A couple of posts ago the topic was lighting equipment and the basic relationship of the two basic sources of light: key and fill. The third light source, entirely optional, comes from behind.
Often called an edge light, it lights up the fringes of the form, separating it from the background. It’s also called a “rim” or “kicker” in the TV business, and it usually requires a source that’s stronger than either of the others. The form has more snap if the edge light doesn’t overlap too much with the key light, leaving the dark turning of the form (sometimes called the “core” or the “hump” of the shadow) intact.
This oil study from the model has the key and rim lights placed just far enough apart so that you can see the core of the shadow on the forehead.
If you want to introduce an edge light source, it should be a different color from the key. By applying contrasting gels to the key and edge lights, you can generate some interesting effects with skin tones. On this portrait I used a warm key light and a cool edge light.
On a future post I’ll suggest some painting methods to make it possible to do these quick oil studies in the standard 20 or 30 minute poses that you get in most sketch groups.