Wikipedia
Introvision was a variation on a front-projection process that allowed film makers to view a finished composite of live action and plate photography through the camera's viewfinder on set and in real time. During its heyday, starting with the feature film, Outland in 1981, Introvision enjoyed the novelty of visual effect compositing in-camera, thus eliminating the need to wait for photo-chemical compositing to determine if the effect shot was successful.
President of Introvision Systems, Tom Naud, explained it this way: "Introvision utilizes a Scotchlite screen - ours happen to be thirty feet tall by sixty feet wide—so in that regard, we're front screen projection. But the finished piece of film produced on our system bears no other resemblance to standard front projection.'
Another benefit to the Introvision process was the ability to place an actor 'inside' a plate, meaning an actor could walk vertically or laterally inside a two-dimensional background image and seemingly go behind objects within any given environment. The actual background on the set was black, so the actor would have to pantomime walking through and around certain objects. Done well, the illusion was nearly perfect, particularly with precise lighting and careful miniature set construction (or previously photographed images, which was done in The Fugitive with a train superimposed behind actor Harrison Ford).
The system faded from use around 1994, due to the widespread adoption of Digital compositing and Matchmoving, which allowed live action characters to be placed in fully or partially computer-generated backgrounds, or computer-generated characters to be combined with live action characters—and sometimes both at the same time.