Light-field camera snaps dusty plasma

Dusty plasmas consist of charged microparticles embedded in a lightly ionized gas (see Physics Today, July 2004, page 32 ). In space, such plasmas pervade planetary rings. In the lab, where higher microparticle densities can be achieved, the plasmas form crystals, melt, carry waves, and exhibit other collective behavior. Observing the microparticles, which dominate the dynamics, is tricky. Although individual particles are large enough to resolve using optical techniques, monitoring all of them requires a combination of magnification and depth of field that standard cameras lack. Péter Hartmann of the Institute for Solid-State Physics and Optics in Budapest, Hungary, and his colleagues are tackling the problem by using a nonstandard camera, the Lytro. Thanks to an array of microlenses that sits between the camera’s main lens and detector, the Lytro and its built-in software can determine not only the direction of incoming light (corresponding to the x and y of a two-dimensional image), but also where in the scene it originated (x, y, and z; the light field). Processing the three-dimensional data yields a set of sharp images for all values of z. Despite the novel optics, a typical plasma cloud, which measures a few tens of millimeters in diameter, still exceeds the camera’s depth of field. Nevertheless, peripheral particles are resolved well enough for an algorithm that Hartmann devised to crunch through the images and reconstruct the 3D particle distribution. The accompanying figure shows one such reconstruction: of a flat, circular cloud about 14 mm across and containing 60 particles. (P. Hartmann, I. Donkó, Z. Donkó, Rev. Sci. Instrum. 84, 023501, 2013.)