The charge-coupled device (CCD) was invented in the late 1960s
by researchers at Bell Labs. Originally conceived as a new type
of computer memory circuit, it soon became apparent that the CCD
had many other potential applications, including signal processing
and imaging—the latter because of silicon's light
CCDs begin on thin wafers of silicon processed with a series
of steps that define the various functions within the circuit. On
each wafer lie several identical devices, or die, each capable of
yielding a functional device. Selected die are then cut from the
wafer and packaged in a carrier for use in a system.
Like the engine of a car, the CCD sensor in a digital camera
acts as the primary tool to capture an image. In its most elementary
form, the CCD sensor is like the camera's "electronic eye"—collecting
light and converting it to charge, and subsequently emitting the
signal that results in a digital image.
Kodak's patented CCD sensors are comprised of thousands
of pixels grouped in either a linear or matrix array to register
the overall light intensity of each point in a scene.
CCD Architecture from Kodak
Below is an overview on the methods of image capture taken by
CCD sensors and their real-world applications.
Area arrays allow photographers to capture moving objects in
one exposure at any shutter speed.
Linear arrays use a single row of pixels that scan across the
image, making three separate exposures—one for each of the
red/green/blue (RGB) filters. As the name suggests,
linear sensors capture one-dimensional images. They are primarily used
to capture still images for use in advertising. Linear arrays, while
possessing the capability to deliver high-resolution images, are
limited to motionless objects that must be continuously lit.
In the tri-linear sensor, three parallel linear CCD elements
are separately coated with RGB filters. When the colored image is
captured, it is built up line by line allowing for full color image
capture. Tri-linear CCD sensors are used in high-end digital cameras
to give the highest resolution and spectral gamut.
This type of sensor utilizes separate arrays for image capture
and charge transfer, allowing images to be read out while the next
image is captured. Interline transfer CCD sensors are commonly used
in lower-end digital cameras, video cameras, and broadcast cameras
for motion capture.
Allowing for more charge capacity, better dynamic range, reduced
noise and delivery of optical resolution, Kodak's full-frame
sensors allow for the full RGB matrix to be captured instantaneously.
Full-frame CCDs consist of a parallel CCD shift register, a serial
CCD shift register, and a signal-sensing output amplifier. In a
full-frame CCD, the exposure is controlled by a mechanical shutter
or strobe to preserve scene integrity, because a parallel register
is used for both scene detection and readout.
Images are optically projected onto the parallel array, which
acts as the image plane. The device takes the scene information
and partitions it into discrete elements, which are defined by the
number of pixels "quantizing" the scene. The resulting
rows of information are then shifted in a parallel fashion to the
serial register, which shifts the information to the output as a
serial stream of data. This process repeats until all rows are transferred
off chip. The image is then reconstructed as dictated by the system.
In KODAK PROFESSIONAL DCS 520 Digital Camera, full frame provides
the following resolution: 1736 X 1160 pixels.
Increasing the Blue Channel Response
In the DCS 520 Camera, Kodak Professional is the first to commercialize
and patent its unique use of Indium Tin Oxide (ITO). Leveraging
ITO, an advanced technology to replace the polysilicon sensor in
digital cameras, has resulted in a significantly enhanced blue channel.
This is a result of the ITO sensor's ability to be more
transmissive than the polysilicon sensors of the past. The key benefit
of the ITO sensor is its ability to increase the spectral response
from the camera, allowing two and one half times more blue light
to reach the sensor, therefore, improving the color accuracy and
reducing image noise.
CCD Sensors and Image Capture
Below is a step-by-step explanation of the CCD sensor and its
role in the digital image capture process.
Mechanical shutter opens, exposing
the CCD sensor to light.
Light is converted to charge in the CCD.
The shutter closes, blocking the light.
The charge is transferred to the CCD output and converted
to a signal.
The signal is digitized, and the digital data is captured
The captured image is processed and displayed on the
camera LCD or computer.
CCD imaging is performed in a three-step process:
Exposure, which converts light into
an electronic charge at discrete sites called pixels.
Charge transfer, which moves the packets of charge within
the silicon substrate.
Charge-to-voltage conversion and output amplification.
Converting Light to an Electronic Charge
An image is acquired when incident light in the form of photons
falls on the array of pixels. The energy associated with each photon
is absorbed by the silicon and a reaction takes place that creates
an electron-hole charge pair (for example, an electron). The number
of electrons collected at each pixel is linearly dependent on light
level and exposure time, nonlinearly dependent on wavelength.
Charge Transfer Techniques
Once the charge has been integrated and is held in the pixel
architecture, there must be a means of getting the charge to the
sense amplifier, which is physically separated from the pixels.
As the charge associated with one pixel move, at the same time,
the charge in all the pixels associated with that row or column
move as well.
The packets of charge are eventually shifted to the output sense
node, where electrons are converted to voltage. Conventional techniques
usually use a floating-diffusion sense node followed by a charge-to-voltage
amplifier, such as a source follower.
Source followers are used to preserve the linear relationship
between light in, electrons generated, and voltage output.
Kodak and Kodak Professional are trademarks of Eastman
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