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Technology

CMOS Image Sensor Technology

Glossary

CMOS (Complementary Metal Oxide Semiconductor) is mainstream semiconductor technology, used to fabricate a wide range of integrated electronic circuits. Image sensors based on this technology can take full advantage of the fast pace of technological innovation in the wider electronics industry, permitting reductions in feature size, increased operating frequencies, and integration of complex components. Equally, CMOS image sensors can profit from the cost advantages of: very large scale integration (VLSI); system on chip integration (SOC); and automated high-volume production.

CMOS image sensors offer imaging applications in computer vision a range of features that can only be implemented in this technology — where each image point (pixel) integrates not only the photo detector but also some active elements, in so-called Active Pixel Sensors (APS).
In Figure 1 The basic principle of a active pixel CMOS image sensor is plotted.

• Figure 1: Active Pixel CMOS image sensors. APS
• Figure 2: Principal schematic of CMOS image sensor pixel
  In Figure 2 the basic structure of a CMOS image sensor pixel is drawn

The first advantage of the CMOS image sensor technology that AWAIBA has developed is its high resistance to illumination overload. In other vision technologies, such as Charge Coupled Devices (CCDs), this would lead to artifacts such as blooming and smear.
The active control of photodiode saturation in CMOS image sensors built with AWAIBA’s technology enables a very high in-scene contrast to be handled without artifacts or saturation. Figure 3 shows the blooming produced by a CCD image sensor when imaging a high dynamic scene. In contrast, Figure 4 displays a scene with approximately 95dB in-scene contrast acquired by a CMOS sensor. The image shows a human eye under exposure from a ’slit lamp’ in an ophthalmological application.

• Figure 3: Blooming artifacts of CCD image sensors
• Figure 4: High dynamic scene without saturation and artifacts acquired by CMOS image sensor.

A further advantage of AWAIBA’s image sensor technology is the ‘global shutter’ or ’synchronous shutter’ principle. This allows all pixels to be exposed simultaneously, unlike the rolling shutter used in most consumer CMOS cameras, which leads to artifacts when imaging fast moving objects. The difference is easy to see in Figures 5 and 6.

• Figure 5: Image of a fast rotating fan with a rolling shutter CMOS sensor.
• Figure 6: Distortion free image of a fast rotating fan acquired with a global shutter CMOS sensor.
  AWAIBA has also developed techniques to enlarge the spectral response of standard CMOS image sensors slightly, in both directions: towards the UV and the IR.

A typical spectral response can be found in Figure 7.

AWAIBA CMOS image sensor for high dynamic range automotive applications