Electronic Design

Forget About Megapixels—Try A Single-Pixel Camera Instead

The idea of a single-pixel camera may sound downright weird. But Richard Baraniuk, an electrical and computer engineering professor at Rice University, claims that a camera that captures several thousand points of light in rapid succession makes more sense, and is far more efficient, than one that simultaneously grabs several million pixels.

We can take a picture with potentially millions of pixels, but using just a single detector element, he says.

Baraniuk and Kevin Kelly, a Rice assistant professor of electrical and computer engineering, have designed a camera that uses a digital micromirror device (DMD) developed by Texas Instruments. The DMD takes the place of the charge-coupled device (CCD) or CMOS array found in conventional digital cameras.

Used primarily in digital televisions and video projectors, DMDs are chips covered with thousands of tiny, microbe-sized mirrors. Each mirror can face in only two directions, appearing alternately dark and bright, registering its current state as a 1 or 0. The new camera focuses light through a lens onto the DMD (see the figure). A second lens then collects the light reflected from the mirrors and focuses it onto a single photodiode sensor.

This is the single pixel that takes our measurements, Baraniuk says. Each time the DMD mirrors shift, the photodiode records a new pixel value. Baraniuk maintains that pixel overkill burdens most conventional cameras.

Even though your picture consists of 5 million pixel values, most photographs can be described \[via compression technology\] using a lot fewer numbers, he says. At a 100:1 compression ratio, for instance, just 50,000 values would be needed to approximate a picture. You've built these 5 million detectors only to throw away most of the numbers they produced in your compression system.

The new technology promises to eventually lead to cameras that rely less on expensive hardware, such as powerful microprocessors, CCDs, and memory devices, and are therefore cheaper and more power thrifty. The JPEG number crunching drains your camera s battery power, Baraniuk says.

But there's one not-so-slight drawback to the technology: The prototype model requires about five minutes to take a single picture. That s because the camera must blink several thousand times to capture a complete image. We re not claiming we'll replace conventional cameras tomorrow, Baraniuk admits.

Yet Baraniuk notes that the camera already is well suited for some imaging applications at wavelengths outside the visible spectrum tasks that would bust budgets using conventional CCD and CMOS imaging arrays. For example, a 1-Mpixel camera comprised of detectors that each cost $1000 would cost $1 billion, while our setup would cost just over $1000, he says.

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