x-y axes with low beam angle in z in order
to provide a detection area as large as possible. Design and implementation of the firmware, electronic hardware,
mechanics and optics are covered in the paper. Possible future work would include characterisation with alternative
designs of proximity sensors, as this is the component which determines the highest achievable accur1acy of the system.
In ultra-low light conditions the presence of dark current becomes a major source of noise for a CMOS sensor. Standard
dark current compensation techniques, such as using a dark reference frame, bring significant improvements to dark
noise in typical applications. However, applications requiring long integration times mean that such techniques cannot
always be used. This paper presents a differential dark current compensating pixel. The pixel is made up of a differential
amplifier and two photodiodes: one light shielded photodiode connected to the non-inverting input of the opamp and a
light detecting photodiode connected to the inverting input of the opamp. An integrating capacitor is used in the feedback
loop to convert photocurrent to voltage, and a switched capacitor network is present in parallel with the light shielded
pixel, which is used to satisfy the output equation to compensate the dark current. The pixel uses 150 μm x 150 μm
photodiodes and is fabricated in a standard 0.18 μm, 6M1P, CMOS process. The results show that the pixel is light
sensitive and has a linear output as expected. However, the dark current is not predictably controlled. Further work will
be carried out on the pixel design, and particularly the switched capacitor circuit, to determine the cause of the non-predictability
of the pixel output.
Polarized light is a naturally occurring phenomenon that human vision does not discern, yet it can provide useful
supplementary information from an image or optical system. Polarization detection can be implemented using hybrid
sensors where additional polarizing material is mounted onto a standard sensor. However these types of sensor tend to
be expensive, requiring extra manufacturing and materials. Presented is a low cost polarization sensor which is
implemented using standard CMOS technology and manufacturing techniques, without the need for supplementary
implants or optical layers. The polarization sensor is realised using a polarization grating, formed from a standard metal
layer, above a CMOS sensor. To compensate for the loss of photons due to the polarization grating, a high dynamic
range sensor is implemented using large, 110 micron photodiodes. The photosensor is used in a "light to frequency
conversion pixel" where the photocurrent is converted to a digital square wave output with a frequency proportional to
the photon flux density. A modulation depth of 10% is achieved. A rotary encoder application implementing the
polarization sensor is discussed.
KEYWORDS: Video, Analog electronics, Power supplies, Imaging systems, Cameras, Signal to noise ratio, Clocks, Data acquisition, Digital signal processing, Sensors
This paper describes the requirements, design, and results of a modular data acquisition system with a resolution of 12 bits at up to 20 MHz sampling frequency. The modularity enables the analog-digital conversion to be separated from the digital processing/storage. This allows the latest, best performing ADC (analog-digital converter) to be easily integrated into the system by a re-design of only the AD board with the rest of the system unchanged. The converter employed operates at frequencies up to 20 MHz. The complete system produces measured quantization noise figures of -75 dB and integral non-linearity of -72 dB. The unit can sample video or non-video waveforms. For video applications, an active clamping system is used to ensure that the black level is accurately maintained. The framestore is connected externally using a high-speed digital data bus. This facilitates the inclusion of real-time digital processing units. The framestore used is doubly buffered to permit simultaneous acquisition and readout. The store is 8 Mbytes to accommodate HDTV images and has an input data rate of 40 Mbytes per second.
The realization of an integrated, flexible, and robust CIM vision system, suitable for performing quality-assurance surface inspections is discussed. The optimized combination of advanced optics, optomechanics, and flexible image sensor realizes a high 'virtual resolution' without penalizing the pixel transfer rate. High computation rates are obtained by complementing the fractal inspection algorithm with a dynamic hologram, a modular data flow processor, and the system computer. The integrated vision system is validated for the surface quality inspection of concrete tiles in an industrial environment. The overall system performance is discussed in detail and the potential of the system for other application fields will be addressed.
A novel active vision system for CIM production and inspection applications has been developed in the framework of ESPRIT II project No. 5194 (CIVIS). The system consists of a unique, integrated combination of novel components: camera head, data acquisition electronics, a custom digital image processor, control hardware and a commercial framestore, all under the direction of control and processing software on a PC-486 platform. The camera head incorporates a fast zoom lens in combination with a pan/tilt mirror system, allowing region-of-interest acquisition. The special 256 X 256 MOS image sensor offers programmable resolution and random pixel access. The unique combination of optics, optomechanics and versatile image sensor has a high `virtual resolution,' corresponding to more than 1k X 1k pixels but without the overhead of a high pixel transfer rate. The fast computation of the algorithm employed for the fractal inspection of surfaces is realized with an unusual combination of an electrically switchable hologram (for performing all linear operations at the speed of light in the optical domain), a module-based digital processor and the host computer. In this way, active vision for the inspection of concrete tile surfaces has been implemented by acquiring only relevant image data and elegantly processing them in the most appropriate domain.
In the course of ESPRIT II project No. 2103 (MASCOT) a high performance color CCD camera was developed. It is based on a 1K X 1K frame-transfer CCD imager whose pixels are covered with an optimized dielectric filter stripe pattern. A microscanning optical unit is employed to displace the image, with a reproducibility of 1/200th of the pixel period, for programmable color image acquisition with a maximum resolution of 3K X 3K color (RGB, XYZ, etc.) pixels. The CCD's output is immediately digitized to 10 bits using an in- house developed ADC subsystem whose performance of 67 dB S/N at 20 MHz is ideal for this application. The data is stored in one of three fast framestores. The raw data is read out simultaneously from these three framestores at a data rate of 30 MBytes per second and processed, fully digitally, in a special color processor. After non-linear transformations to compensate for detector non-linearities, color matrixing is carried out using one set of 16 matrix parameters which have been optimized for different illumination conditions and color temperatures. They also enable the selection of the type of output data to be generated e.g., RGB for specific phosphors, CIE XYZ tristimulus values, etc. After matrixing, a non-linear table-lookup can be used to introduce gamma correction or other calibration functions. The color processor produces 8-bit color pixels at a rate of 20 MBytes per second, writing these data directly into an 8 MBytes commercial framestore plugged into a PC/AT.
Photosensitive elements with well-chosen geometry, combined with suitable analog and digital circuitry on the same CMOS/CCD chip, lead to 'smart image sensors' with interesting capabilities and properties. All our smart sensors were fabricated with commercially available multi-process wafer services of CMOS process, one of them with a buried-channel CCD option. Measurement of the optoelectronic properties of standard CMOS/CCD processes (wavelength-dependent quantum efficiency, lateral homogeneity of quantum efficiency/photo- conductivity, CCD charge transport efficiency, etc.) show excellent performance. The smartness that lies in the geometry is illustrated with a single-chip motion detector, a 3-D depth video camera, a single-chip planar distance sensor, and a sine/cosine (Fourier) transform sensor for fast optical phase measurements. The concept of problem-adapted geometry is also shown with a dynamic frame-transfer CCD whose pixel size and shape can be changed electrically in real-time through charge-binning. Based on the wavelength-dependent absorption of silicon, all-solid-state color pixels are demonstrated by properly arranging the available pn-junctions in the third (bulk) dimension. Moderate color measurement performance is achieved using an unmodified CMOS/CCD process, with a CIE general color-rendering index of Ra equals 69.5.
A laser beam writing system for the fabrication of micro-optical elements as relief structures in photoresist is described. Using a computer controlled precision xy stage and a modulated, focused laser beam, a wide range of surface relief microstructures has been produced, with typical periods of 10 - 100 micrometers and a maximum relief amplitude of about 5 micrometers . Examples include microlens arrays, kinoforms and other phase structures for applications in optical computing, optical interconnects and micro-optical systems in general.
A CCD camera based optical metrology system has been developed for the accurate measurement of a railway locomotive''s wheel movements with respect to the rails. The system is based on the light-sectioning method implemented with four laser diodes projecting light sheets onto the wheel and rail. A high-resolution CCD camera views the four profiles simultaneously using an appropriately folded and combined beam-path. To minimize the effects of ambient light a special narrow-band dielectric filter was designed manufactured and fitted in front of the camera lens. The desired measurement accuracy requires pixel-synchronous acquisition of the CCD video data. This is realized with a custom-built universal CCD data acquistion system with which profile tracking data compression and storage at 12. 5 Hz (half frame-rate) is made possible. A prototype system was built and tested on railway tracks at up to 140 km/h. In laboratory experiments the system surpassed the required measurement accuracies about fivefold attaining an accuracy of 0. 02 mm in relative position and better than 0. 1 mrad in relative angle. 2.
The use of CCD sensors in optical metrology requires synchronous sampling of the image with a good signalnoise
performance. . A system has been developed to digitize optimally the signals from high-resolution CCD
sensors. The data acquisition system is split into two parts -the first is a storage unit for the IBM PC/AT family
of computers with a fast, digital, input-output interface, with 8-bit transmission speed DC-4OMHz and 16 bit
operation DC-2OMHs. The digitization of the analogue signal is performed on separate units, up to 2m from the
computer. Separating the analogue processing from the computer and using a separate power supply not only
reduces the electrical noise from the digital electronics to a minimum but also allows greater flexibility in designing
custom 'front ends' for a wide range of sensors.
The storage card has two 1M byte banks of memory. These are normally used to provide double buffering of
1M pixel images, but can also be used to store 2M byte images without double buffering.
Practical experience, using 8 and 10 bit video front ends, indicates that the geometrical resolution possible
with modern CCD sensors is approaching 1/100 of the pixel period. The digital signal processing required for this
performance does not depend on the CCD camera's PSF and it is insensitive to variations offocus and orientation.
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