What does back light do in a video production?

Back light serves the critical function of separating the subject from the background in video production. When positioned behind the subject, back light creates a slight halo or rim of light around the edges of the subject. This rim helps to visually distinguish the subject from the background, adding depth and dimension to the shot. It enhances the three-dimensionality of the subject, making it more pronounced against a potentially distracting background. This technique is particularly effective in situations where the background has similar colors to the subject, as it ensures that the subject remains the focal point of the image.

In contrast to this correct explanation, other functions mentioned, such as illuminating the front of the subject or filling in shadows, pertain to other types of lighting. These roles are typically fulfilled by key light or fill light rather than back light, which specifically targets separation within the composition. The mention of adding warmth relates more to color temperature and lighting conditions rather than the defined role of back light in establishing separation and depth.