CMOS Imaging Sensors Capture the Limelight

With developments in semiconductors, the number of choices for imaging sensors has grown. A&S takes a look at these componentswho makes them, where they go and how they used for securityand how they are critical to get the big picture.

Capturing sharp images is one of the main goals of video surveillance. Converting photons of light into photographic images varies greatly, with a proliferation of image sensing vendors offering different capture methods. From the dominance of charge-coupled devices (CCDs) to the rise of complimentary metal oxide semiconductors (CMOS, pronounced SEA-moss), different image sensors suit different security needs. Recording crucial moments can be done efficiently and effectively, thanks to image sensing components.

Different Technologies, Different Results

While both CCD and CMOS sensors were developed in the 1960s, the early CMOS sensors were not as sensitive to light as the CCD ones, which were developed in 1969. Both converted visual images to a digital signal on integrated circuits, but with the CCD holding the advantage in light sensitivitycrucial in low-light situationsCCDs won the first round of the image sensor debate.

"CCD traditionally was the better of the two technologies and enjoyed a greater share of the market up until a couple of years ago," said Michael Hepp, Product Manager at Omnivision. "CCD, though generally a much more expensive technology, used to offer high-performance imaging and flexibility compared to CMOS, yet demanded that system size be sufficiently large to house the components, which are often comprised of expensive multichip solutions. In the past, CCD image sensors featured better light sensitivity compared to previous generations of CMOS sensors. CCDs are also complex and are based on a highly specialized process. Until recently, commercial security camera applications requiring high dynamic range (HDR)that is, the ability to record a greater range of tonal detail in varying light conditionsrelied almost exclusively on CCD imagers for purely technical reasons. As a result, users had to contend with the resolution, range and temperature limitations of CCD sensorsissues that are extremely important in security applications."

It was not until 2003 that CMOS sensors caught up to CCDs in light sensitivity, said Paul Bodell, Vice President of Sales and Marketing of IQinVision, in a previous A&S interview. Progressive scan CMOS sensors also had an edge over interlaced CCD chips, scanning all TV lines in sequence, rather then splitting them into odd and even lines which were then scanned one after the other. Interlaced scans resulted in streaking from objects moving rapidly between the odd and even scans. This was not desirable in traffic situations, where clear, unstreaked images of moving vehicles were required.

Analysts predict continued growth in this sector. Frost & Sullivan said the CMOS image sensor market will experience a compound annual growth rate of 33 percent, while revenues are expected to reach US$4.64 billion by 2007, compared to 6.2 percent growth for CCDs.

Proponents of CMOS sensors tout their power-saving abilities and lower costs. "CMOS image sensors typically use just one-tenth of the power usage of CCD devices, a factor which, when multiplied out over todayˇs large security networks, equates to substantial financial savings for the organization," Hepp said. "CMOS image sensors are less expensive to manufacture and are typically sold at a lower cost, again lowering overall system cost, meaning further cost savings to manufacturers."

Others argue that despite more control and reduced power consumption, CCD light sensitivity is still superior to CMOS. What vendors have to say for themselves reflects the debate in the market over which of the two technologies is best suited for imaging uses.

Fresh Faces on the Imaging Scene

Sony is the leader in the CCD market. While it is also ramping up its CMOS production, according to an announcement in June, the majority of the image sensors it sells are CCDs. Some estimates put Sony's stake of the CCD market at 40 percent. Its closest rivals in the CCD industry are Panasonic and Sharp.

Some vendors aim to compete with Sony directly. Pixim, a relatively new company founded in the 1990s, is aggressively promoting its CMOS sensor. Other major CMOS competitors are Micron Technology and OmniVision, who tout their expertise as semiconductor makers. Micron entered the CMOS market about a year ago, with background in DRAM and Flash memory. In a short time period, Micron now claims 38 percent global market share for its last quarter from December 2006 to February 2007, said John Mullins, Asia Communications Manager.

As the CMOS market develops, players compete for functions and price. Each of them seek to differentiate themselves in terms of functions.

Micron Making Its Move

Having a background in semiconductors gives Micron an advantage over its competitors, said Rene Hartner, Senior Marketing Representative. "The history of Micron is DRAM manufacturing, which minimizes leakage in transistors. This helps us in a unique way to minimize the dark current in our image sensor. In addition, CMOS is by far the most common and highest-yielding chip-making process in the world and can easily be added to the production portfolio of established semiconductor manufacturers, creating economies of scale that far surpass CCDs."

"Second, by combining all camera functions on-chip, CMOS image sensors reduce the part count of digital camera systems, which increases reliability, eases device miniaturization, and eliminates the need for additional chip packaging, assembly and peripheral supportfurther reducing costs," he said.

Micron has been able to make sensors that use less power than CCDs, which is ideal for newer cameras. "We see a new trend for battery-powered cameras, primarily for consumer solutions and homeland security," Hartner said. "Our sensors are excellent in power consumption, enabling our customers to develop new solutions to water, gas and pipeline operators to monitor and protect their assets."

He added high dynamic rage (HDR) creates an advantage by properly displaying a scene that spans from very bright areas to dark corners. Environments such as banks, ATMs or traffic require HDR solutions. Standard CMOS sensors have a dynamic range that span from 70 decibels to more than 100 decibels for an HDR solution. "If you walk in a bank, you typically walk in from the street and then walk into a room that is artificially lighted. If a camera sees a person coming in, then the scene is going from a reasonably well-lit area to a very bright area. With standard sensors, it's hard to see details of the person walking through the door, such as the face or the color of the clothing. The MT9V032 offers not only HDR, but a Global Shutter technology as well, which can simultaneously perform the function of a traffic surveillance camera and a high speed camera to capture the face and license plate of someone committing a traffic violation, such as driving too fast."

Low light performance is one of the most critical parameters for CMOS sensors used in surveillance applications. Micron's sensors have excellent low light performance, a factor for many customers who choose them, Hartner said.

OmniVision Out to Prove Itself

While it may not have the brand recognition of Sony, OmniVision is working to distinguish itself from the competition. "OmniVision is one of the first CMOS image sensor suppliers to integrate the signal processor on to the sensor," Hepp said. "This allows customers to have a single chip solution with the image sensor and signal processor in a single chip, thus eliminating the need for other companion processing chips. This can significantly reduce the cost to our customers."

In its imaging sensors, OmniVision designs in features that are tailored for each application. For example, extended temperature operation and the ability to function under extreme environmental conditions are key in automotive applications. However, some baseline requirements do not change for different applications, Hepp said. ¨The performance of any particular sensor needs to be sufficient to meet or exceed the expectations of the industry. Otherwise there is no point in using that sensor in the first place. Image quality, performance in varying light conditions and at varying temperatures, are all important elements in sensor choice, allied to overall bill of materials, ease of integration into a camera design or security system, and end user installation and operation.〃

Pixim Banking on Prime Solution

One of the best-known CMOS companies is Pixim, hailing from Mountain View, California. "Pixim's image sensor architecture is unique in the industry," said John Monti, Vice President of Sales and Marketing. "We call it Digital Pixel System (DPS) technology. The key invention is the inclusion of an Analog-to-Digital converter in each pixel. This allows each pixel to decide its own shutter speed for each frame independent of every other pixel in the sensor array. The result is Ultra Wide Dynamic Range (UWDR) which has many benefits. Some of these benefits are 120-decibel dynamic range, color accuracy, zero blooming and smear, progressive scan image capture, and global electronic shuttering to reduce motion artifacts."

However, not all solutions require that much dynamic range. "Imaging solutions, how they capture images and display them, is very subjective," Monti said. "We made our solution very configurable so that a camera from GE can look and feel different from Honeywell, or Pelco, or from Lenel. They all have different preferences ... they can be set by the camera manufacturer, or integrator or end user."

A range of technological features is what Monti feels gives Pixim an edge over its competitors. "Analog CMOS sensors have a rolling shutter which causes motion artifacts. They also have very poor low light performance which is limited by fixed pattern noise in the sensor," he said. Pixim's global shutter is able to reduce noise by adjusting for each pixel's gain value. The result is increased sensitivity to light, even in low-light situations.

What Camera Makers Look For

With so many component vendors promoting their solutions, camera makers have ample choice in choosing partners. Several prefer one technology over the other, while others look at the application and vary between CCD or CMOS. For German Mobotix, the choice is clear. "We are using a CMOS sensor because of the high dynamic range of sensitivity and very high range of varying brightness within an image," said Peter McKee, International Marketing Director. "Any camera that has to deal with sustained exposure to direct sunlight, for example, would have to have a CMOS sensor, because a CCD would eventually burn out."

The company did not disclose whether it used a third-party CMOS sensor or who the vendor would be, but said its full-frame CMOS solution provided it with a whopping 960 TV-line progressive-scanned image, free from blurry interlaced images from CCD makers. Software allows Mobotix to manipulate the high-resolution images further, reducing the need for more hardware. "On the mechanical side, the CMOS allows us to use lenses with fixed apertures, and the fact that it has no moving parts fits with the Mobotix concept of 'No moving parts equals no maintenance,'" McKee said. Mintron, on the other hand, uses only CCDs in its cameras. Mars Yang, CEO, said he chose Sony's CCD for its first CCD camera for its sensitivity to light, reduced noise and elimination of blooming.

Mintron later developed its own imaging sensor for wide dynamic range, with a light sensitivity of 1:100,000, allowing it to capture clear images in low-light situations. The chip was built on a CMOS process, effectively combining the sensitivity of CCDs and the benefits of CMOS. While not all camera makers have the manufacturing ability to make their own components, they certainly have no lack of choices when it comes to finding image sensor partners.

Gazing Into the Crystal Ball

The CMOS market has yet to see a clear victor, unlike Sony in CCDs. One thing is certain: It will be a competitive fight to the top.

However, CMOS solutions are being embraced in several markets, allowing more room for the extra rivalry. "We expect to see cameras using CMOS sensors being used more and more on a global basis across many applications, including securitythe functionality, price, size and performance are all there," Hepp said. "We are already seeing this in the Asian and ODM markets, where we have many customers in the Web cam and security camera markets. The commercial security market is now starting to implement CMOS-based technology as the image quality and low light performance has reached (and is set to surpass) CCD levels."

Yang agreed CMOS sensors will approach CCDs in their benefits, but each technology has respective strengths, such as low power consumption and reduced prices for CMOS, while CCD have less noise. One technology will not replace the other.

The debate is far from over, but both sides of the imaging sensor debate have strengths they can bring to the table. The next few years will be interesting to watch for the component industry.
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