Computer scientist Rob Fergus was sitting in a dimly lit Boston bar when a sudden burst of bright light blinded him—the camera flash accompanying a friend's photograph.

While blinking away the bright light, Fergus got to thinking: Because clear photographs require a strong light source, cameras depend on an obnoxiously powerful flash in any environment with low lighting. What if we could circumvent the need for such an intense flash? What if we could take high-quality photos in poor light using an invisible flash?

Fergus got on the case and, a year later of tinkering, Fergus had a working prototype. He calls it dark-flash photography—a camera that emits and records light outside the visible spectrum. There is a flash, but you barely notice it. And the photographs are just as good as those that rely on a regular flash, if not better.

How it Works

The dark-flash camera achieves its subtlety by emitting and capturing wavelengths of light in the ultraviolet (UV) and the infrared (IR) spectra, just outside the range of wavelengths the human eye can easily detect. That's something most cameras can't do.

Ordinary cameras use filters to block any light in the infrared spectrum. When building his prototype, Fergus replaced this filter with another that lets some IR through the lens without flooding the camera's sensors. Since most camera flashes emit IR along with their burst of visible light, the one substitution allowed the prototype to transmit and record infrared light.

The prototype camera model Fergus chose could already detect UV, but getting the camera to send UV light was a little trickier. Few people have deliberately altered a camera to emit UV light, but Fergus got some help from a group of hobbyists who use UV photography to reveal hidden patterns on the petals of flowers—landing strips for insects and pollinators that can detect ultraviolet light. Sanding off a protective coating on the flash unit allows the camera to illuminate its subject in UV.

Even after these modifications, the camera's flash still dazzled with a burst of white light. Tucked away in the corner of an optics catalog, Fergus found a special filter that blocks visible light, but not IR or UV. No scream of white—just a violet whisper.

To use a dark-flash camera, you have to take two different photos of the same scene within seconds of one another: one photo in ambient light (regardless of how dark the environment is) and one in the ultraviolet and infrared light provided by the dark flash. Later, the photographer needs to merge the best characteristics of each photo on a computer. "We take the normal colors from the picture without flash and combine them with the edge structures from the dark-flash picture," says Dilip Krishnan, a second-year doctoral candidate in computer science at New York University, who worked with Fergus on dark-flash photography.

But Is the Picture Any Good?

Combining pictures taken with and without flash actually produces a higher-quality picture than either approach alone. Photos taken in poor light turn out grainy or blurry. That's why cameras use flashes in low-light environments: The flash gives the camera's sensors an extra dose of photons, the particles that make up light, Fergus says. But strong flashes do more than just dazzle one's friends—they distort the quality of the picture.

"Most pictures you take with a flash look quite crappy," says Ankit Mohan, an expert in camera technology at the Massachusetts Institute of Technology says. "They look kind of flat, you get the red-eye effect, and one part of the scene is always much brighter than another part. But the problem of capturing a picture with no flash is that you don't get detail. By combining the two you get the best of both worlds."

Amit Agrawal, a computer scientist and electrical engineer who specializes in computational photography at Mitsubishi Electric Research Laboratories in Cambridge, Massachusetts, confirms that, with modification, digital cameras could handle the photo-merging on which dark-flash photography depends. "It could be done right on the camera itself," he says.

Whether the images are combined directly on a camera or on a computer, the result is a crisp photograph without any of the startling dazzle or unwanted artifacts produced by a bright camera flash. Dark flash still has a way to go, though. Although the technique accurately captures skin tones of all kinds, a red shirt might appear a little pinker than usual, or a freckle might disappear. And, since you need to take two photographs in quick succession, your subject can't move around much.

Cramer Gallimore, a professional photographer based in North Carolina, believes dark-flash photography has great potential. "You might be able to take high-quality photographs of wildlife without disturbing them," Gallimore says, "and for forensic photography, it would be very useful to have technology like this that could switch between infrared technology and visible light photography to record certain traces of human activity at a crime scene."

A pair of images are captured at a blur-free shutter speed, one using a multi-spectral flash (F), the other using ambient illumination (A) which in this case is 1/100th of that required for a correct exposure. The pair are combined to give an output image (R) which is of comparable quality to a reference long exposure shot (L).

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