"If it can be written, or thought, it can be filmed..." STANLEY KUBRIC
From Encyclopedia PRO
The separation light, or hair light, is designed to help visually separate the subject(s) from the background. A separation light is not always necessary, but without the use of this light, it is possible that the subject could blend with the background. Use of a separation light also helps to bring out color and texture in the subject’s hair.
Brightness of the separation light can equal the brightness of the key light source, but for interviews, the separation light is usually less bright than the key. Position of this source can range from directly behind and above the subject to just outside of the frame line to create a side rim-light. As is true with almost all light placements, the effect of the separation light is dramatically altered by its position.
The addition of a separation light also can help to add texture, color and/or added separation for the subject(s) from the background. Direction of the separation light also can help to support key light direction and motivation (i.e. light from a window, etc.).