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Smile and Say, ‘Drone!’

Photonics Spectra
Oct 2014
Any photographer will tell you that if it were possible to grow an extra set of hands, he would. As simultaneous scientists, artists and packhorses, photographers must know their equipment inside and out, but they must also be prepared to change shutter speed and aperture at a moment’s notice – even prophetically. Lenses need to be changed, equipment needs to be carried, and an artistic vision needs to remain intact. And while all that responsibility is churning, light remains the crux of the entire operation. When there’s an active subject, the process of moving and adjusting lighting forces the photographer to aim, quite literally, at a moving target. Whether it’s with an external flash, a reflector panel or a four-leaf barn door, lighting placement is complex, essential and tedious.

Now, a very special kind of photographer’s assistant can ease those pains by taking the reins of lighting – from mid-air. Researchers at Massachusetts Institute of Technology and Stanford University have developed a small, light-equipped autonomous drone system that intuitively moves itself around a photographer’s subject. Using a combination of computational photography and robotically controlled light sources, the drone can create advanced lighting effects for moving subjects by analyzing the photographer and subject’s positions, as well as the amount of light produced.

The scientists experimented with rim lighting, one of the most challenging effects to replicate with a dynamic subject. Rim lighting is created when the light source is placed right next to a person. The edge of his or her body is strongly lit, producing a band of bright white light (the rim) and creating a silhouette.

“[The effect is] very sensitive to the position of the light,” said Dr. Manohar Srikanth, a former postdoctoral scholar at MIT. “If you move the light, say, by a foot, your appearance changes dramatically.”

Normally, this complex light effect would not allow for free-form photography. But with the drone, the photographer can wander around, and the device automatically flies to the necessary location to produce the specified rim width. The drone’s onboard lidar locates the subject’s position, and a control algorithm evaluates the produced rim width while adjusting the robot’s position. As the photographer and subject move around, the drone moves, too.

Weighing less than one pound, the aerial robot is an off-the-shelf quadrotor, with four propellers providing four controllable degrees of freedom. A continuous light source on the drone optimizes position; an onboard flash strobe is triggered when the photographer presses the camera’s shutter.

“The challenge was the manipulation of the very difficult dynamics of the UAV (unmanned aerial vehicle) and the feedback from the lighting estimation,” said Dr. Frédo Durand, professor of computer science at MIT. “That’s where we put a lot of our efforts, to make sure that the control of the drone could work at the very high speed that’s needed just to keep the thing flying, and deal with the information from the lidar and the rim-lighting estimation.


GLOSSARY
camera
A light-tight box that receives light from an object or scene and focuses it to form an image on a light-sensitive material or a detector. The camera generally contains a lens of variable aperture and a shutter of variable speed to precisely control the exposure. In an electronic imaging system, the camera does not use chemical means to store the image, but takes advantage of the sensitivity of various detectors to different bands of the electromagnetic spectrum. These sensors are transducers...
lidar
An acronym of light detection and ranging, describing systems that use a light beam in place of conventional microwave beams for atmospheric monitoring, tracking and detection functions. Ladar, an acronym of laser detection and ranging, uses laser light for detection of speed, altitude, direction and range; it is often called laser radar.
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