What Are Image Sensors?

CMOS and CCD sensors perform similar roles using different technology

a digital camera on a table
Digital cameras are packed with technology.

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All digital cameras have an image sensor that captures information to create a photograph. There are two primary types of image sensors — CMOS and CCD — and each has its advantages.

How Does an Image Sensor Work?

The easiest way to understand the image sensor is to think of it 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 consist of pixels that collect photons (energy packets of light) that are converted into an electrical charge by the photodiode. In turn, this information is converted into a digital value by the analog-to-digital converter (ADC), allowing 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 and CCD.

What Is a CCD Image Sensor?

Charge Coupled Device 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 their often higher cost.

There are some distinct advantages to a CCD sensor 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

What Is a CMOS Image Sensor?

Complementary Metal Oxide Semiconductor 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 like auto exposure, color encoding, and image compression directly in the chip
  • Prevents "smearing" when an image is overexposed
  • Less expensive manufacturing process
  • Quality has improved significantly since their introduction

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 only able to measure one color. The other two colors are estimated by the sensor based on the surrounding pixels.

While this approach can affect image quality slightly, it is 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, which each measure one of the primary colors. An image is produced by combining these three layers to produce a mosaic of square tiles. This is still a fairly new technology that is in use on some Sigma cameras.