Willard S. Boyle
Willard S. Boyle
Co-Inventor of the Charge Coupled Device for which he won the 2009 Nobel Prize in Physics
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Boyle’s branch of science is called solid state physics or condensed matter physics, and it involves the behaviour of materials that are solid — things such as crystals, metals and rocks. In particular, he worked on semiconducting materials such as the element silicon. He invented many things while at Bell Labs, but his most famous invention was the Charge Coupled Device or CCD.
1. At the heart of many camcorders and digital cameras is a charge coupled device (CCD), typically about a square centimetre in size.
2. Light in the form of incoming photons enters through the lens of the camera and falls onto the surface of the CCD chip, often passing through a colour filter array. This generates free electrons in the silicon of the CCD, more where the light is brighter and fewer where it is less intense. These electrons collect in little packets created by the geometry of the silicon and surrounding electrical circuitry, laid out in a two-dimensional grid on the chip. Typical CCD chips have from one to five million such packets of charge, which can also be pictured as buckets on a conveyor belt.
3. The CCD operates on the principle of charge coupling. The packets of charged electrons can be moved one row at a time by varying the voltage of adjacent rows, thereby creating a potential well that couples two rows and causes the charge to move over.
4. Imagine buckets on conveyor belts catching falling rain, to represent photons of light. Each bucket (packet) contains a different amount of water (charge), depending on how much rain fell on that part of the array. The buckets are shifted in an orderly fashion to a collecting row, then to a final measuring device at the front. In this way the quantity of water in each bucket is counted. In a typical CCD this can happen very fast: about 30 times per second for every one of millions of “buckets” on the CCD.
Modern CCDs have colour filters (red, green, blue) arranged in a pattern over the chip so that colour images can be collected. The output of the CCD is a string of numbers that define the intensity and the colour of light over the entire image. A computer or camcorder can store these numbers or use them to recreate the image on any kind of viewing screen or printer. For more on how a CCD works visit the Molecular Expressions website.
In recent years, complementary metal oxide semiconductor (CMOS) imagers are replacing CCD chips in some imaging devices. These are not based on the CCD principle but are rectangular arrays of individually addressable pixels. CMOS is the dominant technology for all microchip manufacturing, so cmos image sensors are cheaper to make. In addition, supporting circuitry can be incorporated onto the same device in a single manufacturing process. CMOS sensors also have the advantage of lower power consumption and better infrared sensitivity, or heat imaging, than CCDs. However, for many high-end cameras and camcorders a CCD is still preferred because of its sharper, cleaner images for most photographic applications.
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Boyle expects that many cosmological mysteries will yield their secrets due to the superior imaging power of CCDs, which are now used in virtually every telescope. “We’re going to see much greater understanding of the origin of our universe by having the ability to see things that are eight billion light-years away,” he says. “The mother of all mysteries is the origin of the universe.”
James R. Janesick, Scientific Charge-Coupled Devices, SPIE Press Monograph, vol. PM83, 2001.