Rapid improvements in the quality, size, and cost of image sensors are making an impact on consumer markets. It's now possible to purchase a reasonably good digital camera for less than $200. But even as pixels overtake film in the world of photography, much more looms on the horizon.
The real promise of digital imaging lies in emerging embedded camera applications. One example is cellular phones. According to Nokia, half of all mobile handsets could incorporate cameras in the near future. Predictions are that as many as one billion mobile handsets could be produced in 2004, creating an incredible new opportunity for imaging ICs.
But as engineers, we have to be practical too. There are a number of limitations that hobble the proliferation of digital imaging in wireless devices. Chief among them are power/battery-life considerations, and the quality and number of pixels required to produce compelling images that can be easily transported across wireless data networks.
CCDs rule the high end of digital photography. Analog, expensive, and power-hungry (12 to 20 V), they're unlikely to be a major player in portable devices. CMOS sensors dominate the low-end imaging market. Although image quality of the best consumer CMOS sensor is still less than a disposable film camera, they're far better suited to portable applications than CCDs.
CMOS sensors, however, suffer from inherent weaknesses that degrade image quality, making them unsuitable for system-on-a-chip (SoC) integration—an unacceptable tradeoff for small, lightweight portables like phones. One limitation is image creation. Their analog signals, which must be digitized by an analog-to-digital converter (ADC), are prone to digital noise that creates artifacts in the finished photo.
Another fault is image quality. CMOS sensors traditionally have low dynamic range, resulting in unnatural looking pictures with incorrect colors, regardless of resolution. Although some of the latest CMOS sensors have more pixels, their image quality remains suspect. The low-dynamic-range pictures of CMOS sensors are analogous to early grayscale pictures. With an insufficient number of grayscale divisions, the pictures looked unnatural. Increasing the amount of grayscale dramatically improved them. Dynamic range does the same thing in color. More dynamic range means better looking pictures, regardless of how many pixels.
For small-form-factor devices like cell phones, a new type of CMOS sensor is needed, one that increases image quality without requiring more and more pixels. This is important for future-generation wireless data networks. Imagine how such networks would be overwhelmed if everyone were sending and receiving multimegapixel images at the same time! Fortunately, this technology is coming to the fore. It has the potential to put a camera in every cell phone and quickly displace current sensors in virtually all applications.
An example is the Digital Pixel System (DPS) technology, developed at Stanford University and now commercialized by Pixim Inc. This technology raises image quality for each pixel and turns it into an image processor. DPS combines the ADC with the pixel itself, virtually eliminating noise-related image artifacts, greatly reducing power consumption, and making the imaging device easier to integrate with other IP on an SoC. Better pixels create better images in smaller data packages, which is ideal for bandwidth-constrained wireless networks.
A camera in every cell phone? With improvements in technology, I think it's right around the corner. Let's see if a picture really is worth a thousand words.
This article was in Electronic Design, Oct. 29, 2001