The Asphyx

The Asphyx is a 1972 British horror film directed by Peter Newbrook. Also known as Spirit of the Dead and The Horror of Death, it stars Robert Stephens and Robert Powell.

At a party to celebrate his recent engagement, Cunningham is making home movies with a primitive video camera of his own invention when his fiancee and son are killed in a boating accident. When Cunningham views the film, he sees that not only has he captured the blur, but that it is moving towards his son, and not away from him. From this, Cunningham concludes that the blur is not the soul but a force known in Greek mythology as an Asphyx, a kind of personal Grim Reaper that comes for every individual at the moment of his or her death.While filming a public execution as a protest against capital punishment, Cunningham activates a spotlight that he has crafted using phosphorus stones beneath a drip irrigation valve. Later, when viewing the film with his ward, Giles, Cunningham sees that the condemned mans asphyx was briefly held suspended in the spotlights beam. Concluding that an individuals asphyx is an organic force and therefore subject to the laws of physics, Cunningham theorizes that some property of the energy released by the combination of phosphorus and water renders the asphyx immobile. If correct, this would mean that an asphyx could be trapped, and that an individual would be immortal so long as their asphyx remained imprisoned. ........

Source: Wikipedia