Collecting light for imaging used to be a lengthy business for astronomers trying to capture pictures on photographic film of far away stars and galaxies. All this changed with the electronics revolution and the invention of the charge-coupled device, a light-sensitive silicon chip roughly the size of a postage stamp. Now astronomers can observe objects at the edge of the known Universe with ease, and the charge-coupled device, or CCD, has been found in wide-ranging applications from satellites to supermarket checkouts.
The CCD was first developed at Bell Laboratories in the United States in 1969. Originally it was intended for use in a video telephone that needed a solid-state miniature camera. Video phones did not catch on at that time, but the technology soon found a new home in the video camera. In 1975, Bell demonstrated the first CCD camera with an image sharp enough for broadcast television.
CCDs work by storing charge when light falls onto a transistor gate, or capacitor, called a pixel. Pixels are square pockets of doped silicon arranged in rows. Light falling onto the pixel frees electrons in the doped silicon, and the charge accumulated by each pixel is proportional to the amount of light it receives. The charge stays in the pixel for a fixed amount of time, until the CCD is "clocked." When this happens, the pixel transfers its charge to the next pixel in the row. The process is repeated every time the CCD is clocked until the charge reaches the end of the row. On arrival, the charge is transferred to a low-noise amplifier that converts it to a voltage signal. These signals can be read out or stored on computer.
One advantage of CCDs, their linear response to light, makes it possible to tell how many photons were collected in each pixel, which is useful in astrometry, as astronomers can measure the distances and positions of stars direct from the CCD image. Astronomers usually work with just one CCD at a time, giving a black and white view. However, because the information is stored digitally, it can be manipulated by computers with false colors assigned to specific intensities of light. Home video cameras, on the other hand, have three CCDs, each with a different colored filter. Combining the outputs from the CCDs produces a color picture.
Early CCD arrays were subject to readout noise and were less than 100 by 100 pixels in size. NASA’s Jet Propulsion Laboratory, which had picked up on the potential for using CCDs in astronomy, began development of bigger CCDs, culminating in the 800 by 800 pixel units used on the Hubble Space Telescope when it was launched in 1990. Since then, the standard format for second generation CCDs has reached 2,048 by 2,048, with over four million pixels. These state-of-the-art devices can detect photons at as low a rate as one per minute, making them ideal for viewing very faint objects. Multiple arrangements of CCDs known as mosaics have improved the resolution even more.
CCD technology has not been confined to astronomy and video cameras. Today CCDs can be found in other video applications such as the endoscopes used in surgery, security monitoring, videoconferencing, and high-definition television. Other optical technologies have also made use of them—photocopiers, fax machines, barcode readers, and photographic scanners all use the CCD’s ability to turn patterns of light into digital images. Digital cameras are the latest application, with photographers able to download their pictures directly from a disk to a computer.
