Published: 26 May, 2005

Photoelectric effect is the reason digital cameras work

How does the CCD work in a digital camera?

Most digital cameras work using a light sensor called a CCD - or "Charge couple device" - which contains a grid of millions of tiny devices called capacitors. When a picture is taken with a digital camera, the shutter opens and an image is projected onto the CCD. The light that hits each capacitor can bump electrons out of their atoms causing the capacitor to become charged. This is called the photoelectric effect. The more intense the light is at each site (or pixel) in the grid, the more charge will be held in that capacitor. You can think of the capacitors as buckets and the light as rain. After the shutter has been closed, the charge that has accumulated in each of the capacitors has to be measured in order to get the image data. First a circuit reads how much charge is in each of the capacitors in the bottom row. Then the charges in each row of capacitors are moved to the row below, the bottom row is read out again, and this process is repeated until all of the image data is read.

This information tells us how bright the light is at each pixel but that would only give us a black and white image. To get a color image, some digital cameras use a device called a beam splitter, which splits up the light into red, blue, and green beams and sends each one to a different CCD. This kind of camera can generate very high quality pictures because it closely measures the color of the image at each pixel. But because they constain 3 CCD's, these cameras are bulky and expensive. Most cheaper digital cameras use what's called a Bayer mask to generate a color image using only one CCD. For each group of four pixels in a CCD with a Bayer mask, one is filtered to measure red light, one blue, and two green. After the data is read from the CCD, a computer chip finds the actual color and intensity of each pixel by averaging the amount of red, green, and blue in the nearby pixels. This process is called interpolation.

The number of "megapixels" a camera has refers to how many millions of capacitors there are in the CCD to measure the intensity of light. A 4 megapixel camera would have approximately 4 million capacitors on the CCD. A typical CCD in a digital camera is about 4 by 5 millimeters. That means that each capacitor on a 4 megapixel CCD is only about than 2/1000 of a millimeter or 2 microns long.

CCDs are also used by astronomers to carefully measure the intensity and frequency of light coming from objects in outer space. In fact, most of the images from the Hubble Space Telescope are taken with CCDs.