What is a CMOS Sensor?

Topics Covered

Working Principle
Construction and Types of CMOS Sensors


Complementary metal oxide semiconductors are known as CMOS sensors. These sensors were invented in the 1970s. Another term used to refer to CMOS sensors is active pixel sensing (APS). CMOS technology produces images of high quality, and is more specialized to obtain.

CMOS technology scores over similar imaging sensors mainly due to lower voltage and power consumption, faster read out, and low output noise. Apart from being used as image sensors, CMOS technology is used for data conversion and as transceivers. This article will elaborate on the working principle, construction, and application of CMOS sensors.

Working Principle

Pixels are the light detectors of CMOS sensors. Each pixel is made up of a photodiode, capacitor, and transistors. The capacitor is charged to a known voltage prior to the exposure period. On exposure to light (image), the capacitor drains out through the photodiode.

The rate at which the charge is drained is proportional to the intensity of the light. At the end of the exposure time, the remaining charge is read out and converted into a digital signal.

Construction and Types of CMOS sensors

CMOS sensors consist of active pixel sensors. Each of these pixels is an integration of a photodiode, capacitor, and transistors. The photodiode array represents a Bayer color filter pattern for capturing the color information.

The two types of CMOS image sensors are as below:

  • Passive pixel – these devices have charge amplifiers at the bottom of each pixel, and each pixel has a photodiode, capacitor, and a single transistor
  • Active pixel – these devices implement an amplifier in every pixel


Some of the applications of CMOS sensors are listed below:

  • Scanners, fax machines, security cameras, toys, and cameras
  • Bar code readers, cell phones
  • Medical diagnostics



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