Purpose of subliminal skull in Return of the Jedi?

In the climax of Star Wars: Return of the Jedi Darth Vader throws Emperor Palpatine over a precipice and just as he does so, a subliminal image of a skull is flashed on the screen over Vader's face. The image only appears for a fraction of a second:

Has this subliminal image been explained by any of the makers of film as to its origin or purpose?

I know that in the 1960s it was considered cool to insert single stills into movies that were either pornographic or images of death, like skulls. Is this some kind of homage to that phenomena by Lucas?

Best Answer

As Oliver has pointed out, the shot is not subliminal at all, as most people, (including myself when I first saw the film in 1985 as a 7 year old) clearly notice the extremely brief but overtly visible 'skull', through Vaders mask and helmet.

Again, as has been pointed out in the comments section, the Emperors use of force lightning fries Vader to the extent that it causes or contributes to what are ultimately mortal injuries, presumably by damaging the life support component of his suit and creating a type of X - ray effect, with the intention likely to be in order to both demonstrate how badly affected Vader was and on a more thematic level, it is the second of only 3 scenes in the original trilogy that demonstrate that Darth Vader is a physical human being.

Subliminal images do not register consciously where as this image was, to my knowledge, noticed by most who have seen the film.