Electronic Design

Image Sensors Sharpen Focus

Sensors continue to penetrate existing and new markets thanks to technological improvements. In imaging, end users are reaping the benefits of smaller image chips with higher levels of resolution—not to mention lower costs. And CMOS process technology has been a key driver.

Toshiba has shrank CMOS image sensors for mobile phones to tinier sizes while maintaining high resolution. Its 3.2-Mpixel ET8EF6-AS and 2-Mpixel ET8EF2-AS silicon-on-a-chip devices are now being manufactured in pixel pitches of just 2.2 µm in a 1/4-in. format. Previous imaging chips measured 2.7 µm with a 1/3.2-in. format.

To extend camera-phone battery lifetimes, Capella's line of ambient-light sensors eliminates excessive backlighting. The CMOS3000/3200 Filtron devices also can be used in notebooks, DVD and MP3 players, PDAs, and GPS units. Each chip is a highly integrated device with an optical filter, a photodiode, a digital filter, and a 9-bit digital-to-analog converter (DAC).

Imaging advances have reached the security and surveillance camera market with OmniVision Technologies' single-chip OV7720 Camera-Chip IC. This 1/4-in. format VGA device targets high-sensitivity surveillance and IP/3G cameras. Unlike conventional analog cameras, it doesn't require an analog-to-digital converter (ADC). It features a resolution of 640 by 480 pixels and a 0.25-µm pixel pitch. It also operates at 60 frames/s.

For sheer levels of high resolution, Foveon's X3 concept has yielded a 14.1-Mpixel image sensor for professional digital single-lens-reflex (SLR) photography. The company says this development combines the power of digital image processing with the essence of photographic film.

In the X3's layered sensor design, each location in the grid has layered photosensors sensitive to all three primary colors. In contrast, digital camera sensors commonly use a mosaic Bayer filter sensor where each location is a single photosensor (pixel) sensitive to only one primary color.

The X3's image sensors have three layers of pixels. These layers are embedded in silicon to take advantage of the fact that red, green, and blue light penetrate silicon to different depths, forming the first and only image sensor that captures full color at every point in the captured image.

The present approach to higher-resolution imaging is to increase the number of pixels per given area, which is now in the megapixel range. But a technology that has been kicking around for a couple of decades is now in the news.

Researchers at Rice University are proposing a 1-pixel camera based on a single-photon detector that takes thousands of "rapid-fire" shots to capture the equivalent of a million pixels. They say their idea simplifies the hardware while enabling the imaging of a much wider light spectrum than present megapixel imagers (see the figure).

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