CCDs (charge-coupled devices)

Last updated: May 12, 2008.

When you look at an amazing photograph, what do you see? Stunning colors, amazing light effects, and a dazzling scene that reveals the world to you in a surprising new way. What you don't see is the clever computer trickery that allowed the camera to store all this information in the first place. Tucked away inside every digital camera, there's an amazing, light-sensitive computer chip called a CCD (charge-coupled device), which turns the scene you look at through the camera lens into numbers a computer can store and manipulate. CCDs are the silent heroes of digital photography. We all have them, but how many of us know about them or understand how they work? Let's take a closer look!

Photo: A low-cost CCD chip from inside a webcam.

Painting by numbers

Our brains are built to handle visual information, but computers work a different way—they're made to store and process numbers. Old-style film cameras store images of the world pretty much as we see them by "burning" patterns of light into chemically treated rolls of plastic. Digital cameras don't necessarily create better photographs, but they certainly have other advantages: you can instantly copy your photos onto your computer, edit them, upload them onto the Web, or print them out. What makes digital cameras different is the way they turn your photos into a numeric (digital format) that computers can understand. They do that with an extremely clever chip called a CCD (charge-couple device) positioned directly behind the camera lens (where the film in an old-style camera would be).

A CCD is the heart of a digital camera. It is made of millions of tiny, light sensitive squares arranged in a grid pattern. These squares are called pixels. Good cameras use CCDs with many more pixels and this is why cameras are compared by how many megapixels (millions of pixels) they have. A camera rated as having 6 megapixels has 6 million pixels in its CCD—probably arranged in a rectangle with three thousand across and two thousand down (3000 x 2000 = 6 million).

When you take a digital photo, light from the thing you are photographing zooms into the camera lens. This incoming "picture" hits the CCD, which breaks it up into millions of pixels. The CCD measures how much light is arriving at each pixel. This information is turned into a number that can be stored on a memory chip inside the camera. Thus, taking a digital photograph converts the picture you see into a very long string of numbers. Each number describes one pixel in the image—how bright or dark and what color it is.

How charge-coupled devices (CCDs) work

Artwork showing how digital camera CCD works

Step by step

Light from the object (in this case, a red bicycle) enters the camera lens.
The CCD inside the camera splits the image up into millions of pixels (squares). An LCD display on the back of the camera shows you the image that the CCD is capturing—not an image of the object seen through a series of lenses (as with a conventional camera), but a redrawn, computerized version of the original object displayed on a screen.
The CCD measures the color and brightness of each pixel.
The color and brightness are stored as numbers in the camera's memory card. When you connect your camera to a computer, these numbers are transmitted instantly down the wire.

Here's another photo of the CCD from a webcam. The CCD is the square chip in the middle of this circuit. Only the tiny, central part is light-sensitive: the rest of the CCD chip is concerned with connecting the light detector to the bigger circuit that surrounds it. If you tilt the CCD slightly in the light, you can get a sense that there are lots of light-sensitive squares lurking inside, ready and waiting to generate your pixels!

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