Diendorfer G., Viehberger M., Mair M., Schulz W.:
An attempt to determine currents in lightning channel branches from optical data of a high speed video system
International Conference on Lightning and Static Electricity (ICOLSE), Blackpool, UK, 2003
Direct measurement of lightning currents is only possible at the point of impact using a shunt or other measuring instruments (e.g. Rogowsky-coil). On the other hand, the brightness of the lightning channel can be determined by means of optical measurements. So, if there is any correlation between lightning current and channel brightness, there is also an option to estimate the channel current from the measured brightness of a lightning flash.
We have developed a software tool Flash Analyzer (FA) to verify the correlation between the lightning current measured at the top of an instrumented tower and the simultaneously recorded brightness changes of the lightning channel. The optical data are taken by a high speed video system that is installed at a distance of about 200m to the tower. The camera system is operated with 500 frames per second i.e. an image of the lightning channel is taken every 2 ms. The mean value of the measured current during the frame exposure time of 2 ms is calculated from the simultaneously recorded current values. The 2 ms exposure time of the high speed video frames only allows analyses of the initial continuing current (ICC) phase of the upward initiated flashes to the tower.
The FA software has the option to specify up to four arbitrary lines per image, defining cross sections of the lightning channel. The basic idea of the analysis is to place the lines across the lightning channel and to determine the brightness along these cross sections. The result is a plot of the channel brightness as a function of time. This sequence is at first sight (purely visually) very similar to the plot of the measured lightning current. Several analyses under most different conditions have proven the usability of this procedure. Different parameters effecting the correlation between brightness and current (e.g. displacement of the lightning channel due to strong wind, the use of different camera lenses or background light on images taken during daylight time have been analyzed and corrected. For most of the examined lightning flashes coefficients of determination r2 > 0.5 were determined for a linear regression between channel brightness and lightning current. By placing the cross section lines across the individual branches of the lightning channel, we were able with this software tool to assess the corresponding lightning current in the individual branches of the channel. In this paper we show some results of the analysis of lightning flashes to the Gaisberg tower in the years 2000 to 2002.
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