It&#x27;s not a filter, as others have suggested, but rather a standard artifact of vacuum-tube-based television cameras that were in use at the time.  (I don&#x27;t know why this lecture wasn&#x27;t filmed, but the extensive dark halo effect makes it clear this was shot with a TV camera.  This was the early era for magnetic video tape, but I assume that&#x27;s how the recording was preserved.)<p>Anyway, the point is that these TV cameras are based on the fact that incoming light will dislodge electrons from a thin plate in a vaccum tube in an amount proportional to brightness.  A very bright spot in the image produces a shower of electrons that is more powerful than the rest of the tube (the part that detects the electrons) can deal with.  The net result of this &quot;splash&quot; of electrons is a mild desensitization of the detection apparatus around the bright spot.  This makes the nearby stuff appear darker.<p>You mentioned that dark spots also seem to have a bright halo, but I don&#x27;t see that in the video, and it isn&#x27;t consistent with the usual artifacts of these cameras.  Are you sure?