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Charge coupled device CCD chip from a webcam

CCDs (charge-coupled devices)

by Chris Woodford. Last updated: December 10, 2013.

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. The light sensitive part is the greenish square in the very center. It's about as big as the fingernail on your little finger.

What does a CCD do?

George Eastman's original camera patent #388,850 dated September 4, 1888

Inside a digital camera showing the main components

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-coupled device) positioned directly behind the camera lens (where the film in an old-style camera would be).

Photos: Cameras, then and now. Left: This is how George Eastman conceived the camera back in the 1880s. Light enters through the shutter at the front (red), hits the light-sensitive film (green), and leaves behind an image, which is rendered onto paper by the developing process. The rolls at the back (orange) typically store enough film to take 24-36 photos: that's the camera's memory. Patent drawing courtesy of US Patent and Trademark Office (I've added the colors and film so you can see the details).

Right: In a modern digital camera, a CCD chip and flash memory card (blue, at the front) replace the film. Instead of developing, you simply download the images to your computer. In this camera (and most other digital cameras and webcams), the CCD is positioned immediately behind the lens. You can see a bigger version of this photo in our main article about digital cameras.

What is a CCD?

A CCD is the "electronic eye" of a digital camera. It's a semiconductor chip 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). A better camera rated at 12 megapixels would have a 4000 x 3000 pixel CD. Take a photo the same size with those two cameras and the 12 megapixel one is going to give you 1000 more dots horizontally and 1000 more vertically—smaller dots giving more detail and higher resolution. A single pixel in a CCD is something like 10 micrometers (10μm) in diameter (5–10 times smaller than the diameter of a typical human hair)!

How charge-coupled devices (CCDs) work

Artwork showing how digital camera CCD works

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 individual 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.

Step by step

  1. Light from the object (in this case, a bicycle) enters the camera lens.
  2. 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.
  3. The CCD measures the color and brightness of each pixel.
  4. The color and brightness are stored as binary numbers (patterns of zeros and ones) in the camera's memory card. When you connect your camera to a computer, these numbers are transmitted instantly down the wire.

Here are some more photos of the CCD from a webcam. The CCD is the square chip in the middle of this circuit. Only the tiny, green-colored central part (shown in closeup on the right) 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!

Webcam CCD or charge coupled device Webcam CCD or charge coupled device

Who invented CCDs?

The CCD we all use today was invented in fall 1969 by Canadian-born Willard S. Boyle (1924–) and American George E. Smith (1930–), two colleagues working at Bell Laboratories (a famous American research center in New Jersey responsible for all kinds of amazing inventions, most famous of which is the transistor). Boyle and Smith were trying to develop a new kind of computer memory—in their notes, originally called a charge "bubble" device—but what they actually invented proved far more useful for capturing and storing images in digital form.

The science behind the CCD (turning light energy into electrical energy) dates back much further—to 1905. Known as the photoelectric effect, it was the first major scientific discovery by Albert Einstein (1879–1955). Einstein showed how a light beam could give up its energy when it hit the surface of a material, knocking out electrons that would then form an electric current—and a quantity of electrons that could be related directly to the intensity of the incoming light. It was for this early piece of work (and not his much more famous later work on relativity) that Einstein was awarded the Nobel Prize in Physics 1921. Boyle and Smith earned their own place in history almost 90 years later when they won the Nobel Prize in Physics 2009 (shared with fiber-optic pioneer Charles Kao).

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Text copyright © Chris Woodford 2008, 2011. All rights reserved. Full copyright notice and terms of use.

The black-and-white bicycle artwork used in this article appears on UK road signs and comes from the UK traffic signs images database. It is Crown copyright, but reused here under the UK's Open Government Licence.

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Woodford, Chris. (2008) CCDs. Retrieved from http://www.explainthatstuff.com/howccdswork.html. [Accessed (Insert date here)]

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