msb@lsuc.UUCP (Mark Brader) (08/10/85)
(For net.video readers: The subject is misspelled because this is a followup to a net.physics article where it was spelled that way.) I happened to videotape a TV news show a couple of weeks ago that included a view of a lightning strike (in Wyoming). It was interesting to play it back in slow motion. The bolt was more or less vertical. In the first frame where it appeared, it occupied only the bottom 1/5 of the frame; in the next frame, it occupied the whole height of the frame. In both of these frames there was a bright aura splaying out from the bolt; the second frame was close to washed out. (Incidentally, this passed by too fast to notice at normal speed. It was much more dramatic in slow motion.) I presume the aura was simply reflected light from the cloud and rain. Anyway, in the first of the two frames, both bolt and aura were sharply cut off by a horizontal boundary. I take this to mean that the light rose from zero to full intensity in a time comparable to that between two scan lines -- i.e. about 1/15,000 second -- if not shorter. (I must admit that I didn't look closely at the boundary, and if it was spread over a few scan lines I might not have noticed. That's why I said "comparable to" rather than definitely "less than".) The frame after the washed-out frame had no lightning in it, though there was a powerful afterimage (artifact of the TV camera, I assume) that faded in about 1.5 seconds. Therefore the total time of the bolt was no more than 1/30 second. It could have been much less, of course; once the image is in the camera it is going to persist for a little while. I'm sure there's no new science in this message, but I found it impressive that I could make observations on the millisecond level using apparatus found in my home! (And it only took me weeks to realize that I had done so, and meanwhile the tape has been reused...) Mark Brader
brown@nicmad.UUCP (08/15/85)
In article <747@lsuc.UUCP> msb@lsuc.UUCP (Mark Brader) writes: >(For net.video readers: The subject is misspelled because this is a >followup to a net.physics article where it was spelled that way.) > >The bolt was more or less vertical. In the first frame where it >appeared, it occupied only the bottom 1/5 of the frame; in the next >frame, it occupied the whole height of the frame. In both of these >frames there was a bright aura splaying out from the bolt; the second >frame was close to washed out. (Incidentally, this passed by too fast >to notice at normal speed. It was much more dramatic in slow motion.) >I presume the aura was simply reflected light from the cloud and rain. I hope you aren't misled by the structure of television scanning. Now, if the lightning bolt was shot on video tape, then what I have to say will be appropriate. But if the bolt was shot on film the what you saw in slow-motion makes sense. Now, every 1/60th of a second, one tv field is scaned. But, because it takes time to scan from the top to the bottom, part of the field could be done before it picks up something that heppens real fast, ie, the bolt you saw, using slow-mo, could have been to the top of the field after the time it started and the point at which the camera 'sees' it. Because of the time it takes to get back to the top of the field, the camera can and will miss things that are happening. This is the alias effect. Examples of this can be seen when TV camera shots of lasers and strobes are done. So, you can't pay too much attention to the fact that you saw only part of a bolt on a frame. The best way to see lightning is with a high speed film camera. Then the resolution, in the time domain, is much finer. TV and normal film cameras have too course a time resolution. -- Mr. Video {seismo!uwvax!|!decvax|!ihnp4}!nicmad!brown