CMOS Image Sensor

CMOS image sensors have been in existence since the late 1960s. The basic structure of a pixel consists of a light-sensitive element and a MOS transistor acting as a switch. The early design had a photodiode as the photo-sensing element. However, due to the issues of large fixed-pattern noise (FPN), scal ability, and difficulty to obtain fast read-out, MOS sensors have lost out to CCDs as the sensor of choice. In the past few years, CMOS sensors have attracted a lot of attention from the imaging industry. The demand for low-cost, low-power, and compact imaging systems and the significant improvement in device scaling are the major contributing factors to the renewed interest in CMOS sensors.

Advantages of CMOS Image Sensors

The advantages of CMOS image sensors are

• low-voltage operation and reduced power consumption 
• the ability to integrate timing, control, and image processing circuitry onto the sensor chip
• random access of the image data that allows electronic windowing, pan and zoom
•  reduced silicon processing cost, benefits from the compatible fabrication process, and the huge volumes in the generic CMOS industry

Drawback of CMOS Image Sensor

One major problem associated with CMOS sensors is the relatively large fixed-pattern noise (FPN). Unlike CCD sensors, each pixel in a CMOS sensor has its own amplifier. Variations of amplifier gain and offset result in a static pattern noise in the background of the image.

Applications of CCD and CMOS Image Sensor

CCD image sensors have been used in a variety of con sumer, commercial, medical, scientific, and military applications. In the consumer market, the home-use CCD-based camcorder has been the dominant product for many years. Only in recent years have low end digital electronic cameras started winning the acceptance of consumers. In the commercial mar kets, high-resolution digital still cameras are widely being used in photojournalism, catalog preparation, advertising, and studio photography for advantages which include good image quality, instant feedback, and ease of editing and management.

 Low-to-medium resolution electronic cameras have been used in insurance, real estate, law enforcement, security and surveillance. Traffic monitoring is an area of appli cation that has been gaining a lot of interest. Scanning applications include document scanning, FAX, graphic arts, digital archiving of movie and artistic works, automated inspection, and sorting. Machine vision can be used in robotics manufacturing, and remote inspection of areas unsafe to humans.

 Ultra high-frame rate cameras have been used in the automotive crash testing and inspection of high-speed moving processes. Medical imaging applications such as microscopy, endoscopy, mammography and x-ray dental detection have been moving towards the use of electronic imaging for better detection instant diagnostics, lower x-ray dose, and increased patient comfort. 

In scientific applications, high-resolution, low-noise, high-sensitivity and high-dynamic range CCD cameras are used in astronomical observato ries, spectroscopic instruments, and space-based systems such as the Hubble Space Telescope. Recently, images of Mars captured by the CCD cameras on the Mars Rover have created worldwide interest on the space programme. Satellite and remote sensing provide monitoring of earth resources, weather tracking, military detection, and identification in the military applications. CCDs have the advantages of small size and ruggedness. They are used for night vision, target identification, and mapping.

With the advantages of low-power consumption, high level of integration and low production cost, CMOS sensors are ideal for cost cameras. Applications such as PC-peripheral digital camera for input to web pages and video conferencing, interactive games, security monitoring, toys, video phone, automobile sensor, fax, and document scanning will see increased adoption of the CMOS imagers as most of them become com mercially available. With the improved performance of CMOS imagers, more sophisticated and high resolution imaging applications will be possible.

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