The other half of this year’s Nobel Prize in physics goes to the inventors of the CCD, a device that converts images into electrical signals, thereby revolutionizing photography and digital imaging.
While Kao’s work grew out of a concerted effort to find a better telecommunications medium, Boyle and Smith’s was unanticipated. They developed the CCD at Bell Labs in 1969, after having sketched out the basic design during an hour-long brainstorming session. The principle behind the CCD is the photoelectric effect, which was in part theorized by Albert Einstein, earning him the Nobel Prize in 1921. When bombarded by a photon, some materials emit an electron. Boyle and Smith’s design is a silicon chip whose surface is covered with a grid of capacitors that store the electrons created when the chip is illuminated. Each capacitor is a pixel. The number of electrons stored at each capacitor is proportional to the intensity of the light in that part of the image. The image can be read out by pulling the charges off the CCD.
The advantage of the CCD over light-sensitive chemical films and even the human eye is its high sensitivity. Over the entire spectrum of light, from infrared to x-rays, CCDs can capture 90 percent of incoming photons. The eye or a film camera captures only 1 percent of these photons.
A year after their invention, Boyle and Smith made a video camera based on the digital-image sensor; in 1981 Sony brought to market the first CCD camera, the Mavica. Astronomers were early adopters and have used the sensors to capture images of distant celestial objects that were heretofore invisible.
Today the CCD faces some competition from another digital-imaging chip invented around the same time–CMOS (complementary metal-oxide-semiconductor). Both devices rely on the photoelectric effect. While the CCD directs electrons off the chip in a single stream to be read out, data from CMOS pixels are read out on site, which saves power and prolongs battery life. However, CMOS is not as sensitive as CCD, which still has advantages for advanced applications like astronomy and medical imaging.