Matte

Most visual effects in 1980's animation were complex, even seemingly simple ones such as making the Gundam's eyes glow in the first episode of Mobile Suit Gundam. How they used to do it, in the dark ages before the computer, was to expose the film twice. The first time, the sequence was filmed with the cel itself, but with the glowing element (or other optical effect) cut out replaced with a black area to prevent light leakage. The second pass would be a "negative" of the first, with the entire cel blacked out except for the visual effect's area (a "matte"). In the case of a glowing area, the cameraman would put a Kleig or other bright light with a colored gel underneath the matte, and expose the film a second time using that light. Being very strong, the light would go through the clear areas of the cel with some diffusion around the borders, thus producing the desired 'halo' glowing effect.