The third light in a three light setup is called the "back" light. It is positioned behind the subject and points in the general direction of the camera.

The purpose of the back light is to put a splash of light on the subject's shoulders and hair. This glow has an important purpose: it separates the subject from the background.

In real life, it's obvious where a person ends and the background begins. But in the two-dimensional world of television and film, those distinctions are not so clear. So we add the back light to provide separation.

Ideally the back light would be directly behind the subject, but often that would mean a visible light stand in the background. So it is OK to put the back light off to the side a bit.

Back lights also add a bit of glamour to your subject. The shiny, glowing hair is a staple of the golden age of Hollywood--and modern day fashion photography.