CCD - Reliable with supreme quality

Sensors based on CCD (Charge-Coupled Device) technology helped pioneer the development of digital machine vision. Light sensitive areas (photosites) in each of the pixels of the CCD chip convert incoming photons to electric charge. Compared with CMOS technology, CCD sensors generally have a larger light sensitive area. This simplifies the illumination requirements in your application.

Simplicity is the strength of CCDs

The collected charge is transported in a bucket-brigade manner to the Analog-to-Digital (ADC) converter. The most commonly used CCD sensors have a single ADC, with negligible effect on Signal-to-Noise ratio. CCD technology is still preferred for high performance imaging.

CCD based SVCam models from SVS-Vistek

SVS-VISTEK provides a broad selection of CCD cameras - with resolution ranging from VGA up to 47 megapixel. Choose the one that best suits your needs, or contact us with your specific requirement.

 

Cameras from SVS-VISTEK with CCD technologie (120)

Worth knowing about CCD technique

The history of CCD

The original use for CCD elements (Charge-Coupled Devices) was solely for the transportation of electric charge. As early as in the 1970s the photoelectric properties of the semiconductor material found practical use. This permitted the transport of light-generated charge in the CCD.

CCD was worth a Nobel prize

In 2009 the minds behind the CCD technology received the Nobel Prize in Physics. The two laureates, George E. Smith and Willard S. Boyle, focused their research at Bell Laboratories on generation and transport of electric charge since 1965. Their work resulted in a patent for the technology being filed.

Mass produced by Sony in Japan

Mass production of CCD-chips proved to be more difficult than anticipated, with only small-scale implementation in the USA. At Sony, Kazuo Iwama lead a team of engineers to the goal of commercially mass-produced CCD sensors. Sony paved the way for the future. A CCD sensor still today adorns his tombstone.

The inner workings of CCD

The bucket-brigade principle

The analogy of buckets collecting raindrops is often used to explain how a CCD collects and transports electrons. Photons cause electrons to be released from the semiconductor material, and are stored as negative charge in a “bucket”, called a potential well. The negative electrons are trapped in the potential well by applying a positive voltage to a gate. By shifting the voltage over the wells in a sequence, the charge is moved from one well to the next. Very much like water moved in a bucket brigade.

CCD sensor readout

At the end of the exposure time, the charge is transported from the light sensitive area to the vertical shift registers. Organized as columns, they transport the charge downwards in a bucket-brigade fashion. Line by line, the charge is moved to the CCD sensor’s horizontal shift register that operates a much higher speed. At the end of the horizontal shift register an amplifier and an Analog-to-Digital (A/D) converter are located.

The above-described architecture is referred to as “interline frame transfer”.

The bucket-brigade principle

CCD data rates

Due to the inherent design of CCD sensors, their readout rate is slower compared with CMOS sensors. By applying additional readout structures, commonly referred to as “taps”, the readout rate can be doubled (dual tap sensors), quadrupled (quad tap sensor) or made even faster with additional taps. (See also the “PIV mode” section for additional ways of increasing CCD speed.) Consult us to learn more about how advanced CCD timing schemes can be put to use in your specific application.

The bucket-brigade principle on a multi tap sensor

Performance limiting effects

Amplifiers and Analog-to-Digital (ADC) converters typically have the largest influence on performance. Their operation is not strictly linear. Most CCD sensors only have a single set of these elements, and therefore only have a minimal impact on the image. Sensors with multiple readout structures (taps) subsequently have several amplifiers and ADC converters. Due to non-linearity, additional signal processing may be required to adjust the output levels (tap balancing).

Blooming

Blooming can be compared with a bucket overflowing. When the photosite accumulates too many electrons, charge begins to spill into the neighboring pixels. On the captured image, blooming manifest itself as a large circular blob, resembling a blossoming flower centered over the oversaturated area. This can be overcome with an anti-blooming drain, limiting the maximum charge in a pixel. Much like having a drain hole just below the rim of a bucket.

Smearing

With high light intensities, you might see vertical lines in the image. These are called Smearing. Smearing is created directly on the vertical shift registers. After image readout, the shift registers on which the CCD pixel information is travelling to the amplifier themselves might be sensitive to light.

Photo cameras avoid this with a mechanical shutter protecting the chip while readout. Machine vision cameras try to avoid this by chip design.

Feel free to consult us on the various options for reducing performance limiting effects.