The Key Light is the main light source, it illuminates the subject the most and it defi nes the overall lighting design of the scene. Key light casts the strongest shadows to the subject and represents the main light source. In real life the main light source usually shines from above, so in a virtual studio it should be positioned above the face.
The Fill Light is less bright and is used to illuminate parts of the subject that cannot be reached by key light. Fill light is also softens the shadows of the key light and decreases the contrast of the subject’s surface.
By-the-way, the intensity of fill light is always much less than the intensity of key light, so that it only affects the diffuse characteristic of the surface.
Lastly, the Back (or Rim) Light is used to illuminate the edge of a subject from above and behind in order to separate the subject from the background. The back light can be very bright because it only highlights the edges of the subject and doesn’t compete with the key light. A back light is what helps to give a picture depth, and make the subject stand out from the background.
It’s often a good idea to test each of these light sources one at a time. If you turn them all on at once it can be difficult to work out how individual sources are affect the illumination.
In a virtual studio it’s useful to have the back lighting on the same levels as the screen lighting to help ensure that the subject doesn’t stray into the screen lighting, thereby creating unwanted shadows.
When lighting full figure subject shots, the lighting for the subject should match the pitch, intensity, and angle of the source light in the background shots.
The type of lighting you use, whether Tungsten, Fluorescent or LED is largely irrelevant so long as it’s fit for purpose. In general, larger, softer sources are best for screen and the fill lighting, with smaller, more directional lighting for the keys and back lights.