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A Single-Pixel Digital Camera

A single-pixel digital camera, scientists at Rice University believe, will reduce power consumption and storage space without sacrificing spatial resolution. The new approach aims to confront one of the basic dilemmas of digital imaging, namely the huge waste factor.

Consider that a megapixel camera will, when you take the picture, capture and momentarily store a million numbers (the light levels from the pixels).

No camera can store that much information for hundreds of pictures, so an immediate data compression takes place right there inside the camera.

A tiny microprocessor performs a Fourier transform; that is, it converts the digital image into a weighted sum of many sinusoid waves.

Instead of a million numbers, the representation of the image can now been compressed into something like 10,000 numbers, corresponding to the most important coefficients from the mathematical transformation.

These are the numbers actually retained for later processing into pictures. The Rice camera saves space and energy by eliminating the first step. It gets rid of the million pixels.

Instead it goes right to a transformed version (about 10,000 numbers rather than a million) by viewing the scene prismatically with a single pixel.

No, the light from the object doesn’t go through a prism, but it is viewed about 10,000 different ways.

The light, in a quick succession of glances, bounces off the myriad individually driven facets of a digital micromirror device, or DMD (http://en.wikipedia.org/wiki/Digital_micromirror_device).

The mirrors of a DMD (only a micron or so in size) do not image an object or record data but merely steer light; they can be individually angled in such a way that the light strikes a photo detector or not, depending on whether the light is representing a digital 1 or 0 at that moment. The main idea is that the DMD is acting as a sort of analog optical computer.

Each time the pixel views the object, a different set of orientations is imposed on the array of micromirrors.

And, in an interesting twist, the Rice camera uses random orientations. Looking like the haphazard splotch of black and white squares of a crossword puzzle, the DMD’s surface is reflective here and dark there; some of the mirrors will faithfully reflect light from the object to the pixel while others will, in effect, appear black. Then the object is viewed again with a different micromirror activation pattern; again the pixel will record an overall light level.

This process recurs about 10,000 times. Later, offline on a computer, the single pixel light levels, along with the micromirror patterns are processed using new algorithms to reconstruct a sharp image.

This isn’t quite the old type of imaging process, the kind used in x-ray crystallography or CAT scans (which also convert pinpoints of data into images), but a new kind of imaging called compressive sensing that is only about two years old. To summarize, the acquisition of imaging data is reduced many-fold (saving on data storage), only a single pixel is needed (freeing up valuable space in the primary detector), and the bulk of the processing can be offloaded to a remote computer rather than a chip inside the camera, thus greatly reducing power needs and extending the usefulness of batteries.

Rice researchers Richard Baraniuk (richb@rice.edu) and Kevin Kelly (kkelly@rice.edu) say that an additional virtue of the camera is that with only a single pixel, the detector (a photodiode) can be as fancy as you want.

It can even accommodate wavelengths currently unavailable to digital photography, such as x ray, terrahertz waves, even radar.

A working camera prototype has been built.

One of the main tasks is to reduce the time it takes to record an image; the price for compressing space, pixels, and power is to spread everything out in time since the cyclops-like pixel must blink ten thousand or more times to capture the image.

As Baraniuk says, the Rice form of photography is multiplexed in time. The Rice results were reported last week at the Frontiers in Optics Meeting of the Optical Society of America (OSA) held in Rochester (www.osa.org/meetings/annual/) (For a picture of the setup and the imaging results, see the web page http://dsp.rice.edu/cscamera and the research paper at http://www.dsp.ece.rice.edu/cs/cscam-SPIEJan06.pdf )