The most important light in three-point Hollywood lighting is the key light, which illuminates the object that's the central focus of the frame or shot. Key lights were theater spotlights until strong incandescent bulbs were invented. The key light makes the main actor or object stick out and almost glow so that the eye is drawn to it instead of the rest of the frame. Strong key lights were the reason actors in old Hollywood had to wear such heavy makeup. The high-powered light revealed any minor skin imperfections. The key light usually illuminates the subject from above.
The fill light is less bright than the key light and shines from ground level off to one side of the subject to literally fill in the rest of the space so that there is texture to the set and other objects. Sometimes cinematographers use multiple fill lights for larger sets or to eliminate small shadows. With very dim fill lights, there is more contrast between light and dark, because shadows are sharper and stand out against the bright fill light. This is called low-key lighting.
In order for the subject to appear in three dimensions, Hollywood lighting uses a back light. Back lights work from behind the set like a slightly dimmer fill light to provide a sense of roundness and texture. Without a back light, subjects appear flat or blend in with the background. Back lights can shine from the side, above or ground-level, depending on the needs of the film makers.
Even before the advent of color film technology, Hollywood lighting saw developments that changed the way films could look. Cinematographers began using lights with diffusers, which changed the quality or harshness of lights. Diffusers could make a shot appear fuzzy and angelic or crisp and clear. Reflectors and light absorbers help eliminate shadows and amplify back lights for texture and depth by bouncing light from to the set or objects.