Stereo-photography of point-plane streamers in air
  Standard photographs of streamer discharges show a two-dimensional projection. We here present stereo-photographic images that resolve their three-dimensional structure. We describe the stereoscopic set-up and evaluation, and we present results for positive streamer discharges in air at 0.2 - 1 bar in a point plane geometry with a gap distance of 14 cm and a voltage pulse of 47 kV. In this case an approximately Gaussian distribution of branching angles of 43 degrees is found; these angles do not significantly depend on the distance from the needle or on the gas pressure. 
S. Nijdam, J. S. Moerman, T. M. P. Briels, E. M. van Veldhuizen, U.   Ebert