Rakefet Ben-Ari
Hunter College
Physics Major
1 of 3
A terrorist organization has deployed one of its agents to carry out an attack. The tall bearded
man is en route to his target - a strip mall in New York City. As he walks by a convenience store,
his gun falls from his jacket. He quickly grabs it and conceals it under his coat. A bystander,
having seen the incident, runs into the convenience store to call the police. Using a security
camera monitoring the store's entrance, the police identify the man and arrest him moments
before he is able to cause devastation. Given New York's high population density, a simple
description of the attacker would not have sufficed to identify him, but a clear digital image was
able to save lives. The innovative imaging device that captures light and stores it as information,
ultimately saving lives, preserving our most precious memories, and exploring the depths of
space, earned its inventors the Nobel Prize in Physics in 2009.
The creation of the charge-coupled device (CCD) began in October 1969 with a discussion
between the joint laureates, Willard Boyle and George Smith of Bell Laboratories. Their goal
was to compete with a similar project being conducted in another Bell department. Here we will
explain the physics behind the device's operation.
First, one must understand the photoelectric effect. Packets of energy known as photons interact
with the physical matter around us, acting both as waves and as particles. This property is called
the wave-particle duality. The most direct example of the photoelectric effect is that of a photon
leaving a light bulb and entering your eye. When the photon makes contact with special cells in
your eye, the cells send a message to your brain based on the frequency and intensity of the light.
The color of the light is determined by its frequency, or the number of cycles the wave
completes in a given amount of time (ie, cycles per second). Photons emitted at a higher
frequency have more energy per photon than those emitted at a lower frequency. The brightness
is determined by the light’s intensity, which depends on how many photons the source emits. A
larger number of photons creates light of greater intensity, though a higher intensity light does
not contain more energy per photon than light of the same frequency at a lower intensity.
After the photon reaches its destination, in this analogy your eye,
electrons in special molecules of your eye cells absorb the photon
and, with the gained energy, leave the molecule. Any given
electron is sensitive to a specific photon frequency. Therefore,
your eye transmits data to your brain based on which electrons are
being displaced and how many. The frequency, and therefore the
color of the light, is revealed by which electrons are being
displaced, and the number of electrons reveals the light’s intensity
or brightness. The phenomenon of photons ejecting the electrons
that absorbed them is called the photoelectric effect (Fig 11).
Fig 1: Photoelectric Effect1
How does the light from the original image, which is large, fit into the small area of your eye?
Note that your eye, like a camera, is covered by a lens. A lens is an asymmetrical, transparent
object that can either condense light or disperse it. In the case of a projector, for example, the
lens disperses highly concentrated light to a larger image. Other apparatus, such as eyes and
cameras, do the opposite: they take dispersed light and condense it into a much smaller space.
Rakefet Ben-Ari
Hunter College
Physics Major
2 of 3
The CCD is able to record images in a similar
fashion to your eye. Its basic construction follows
the metal-oxide semiconductor (MOS) model (Fig
22). The basic MOS model consists of three layers.
The uppermost layer is made of organized, square
metal electrodes, which act as gates through which
photons can pass. They rest on a thin oxide layer,
2
Fig 2: Basic MOS model
which in turn rests on a thick semiconductor, in this
case silicon. When a positive charge is applied to a
specific gate, electrons from the silicon layer gather under the oxide layer in the region below the
gate. Due to the high concentration of electrons in one area, other electrons are repelled from the
surrounding area. This phenomenon creates a potential well, or a defined region in which a
specific type of energy (in this case, electric potential) cannot convert into other forms of energy.
When the shutter of the camera is opened, photons from the external light are absorbed by some
of the electrons in each well, leaving behind the information required for a single pixel. A pixel
is the smallest unit of an image, and is usually represented by squares. Very quickly, as to
prevent overexposure, either the shutter must be closed or, in digital cameras, the pixel is
transferred away from the region of sensitivity.
The CCD's ability to transfer the analog pixels to a digital
information storage device is one of its most impressive
features. Having been exposed to light, there is a single
pixel stored in each well of the CCD. Recall that only
some of the gates had a positive charge, while the
neighbors of those gates did not. Therefore, each well is
isolated from every other well, and furthermore the
electrons cannot escape the wells in which they are
contained. When a positive charge is applied to a gate
neighboring a well, the well expands so it extends under
two gates and the electrons spread out as to fill the
enlarged well. The positive charge is then removed from
Fig 3: CCD Potential Well Transport3
the original gate and all the electrons gather in their new
well under the neighbor gate, and thus the pixel has shifted one space (Fig 33). The pixel is then
shifted again until it reaches the Analog/Digital (A/D) converter, a device capable of
interpreting analog pixels and transmitting them in the form of digital information to a memory
storage device.
Imagine now a CCD with 2000 columns, each with 2000 gates. (To be exact, 2048x2048 pixels
are standard in modern CCDs.) When the shutter is opened, some of those gates are prepared to
accept a pixel, which will be transferred, one column at a time, to the A/D converter and
ultimately to a memory storage device. Thus far, we have only described how a black and white
picture can be captured. We will now explain the additional process used for creating a color
image.
Rakefet Ben-Ari
Hunter College
Physics Major
3 of 3
Creating a color image requires installing a color filter. A rather ubiquitous
filter used is called the Bayer filter, which arranges a mosaic of red, green,
and blue (RGB) filters. For each 2x2 set of gates, there are two green
filters above gates diagonally across from each other, with a red and a
green filter above the other two gates (Fig 44). This proportion is used to
4
Fig 4: Bayer Filter take into consideration that the human eye is considerably more sensitive
to green light than red or blue. As discussed previously, a photon's frequency determines its
color. White light is a mixture of photons of all visible color frequencies in approximately equal
proportions, while light of specific colors have abundant photons of some frequencies and fewer
photons of others. When the light reaches the filter, only
the "blue" photons can penetrate the blue filter, and so too
with red and green (Fig 55). Thus, when the CCD is
exposed to light, each set of four gates contains a certain
proportion of electrons based on the intensity of each of
those colors in the image being captured. When the
analog pixels are moved to the A/D converter, the digital
interpretation understands which electrons are coming
from the respective RGB filtered gates, and will present a
color pixel using the correct proportion of each RGB
Fig 5: Color Filtering5
color. For more accurate colors, one could replace one of
the green filters in each set of four with an emerald filter, created a red-green-blue-emerald
(RGBE) filter. Alternatively, other larger arrangements of RGB filters can produce more precise
colors. Kodak is experimenting with several 4x4 filter arrangements. To create more luminescent
pictures, one could use a cyan-yellow-green-magenta (CYGM) filter, though this comes at the
expense of color accuracy. It should be noted that because of the 2x2 RGB arrangement, four
analog pixels from the CCD are required for each digital pixel in the final image. Therefore, a
color photo will have poorer resolution than its black-and-white counterpart (and even more so
with the larger RGB patterns mentioned), though there are ways to overcome this limitation.
Image Sources:
1. Wolfmankurd. Photoelectric effect. Digital image. Wikipedia, 8 May 2007. Web. 11 Nov.
2009. <http://en.wikipedia.org/wiki/File:Photoelectric_effect.svg>.
2. Fig. 1: The basic MOS structure. Digital image. Nobel Prize in Physics 2009. The Royal
Swedish Academy of Sciences, 6 Oct. 2009. Web. 11 Nov. 2009.
<http://nobelprize.org/nobel_prizes/physics/laureates/2009/sciback_phy_09.pdf>.
3. Fig. 4: The sequence of collecting and moving the charge along a column of a CCD
detector. Digital image. Nobel Prize in Physics 2009. The Royal Swedish Academy of
Sciences,
6
Oct.
2009.
Web.
11
Nov.
2009.
<http://nobelprize.org/nobel_prizes/physics/laureates/2009/sciback_phy_09.pdf>.
4. Fig. 5: Bayer filter. Digital image. Nobel Prize in Physics 2009. The Royal Swedish
Academy
of
Sciences,
6
Oct.
2009.
Web.
11
Nov.
2009.
<http://nobelprize.org/nobel_prizes/physics/laureates/2009/sciback_phy_09.pdf>.
5. Cburnett. Bayer pattern on sensor profile. Digital image. Wikipedia, 28 Dec. 2006. Web.
11 Nov. 2009. <http://en.wikipedia.org/wiki/File:Bayer_pattern_on_sensor.svg>.