CCDs (charge-coupled devices)
by Chris Woodford. Last updated: January 28, 2012.
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?
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
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
- Light from the object (in this case, a 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 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!
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).
Further reading
On this site
- Photography: Our introductory article to old-style film photography.
- Digital cameras: Modern photography—the way most of us do it now.
- Light: The science of light—a form of electromagnetic energy.
On other sites
- Nobel Prize in Physics 2009: Masters of Light: Explains the contributions of Willard Boyle, George Smith, and Charles Kao to our modern digital world. Includes a copy of
a page from Boyle and Smith's lab notebook where they talk about their discovery of the "charge bubble device" (as it was originally called).
Books
- Handbook of CCD astronomy by Steve B. Howell. Cambridge University Press, 2006. As the title suggests, this book is mainly about using CCDs for making images of space, but the first half of it (about 100 pages or so) is all about CCDs generally (the different types, the quantum physics of how they work, and so on), so it's worth a look if you're interested in CCDs for other reasons.
- Handbook of Modern Sensors: Physics, Designs, and Applications by Jacob Fraden. Springer, 2010. A detailed (663-page) reference book that compares the physics behind all kinds of analog and digital sensors (with a short section on CCDs).
