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KODAK PROFESSIONAL: Technical Information Bulletin

Technical Overview: CCD Technology
 
 

Contents

 

Introduction

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 sensitivity.

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

Area arrays allow photographers to capture moving objects in one exposure at any shutter speed.

Linear Arrays

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.

Tri-Linear Sensor

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.

Interline Transfer

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.

Full Frame

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.

  1. Mechanical shutter opens, exposing the CCD sensor to light.

  2. Light is converted to charge in the CCD.

  3. The shutter closes, blocking the light.

  4. The charge is transferred to the CCD output and converted to a signal.

  5. The signal is digitized, and the digital data is captured in memory.

  6. The captured image is processed and displayed on the camera LCD or computer.

CCD imaging is performed in a three-step process:

  1. Exposure, which converts light into an electronic charge at discrete sites called pixels.

  2. Charge transfer, which moves the packets of charge within the silicon substrate.

  3. 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 Kodak Company.

TIB4131

Technical Information Bulletins provide information of limited or specific application. Responsibility for judging the applicability of the information for a specific use rests with the end user.