CMOS sensors contend for camera designs
Says Brian O'Rourke, senior analyst with the Cahners In-Stat Multimedia Group (Scottsdale, AZ), "Over the last five years, the image-sensor market has seen unprecedented change that will continue into the near future, and one of the most visible changes will be in the types of image sensors that will dominate the market."
By Andrew Wilson,Editor
For image sensing, CMOS-based cameras focus on low-cost, high-volume vision applications.
Says Brian O'Rourke, senior analyst with the Cahners In-Stat Multimedia Group (Scottsdale, AZ), "Over the last five years, the image-sensor market has seen unprecedented change that will continue into the near future, and one of the most visible changes will be in the types of image sensors that will dominate the market." But while CMOS image sensors continue to make great strides at the expense of CCDs in low-cost applications, in 2000, CMOS sensors consisted of less than 15% of all image sensors shipped.
Because CMOS image sensors cost less, consume less power, and can integrate many functions on-chip, they will experience increasing popularity, particularly in applications such as PC cameras and mobile phones, for which cost, power, or size is a primary concern.
In-Stat also predicts that, despite the increased popularity of CMOS devices, CCDs will continue to dominate in such high-margin, low-volume applications as medical, scientific, and industrial cameras.
Lately, much has been written about the benefits of CMOS-based imagers over their CCD counterparts. Claims that CMOS sensors now surpass the performance characteristics of CCDs have appeared unchallenged on many high-technology Web sites. And many manufacturers of CMOS sensors have used pseudotechnical arguments to reinforce these unsubstantiated claims. Perhaps the most important of these is that CMOS devices can be manufactured using standard CMOS processes for a myriad of foundry sources. In reality, industry research shows that this is not the case.
CMOS sensors are fabricated using either passive or active photosites. In passive imagers, an array of single photodiode and access transistors captures the image. Once this charge is captured, it is transferred to support circuits that convert the charge to a voltage, amplify the signal, suppress noise, and cancel the offset nonuniformity. In active CMOS imagers, an amplifier is incorporated at each photodiode site to amplify the signal. Because additional circuitry is incorporated at every photosite in both active and passive CMOS imagers, the fill factor, or percentage of the imager area that detects light, is never 100%.
To increase the effective fill factor, many CMOS imagers use microlenses to concentrate the incoming photons to the device's photosensitive region. To fabricate color CMOS devices, additional color filters must be deposited at each photosite. So, while CMOS imagers can be manufactured using the same unipolar process as memory and microprocessors products, the specialized nature of the devices requires that the process be tailored to meet the special requirements of CMOS imager fabrication. Says Dave Litwiller, vice president, marketing, at Dalsa (Waterloo, Ontario, Canada), "The CMOS processes required for good imaging performance have limited CMOS imagers to specialized, lower-volume, mixed-signal fabrication processes."
While CMOS imagers may not achieve the same dynamic range, uniformity, and shuttering capability of CCDs, they do offer their own set of advantages (see Table 1). Perhaps the most important of the CMOS processes is its ability to allow peripheral circuitry such as ADCs, DACs, DSPs, and composite video encoders to be integrated on the same device as the CMOS imager. This is especially important in products such as games, digital still cameras, and toys, where millions of highly integrated devices must be produced at the lowest price possible.
MODULES AND CAMERAS
At present, more than ten vendors are competing for a share of the CMOS sensor market. These include such well-known companies as Agilent (Palo Alto, CA), Kodak (Rochester, NY), National Semiconductor (Santa Clara, CA), and Photobit (Pasadena, CA), as well as a handful of startups such as IC Media (San Jose, CA), Pixim (Mountain View, CA), and SmaL Camera (Cambridge, MA). In developing their CMOS imagers, a number of these companies have focused on producing VGA-resolution-type imagers (see Vision Systems Design, June 2001, p. 14). This approach has resulted in similar products that cannot be readily differentiated on product specifications alone.
This plethora of low-cost CMOS imagers has led to the introduction of a number of low-cost camera modules and cameras based on the technology. Interestingly, despite the more than 100 CCD-based camera modules available from Korean, Japanese, and US manufacturers, very few such modules are based on CMOS devices (see "Asian vendors lower the cost of board-level camera designs," p. 11). Of those, both Canadian Photonic Labs (CPL; Minnedosa, Manitoba, Canada) and Koentra (Ulsan, Korea) offer camera modules that use 1/3-in. CMOS image sensors. The CPL-1800N from CPL and the CM320 from Koentra are both color camera modules and feature resolutions of 537 x 597 and 510 x 492 pixels, respectively.
Just as CMOS imager vendors are looking at ways to differentiate their products, CMOS camera vendors also are having a difficult time. Of the few CMOS cameras available, many use standard 1/3-in. imagers that exhibit similar resolutions and sensitivity (see Table 2). To differentiate these products, these vendors have tailored their cameras with different types of interfaces.
To meet the needs of surveillance, video conferencing, and PC multimedia applications, for example, Vigitron (San Diego, CA) has incorporated twisted-pair transmitters into its RC2000 and RC2001 range of monochrome and color CMOS microcameras (see Fig. 1). These cameras can transmit color or monochrome video over unshielded twisted-pair wires at distances up to 5000 ft. Incorporating built-in surge-supression circuits designed to protect other video equipment from damaging voltage spikes, the cameras feature automatic exposure, gain, and white balance.
Interestingly, Photonfocus (Lachen, Switzerland) has chosen machine vision, industrial inspection, and microscopy as the target markets for its MV-D300k CMOS camera. With 640 x 480 x 8-bit resolution, the camera offers a low-voltage-differential-signal (LVDS) (RS-644) output that can be used with a number of currently available LVDS-compatible frame grabbers. To lower the cost of the camera, the IC used in the camera incorporates the image sensor, 10-bit ADCs, and control logic. According to Martin Wäny, CTO of Photonfocus, the benefits of using a CMOS imager include blooming resistance, low power consumption, and monolithic integration.
Other vendors of CMOS machine-vision cameras, however, are taking a more conservative marketing approach. Next month, DVT (Norcross, GA) will introduce its 531, a CMOS-based Smart Camera based on the OV7120 from Omnivision Technologies (Sunnyvale, CA). With an embedded Motorola PowerPC, the camera features 640 x 480 resolution and costs less than $3000. According to DVT, the camera is targeted to production lines where parts come to a stop (or near stop) in front of the camera but not high-speed production lines where lines where parts are moving at 1500 pieces/minute.
DRIVEN BY CONSUMERS
Like many other products in the image-processing market, high-volume, low-cost consumer products will be a driving force behind the development of next-generation imagers. In mobile cell phones, in particular, designers are looking for miniature, low-power, highly sensitive, fully integrated sensors for their next-generation videophone designs. For these reasons, the CMOS imager had been seen as the natural choice for these types of products.
"While CMOS image sensors are currently seen as the leading image sensors for these areas, their sensitivity falls as the device is made smaller and CMOS imagers tend to produce noisy images," explains Minoru Hamada, manager of the CCD development department, MOS-LSI Division, Semiconductor Co., Sanyo Electric (Tokyo, Japan). "CMOS image sensors also have the problem that the hand-held camera shake associated with the longer exposure times required results in degraded image quality. Furthermore, the CMOS-image-sensor shutter function can result in distorted images if the object being photographed moves," he says.
Dissatisfied with the sensitivity of CMOS imagers, the company has developed its iGT99263—a 11.2 x 11.2 x 6.7-mm camera module that combines both its LC99263FB frame-transfer CCD and LC99704 digital-signal processor. The camera module, which has a sample price of x7500 (about $60), requires a 3-V supply voltage, consumes 90 mW at 15 frames/s, and outputs both RGB and YUV color signals. With 370 x 296-pixel resolution, the CCD is a 1/7-in. device with 5.5-µm-square pixels. To support the device, the module's LC99704 digital-signal processor features an on-board drive voltage step-up circuit, CCD vertical driver, CDS, AGC, and 8-bit ADC, autoiris, and white-balance control.
Because of the technological differences between CMOS- and CCD-based cameras, the current sensor market is rapidly dividing into a high-performance, low-volume branch and a low-cost, high-volume branch (see Fig. 2). "In the high-performance branch are applications that will continue to be dominated by CCD technology, but CMOS technology will find market share, too, especially for lower-cost or more-portable versions of these products," says Helen Titus, marketing manager at Kodak.
Most of the CMOS activity will be in lower-cost, more-portable products. "Here, CCD sensors will be replaced with CMOS sensors. These could include some security applications, biometrics, and most consumer digital cameras," she adds. While both CCD and CMOS cameras offer unique benefits, the choice of whether to use one or the other will remain application-specific. For now, it seems, many designers are sitting on the side of the CCD fence awaiting further CMOS developments.
Due to space limitations, this Product Focus does not include all the manufacturers of the described product category. For information on other suppliers of CMOS cameras, see the 2001 Vision Systems Design Buyers Guide (Vision Systems Design, Feb. 2001, p. 82).
Palo Alto, CA 94303
Scottsdale, AZ 85254
Canadian Photonic Labs
Manitoba, Canada R0J 1E0
Canada N2V 2E9
Norcross, GA 30093
San Jose, CA 95112
85356 Freising, Germany
Image Sensor Solutions
South Korea 680-190
El Segundo, CA 90245
Santa Clara, CA 95052-8090
Sunnyvale, CA 94085
Pasadena, CA 91101
8853 Lachen SZ
Mountain View, CA 94043
Sanyo Electric Semiconductor Co.
Tokyo 110-8534, Japan
SmaL Camera Technologies
Cambridge, MA 02138
San Diego, CA 92129