In photography, backlight is illumination from behind. The source of the backlight can be from the sun, from a strobe, or from a constant light.
technical explanation of backlight
Backlight, or back lighting, is any light source that comes from behind your subject. However, backlighting is a more general term, and can be used to describe a broad range of lighting from rim light to a "kicker" light. Whether it is technically correct or not to use the term backlight to describe any and every form of lighting that comes from behind is not as important as simply understanding what another photographer is referring to.
Backlight can definitely come from any light source, including the sun on a bright day, or a wireless flash on a dark night.
The goal of backlighting, from an artistic and technical standpoint, is usually to help separate a subject from a background, and bring emphasis to that subject in dark conditions, having a backlight can be especially helpful, and sometimes it can actually serve as the ONLY major light source in a dramatic silhouette photo.
In post-production, dealing with backlighting can be difficult, if your subject is in deep shadow, however both silhouette images and brightly exposed images can work, if the editing fits the mood of the scene, pose, etc.
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