Object Source Lighting (OSL) is a technique to create reflections of light that are painted onto a figure. For example, a figure carrying a lantern would have the hand and arm brightly lit by the lantern but fade to darker colors farther from the source of light (the lantern). The same concept can be applied to fire or a lightsaber. Of course, on a figure, there is no actual source of light, so the light and reflections must be painted to appear as if glowing.
OSL can easily be overdone and quickly go from enhancing a figure to ruining one. Getting the level of reflections just right takes practice, so if you're trying it for the first time, practice on a figure that is not your current masterpiece.
Below is Darth Vader from Star Wars Legion with some OSL recently finished in the studio.
