Initially, CMOS (Complementary Metal Oxide Semiconductor) was limited because of its inherent noise, however, CMOS imaging technology is now trending to become the dominant imaging technology. The original architectural design was essentially analog, and the concept of integrating the image processing features with System On Chip (SoC) technology had not been considered.
Fundamentally, it is this SoC characteristic of CMOS that is driving the impressive growth of CMOS imaging technology. Over the years, this technology has become more and more competitive. The race to use it commercially began in early 2000, when the big players applied continuous improvements to electro-optical performance.
With CCDs, photonic signals are converted into electron packets and are sequentially transferred to a common output structure where the electric charge is converted to voltage. Conversely, a CMOS imager converts charge to voltage at the pixel level, and most functions are integrated into the chip.
It can be operated with a single power supply and is capable of flexible readout, with regions-of-interest or windowing. CCDs are generally made in NMOS technology, which is dedicated in performance with specifics like overlapping double poly-silicon, anti-blooming, metal shields, and a specific starting material. CMOS imagers are often consumer oriented and are based on standard CMOS process technology for digital ICs, with some adaptation for imaging (e.g. pinned photodiodes).
The system architecture is improved with CMOS, as it generally embeds SoC features like analog to digital conversion, correlated double sampling, clock generation, voltage regulators, or image post-processing.
Next generation products will have to offer application- and sensor-level cost reduction through smaller optics and camera housings. Scanning, identifying, code reading, inspection and measurements of objects in factory automation, logistics, and retail applications, all with higher levels of accuracy will be demanded. At the same time, it will reduce both the cost of ownership and significantly improve productivity.
To respond to these market forces, one clear trend in industrial image sensor developments is the drive toward smaller pixel technologies with the usual necessary features like global shutters, and high-speed outputs, and the like.
Industrial CMOS sensor manufacturers are being forced to implement many of the techniques introduced to shrink consumer pixels without sacrificing Signal to Noise Ratio (SNR) or other critical performance parameters.
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