What Are Image Sensors?

CMOS and CCD sensors perform similar functions using different technology

a digital camera on a table

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All digital cameras have an image sensor that captures information to create a photograph. CMOS, CCD, color filter array and Foveon sensors are the most commonly used.

How Does an Image Sensor Work?

Think of an image sensor as the equivalent of a piece of film. When the shutter button on a digital camera is depressed, light enters the camera. The image is exposed onto the sensor in the same way that it would be exposed onto a piece of film in a 35mm film camera.

Digital camera sensors capture light and other information, which is converted into a digital value by the analog-to-digital converter (ADC). This allows the camera to process the values into the final image.

DSLR cameras and point-and-shoot cameras primarily use two types of image sensors: CMOS (complementary metal–oxide semiconductor) and CCD (charged coupled device).

CCD Image Sensors

CCD sensors convert pixel measurements sequentially using circuitry surrounding the sensor. CCDs use a single amplifier for all the pixels.

CCDs are manufactured in foundries with specialized equipment. This complexity is reflected in a cost that's usually higher than that of CMOS sensors.

A CCD sensor has a few distinct advantages over a CMOS sensor:

  • Less noise and typically higher-quality images, particularly in low-light conditions.
  • Better depth of color, because the dynamic range of the sensor is often twice that of CMOS sensors.
  • Higher resolution and light sensitivity.

CMOS Image Sensors

CMOS sensors convert pixel measurements simultaneously, using circuitry on the sensor itself. CMOS sensors use separate amplifiers for each pixel.

CMOS sensors are commonly used in DSLRs because they are faster and cheaper than CCD sensors. Both Nikon and Canon use CMOS sensors in their high-end DSLR cameras.

The CMOS sensor also has its advantages:

  • Faster processing speed, because the active pixels and ADC are on the same chip.
  • Lower power consumption—as much as 100 times less than a CCD.
  • Integrated camera functions such as auto exposure, color encoding, and image compression directly in the chip.
  • Prevents "smearing" when an image is overexposed.
  • Less expensive manufacturing process.
  • Continuing quality improvements.

Color Filter Array Sensors

A color filter array is fitted to the top of the sensor to capture red, green, and blue components of light falling on the sensor. Therefore, each pixel is able to measure only one color. The other two colors are estimated by the sensor based on the surrounding pixels.

This approach can affect image quality slightly, but it's hardly noticeable on today's high-resolution cameras. Most current DSLRs use this technology.

Foveon Sensors

Human eyes are sensitive to the three primary colors of red, green, and blue, and other colors are worked out by a combination of the primary colors. In film photography, the different primary colors expose the corresponding chemical layer of film.

Similarly, Foveon sensors have three sensor layers; each measures one of the primary colors. An image is produced by combining these three layers to produce a mosaic of square tiles. Some Sigma cameras use Foveons, but these sensors aren't as popular. Photographers attribute this to their slower speed, greater image file sizes, and trickier techniques than Foveon's more common counterparts.

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