The Electronic Bucket Brigade


When a building caught fire in colonial America, fire trucks equipped with high pressure water did not arrive to dampen the flames because they had yet to be invented. But fires still needed to be quenched. Consequently, these early Americans borrowed a practice dating back to the time of the Roman Emperor Nero. Residents aligned themselves in straggling lines that stretched from the site of the conflagration to the nearest sizable water source—a pond, perhaps, or a river. There, from the water’s edge, one person filled a bucket with water and passed it to the person standing next to her. That person passed it to the next person in line, and so on until it reached the person standing next to the inferno.

Bucket brigades were so culturally resonant that in 1969 when F. Sangster and K. Teer of the Philips Research Labs invented a device that took electrical charged packets and moved them from one transistor to another in much the same leaky way that colonial Americans transferred buckets of water from one location to another. They dubbed it the bucket-brigade device or BBD.

To understand what a BBD does, it helps to understand how it’s most commonly used—to create electric guitar effects. A BBD captures an electric guitar’s sound, in the form of a charged packet, and moves the packet from capacitor to capacitor before releasing it to the speaker. The whole process creates the sound effects that are the hallmark of the electric guitar: echoes, reverbs, and vibratos. The BBDs most substantial impact, however, isn’t auditory; it’s visual.

In 1970 Bell Laboratories’ researchers Willard Boyle and George Smith took Sangster’s and Teer’s BBD device and expanded upon it. They created a mechanism that, like the BBD, transports a charged packet from one location to another, bucket brigade style. But while the BBD moved electrons from capacitor to capacitor, Boyle and Smith’s device moved a charge from a silicon plate to a capacitor, a difference that changed the way we see the world. They named the device the CCD, short for Charged Coupled Device.

A CCD captures and converts light into an electrical signal , which is then translated by a processor chip. It’s based on a physical quirk of nature. When light strikes metals and metalloids (elements like silicon that exhibit behaviors of both nonmetals and metals), it releases electrons. This phenomenon, known as the photoelectric effect, is the foundation underpinning CCDs. These devices are essentially sheets of photosensitive grids—a kind of silicon and aluminum waffle. When light hits the squares, like drizzling maple syrup over the waffle, electrons are released due to the photoelectric effect. This electrical information then gets dumped—as if you were to tilt your waffle on an angle so the syrup flowed along the segments ridge, or channel— collected, and characterized by a waiting processor.

The CCD was originally conceived as a device to store information, but the realization that it was far more sensitive to light than film made it a natural fit for photographic imaging. Typically, in photographic film of the sort that was the mainstay of point and shoot cameras a generation ago, only two percent of the light that struck the film was able to trigger the chemical reaction needed to produce an image. Roughly 98 percent of the light is wasted, explaining the true reason behind all of those seemingly perfect shots that were returned as dark splotches upon development. The CCD in your typical digital camera, however, is 35 times more sensitive to light than film; it reacts to 70 percent of the light it receives. If a person were a photograph, someone who ordinarily takes 60 minutes to tan on a cloudy day, would suddenly find themselves tanning in less than two.

Each waffle square—a pixel—of the CCD builds up an electrical charge in proportion to the light’s intensity. More light creates more charge. The charge gets stored in the capacitors, and when the image is done being recorded—when the shutter slams shut—they get dumped, bucket brigade style into the processor chip.

But what about color? A CCD only detects total light intensity. That’s useful for black and white photography, but it doesn’t capture colors like fire engine red. For that, the CCD is covered with a series of filters—red, blue, and green—that the processor can then translate into color images.

CCDs didn’t create digital images—but they revolutionized them. Early digital cameras required every pixel to be directly wired together. Needless to say, the resulting technology was both costly and bulky. CCDs helped lower the cost and make digital imaging ubiquitous. Less than half a century after its invention, the CCD can be found in photocopying machines, closed circuit televisions, and perhaps most importantly for the modern photographer in digital cameras. Not too bad for a technology based, at least in part, on an idea that dates back more than a thousand years.