Sci-Fi lighting is often used to discribe screengrabs so poorly lit that they appear almost black. The term comes from the SCI FI Channel's usage of poor lighting in their original films to lower production costs. Often they will produce a film which features several scenes which is mostly an almost pitch black night sceen with some beast growling. During the time of Caption This! on their website, cappers began to notice this trend and began joking about how poor the lighting was. This evolved into calling all poorly lit screengrabs into having captions like "lighting by Sci-Fi" and such and thus the gag about Sci-Fi lighting was born.
Cappers mainly being Sci-Fi fans, several variations of the Sci-Fi Lighting gag crept in over the years, with inspirations taken from such sources as:
The Hitchhiker's Guide to the Galaxy:
"Every time I push one o’ these weird black buttons which is labelled in black on a black background, a little black light lights up black to let me know I've done it."
"I think my eyes ARE getting better. Instead of a big dark blur, I see a big, LIGHT blur." And variations on that theme. (Usually attributed to Beedo. Incessantly.)