What Is a CMOS Image Sensor?

Sensors impact images more than you realize

CMOS technology is found in some camera image sensors.
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A Complementary Metal-Oxide Semiconductor (CMOS) image sensor is a type of image sensor technology inside some digital cameras. It consists of an integrated circuit that records an image. You can think of the image sensor as being similar to the film in an old film camera. 

The CMOS sensor consists of millions of pixel sensors, each of which includes a photodetector. As light enters the camera through the lens, it strikes the CMOS image sensor, which causes each photodetector to accumulate an electrical charge based on the amount of light that strikes it.

The digital camera then converts the charge to a digital reading, which determines the strength of the light measured at each photodetector, as well as the color. The software used to display photos converts those readings into the individual pixels that make up the photo when displayed together.

CMOS vs. CCD

CMOS uses a slightly different technology from a Charged Coupled Device (CCD)—another type of image sensor found in digital cameras. More digital cameras are using CMOS technology than CCD because CMOS image sensors use less power and can transmit data faster than CCD. However, CMOS image sensors tend to cost more than CCD.

And as image sensors have been increasing in the number of pixels they record, the ability of a CMOS image sensor to move the data faster on the chip and to other components of the camera has become more valuable.

In the early days of digital cameras, the batteries were larger because the cameras were larger, and so the CCD's higher power consumption was not a huge concern. But as digital cameras shrunk in size, requiring smaller batteries, CMOS became the better option.

Benefits of CMOS

One area where CMOS really has an advantage over other image sensor technologies is in the tasks it is able to perform on a chip, rather than sending the image sensor data to the camera's firmware or software for processing. For example, a CMOS image sensor can perform noise reduction capabilities directly on the chip, which saves time when moving data inside the camera.

The CMOS image sensor can also perform analog-to-digital conversion processes on the chip—something CCD image sensors cannot do. Some cameras will even perform autofocus work on the CMOS image sensor itself, which again improves the camera's overall performance speeds.

Continued Improvements in CMOS

As camera manufacturers have migrated toward CMOS technology for image sensors in cameras, more research has gone into the technology, resulting in even strong improvements. For example, while CCD image sensors used to be cheaper than CMOS to manufacture, the additional research focus on CMOS image sensors has allowed the cost of CMOS to continue to drop.

One area where this emphasis on research has benefited CMOS is in low light technology. CMOS image sensors continue to show improvement in their ability to record images with decent results in low-light photography. The on-chip noise reduction capabilities of CMOS have steadily increased in recent years, further improving the ability of the CMOS image sensor to perform well in low light.

Another recent improvement to CMOS was the introduction of back-illuminated image sensor technology. With this design, the wires that move data from the image sensor to the camera are moved from the front of the image sensor—where they can block some of the light striking the sensor—to the back. This help the CMOS image sensor perform better in low light, while retaining the chip's ability to move data at a high speed when compared with CCD image sensors.