Image Sensors

1

What is an Image Sensor?• An Image Sensor is a photosensitive device that

converts light signals into digital signals (colours/RGB data).

• Typically, the two main types in common use are CCD and CMOS sensors and are mainly used in digital cameras and other imaging devices.

• CCD stands for Charged-Coupled Device and CMOS stands for Complementary Metal–Oxide–Semiconductor

IMAGE SENSORS

How Image Sensors Work• Both CCD and CMOS sensors

work by employing photosensitive circuitry that reacts to light and stores the analog signals as digital data, namely an image.

• They both use different methods to achieve this.

• Will take a look at CMOS image sensors.

How Image Sensors Work

Passive CMOS Image Sensors

• Passive CMOS image sensors contained in each individual pixel only a photo sensing element(usually a photodiode) and a switching MOSFET

• These MOS-array sensors offer the advantage of random access to the individual pixels allowing the definition of sub windows or “Regions Of Interest (ROI)

• The noise performance and sensitivity are however lower than those of CCD sensors, mainly due to large capacitances at the input of the output amplifier.

Architecture of CMOS passive image sensors

ACTIVE PIXEL SENSORS

• In Active pixel Sensors (APS) , the first amplification stage is implemented in every pixel, improving the noise figures and thus the achievable dynamic range and S/N ration.

• Also in terms of spatial resolution and sensitivity CMOS APS today present performance close to or better than their CCD equivalent.

• The largest CCD sensor has a resolution of 9216*9216 pixels, whereas the smallest pixels have a size of 2.4*2.4 micro meter. In comparison CMOS sensors with a resolution of up to 4096*4096 have been manufactured, while the smallest reported pixels have a size of 3.3 micro meter.

Architecture of CMOS Active image sensors

FEATURE COMPARISON

• Lower Cost and On-chip integration in CMOS Cameras.• Low power usage in CMOS Cameras• Flexibility of Operation• Higher Quality Images in CMOS Cameras• Better Quality Low Light Images in CCD Cameras.• Faster Frame Rates in CMOS Cameras• Smart functions in CMOS Cameras

USES

CONCLUSIONS CMOS video imagers can be implemented in consumer products such as camcorders, digital

cameras, toys, security cameras and digital cellular phone systems. Business and industrial

applications include video conferncing, machine vision, medical instrumentation, broadcasting

etc.

The key to replacing CCDs is to integrate the necessary on-chip circuitry to eliminate since

CMOS readily facilitates such integration, CMOS cameras are poised to replace the current CCD

based ones.

Future generation of CMOS image sensors will be based on process geometries of 0.25mm and

smaller. These devices will be able to provide resolution of 5 –25 million pixels at video frame

rates, matching the resolution of photographic film.

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