Off-screen space is the area that isn't visible in your shot, such as the space above the camera. The illumination, shadows, and reflections you see in a photograph are often motivated by off-screen sources, rather than by light sources that are visible within the frame. An important part of your job in designing lighting for any scene is to imagine what exists in off-screen space, so that you can light your scene with lights that appear to be motivated by real light sources.
To see how lighting can be shaped by off-screen space, take a look at the four photographs in
Figure 1.1. The objects in the frame don't change, and yet based on the light coming from off-screen, you can tell a great deal about where each picture was taken.
Figure 1.1. You can recognize a scene lit by a lamp (upper left), by light through a window (upper right), under a cloudy sky (lower left), and in direct sun (lower right).