Mark Vargo is a pioneer of cinematography and a huge contributor to the special effects industry.
Watch in the video below how Mark explains step-by-step how ILM processed their blue-screen footage in The Empire Strikes Back.
Vargo mentions they used rear-lit blue screens that emit light in a very specific range of 500 nanometers. Captured blue screen footage was then sent to an optical printer where the footage is then photographed again into a combined composite image.