23.03.10
Digital Camera Sensors Explained
In October last year, three scientists were recognised by the Nobel Foundation for their lifelong contribution to the study of physics. The pioneering work they conducted decades ago formed the foundations that allow today’s photographers to so easily record and share their images. Sean Samuels finds out more.
Professor Charles Kao was awarded one half of the Nobel Prize in Physics in 2009. His development of fibre optic cables made of pure glass is the reason why information across all mediums can now be sent and received, literally at the speed of light.But perhaps more important to photographers was the work of George E Smith and Willard Boyle, who share the remainder of the prestigious prize for inventing the device capable of recording images in the first place — the Charge Coupled Device or CCD sensor.
Modern digital imaging would not be possible without the research they conducted almost 40 years ago while working for Bell Laboratories in the US. Boyle joined in 1953 and Smith joined in 1959. Both were fortunate to enter a unique corporate atmosphere that granted its employees the freedom to think and invent, and it was during an afternoon of brainstorming in 1969 that Boyle and Smith devised the sensor. As researchers, they had been tasked with finding a creative solution to information storage, an all-consuming problem even in those times. The idea they came up with was an image sensor based on Albert Einstein’s photoelectric effect, for which he was awarded the Nobel Prize in Physics in 1921. Boyle and Smith’s development looked at the way photocells emit electrons in amounts proportional to the intensity of incoming light. In relation to photography, this meant an optical image going in could be transformed into a digital one. The challenge was to transform this information as accurately as possible in a short period of time. The desire was to recreate the optical image as a large number of image points or pixels.
As happens occasionally, technological solutions do not solve the problems that gave rise to their invention, and so it was with the image sensor. CCD was not to have a future in memory storage, but it did lead to an explosion in digital imaging. Made from silicon, CCD sensors hold millions of photocells arranged horizontally in rows and vertically in columns that are sensitive to light. When light hits a sensor, negatively charged electrons within the photocells are displaced and gathered up at the edge. When voltage is applied to the collected electrons, they form a more organised structure that can be analysed. What the sensor has done is transform the optical image into electric signals that are subsequently translated into digital ones and zeros, which can be read as pixels. It is an electronic eye, and the more light that passes through the better because more electrons are displaced, which means more pixels. This is why noise is more of an issue on most entry-level cameras than high-end DSLRs, because they house smaller sensors, over which less light passes.
Because CCD sensors register optical images in black and white, filters are required to produce colours. These filters contain red, green or blue colours and are placed over every photocell. The number of green pixels required need to be twice that of blue or red because of the sensitivity of the human eye.Just one year after its creation, Smith and Boyle demonstrated a CCD in a video camera for the first time, but it would not be until 1981 that the first camera with built-in CCD appeared on the market. Five years later, the first 1.4 megapixel image sensor emerged, and a further nine years on, the world’s first fully digital photographic camera was born, incorporating a shutter to cut off the amount of light hitting the sensor to avoid overexposure, and the ability to transfer the digitalinformation to memory, enabling another image to be taken in sequence.
Today the CCD faces competition from another technology - Complementary Metal Oxide Semiconductor (CMOS), which was invented at about the same time as CCD. (Indeed Willard and Boyle based the CCD on MOS technology.) CMOS sensors are used in cameras such as the Nikon D3s and Canon EOS 1D MkIV and both are capable of extraordinary photographic feats. For the foreseeable future, CMOS will be the technology of choice for many manufacturers. However, CCD sensors are used elsewhere, from astronomy to fax machines and medical scanners. Astronomy in particular makes maximum use of the technology in tools such as the Hubble Telescope for capturing images of distant galaxies.
For this reason, the work of Willard and Boyle should not be forgotten. It may not have been their intention to usher in a new era of photography, but both are absolutely aware of the impact their work has had. In his Nobel Banquet speech, Smith said it was heartening to see such a widespread application of the technology he and Boyle developed. “Think of snapping a photo with your cell phone,” he said “and instantly sending it to a friend thousands of miles away instead of finishing the roll of film, having it developed, putting it in an envelope and posting it to a far away country.”Expressed in this way, the world of photography would indeed be a lesser place were it not for the invention of the Charge Coupled Device.
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