Advanced Imaging


Advanced Imaging Magazine

Updated: January 12th, 2011 10:01 AM CDT

Meeting New Demands of Image Capture

The design of CMOS imagers with global shutter and high dynamic range is determined by multiple parameters
Pixel structure migratio
Figures 1a, 1b, 1c. Pixel structure migration from three-transistor rolling shutter to six-transistor fast pipelined global shutter.
Figure 2
Figure 2. Linear segments of a sensor characteristic established by partial reset of the photodiode result in a wide dynamic range.
Figure 3
Figure 3. Pixels with smart reset capability through internal feedback yield a very wide dynamic range.
Figure 4
Figure 4. Large pixels capture dark image areas; small-area pixels are for light areas.
Figure 5
Figure 5. Transfer gates multiplex the different exposure times of four-color pixels in one readout circuit.

By Guy Meynants, Ph.D., CMOSIS

The majority of motion analysis and industrial imaging applications require synchronous image acquisition of all pixels while often operating under difficult lighting conditions. Through innovative pixel design, CMOS image sensors meet these preconditions, establishing a solid basis for future requirements. Custom design enables sensor solutions optimized for specific applications.

Over the last several years, CMOS sensors have established themselves for fast and high-resolution image acquisition. They can be produced at low cost on standard manufacturing lines, monolithically integrating analog and digital functions on the same chip. By saving external components with their inherent board space overhead and mounting expenditure, such subsystems primarily reduce cost. At the same time they consume less power because drivers for external circuits and components are obsolete.

The advantages and successes of industrial image processing are widely known—leading to ever more novel applications. These novel applications put even stronger demands on image capture systems. The new requirements surpass speed or resolution (number of pixels). In the industrial arena there is a growing demand for synchronism and operation under unfavorable lighting conditions.

Image acquisition systems featuring a global shutter and the widest possible dynamic range are in demand. This demand shifts the center of the design targets of application-specific CMOS image sensors from integration of additional functionality to novel ways of shaping the pixels and the electronic circuitry in close proximity.

Global Shutter

Industrial image processing, when applied to process control, requires the capture of the state of a process at a certain point in time to detect a fault and determine the appropriate corrective measures. This requires cameras with synchronous acquisition of all pixels and holding this information uncorrupted until the entire image information has been read out. This was feasible with interline transfer CCD imagers without many problems. The CCD photodiodes, after exposure, transferred their charges simultaneously to CCD storage elements. The photodiodes were ready for the next exposure immediately afterwards, enabling pipelined global shutter operation. In this scheme, every pixel has two storage elements: the diode for integrating the photon-induced charges, and one within the CCD MOS capacitor chain.

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