A commonly used studio lighting unit is the spotlight. Spotlights shine straight and focused rays of light at the desired subject or location. The intensity of spotlights are measured in watts. The lower the wattage of the bulb, the less light it shines. Wattage also indicates the physical size of the spotlight itself. For instance, 100 to 200 watts is very small, and 5,000 watts is very large. Spotlights can be of different shapes, such as circular, rectangular and square.
"Shot By Shot: A Practical Guide to Filmmaking" states that light can be measured in two ways: intensity and quality. The intensity of the light means how brightly or darkly the subjects are lit. Light quality means the texture of the subject or how much shadow it produces. Soft quality of light has slight, soft-edged shadows, and hard quality has long, dark and crisp shadows.
The brightest and most significant studio light is the key light. Key lights point downward at the subject from a diagonal or 90-degree angle. Pointing the key light downward eliminates awkward, long shadows from appearing behind the subject, regardless of the light quality. The key light also establishes for viewers where the direct source of light is coming from for continuity sake. If it is unaided by any other lights, the subjects are usually half-lit and almost completely dark on the other half of the body, or the side of the face, if the subject is a person.
The fill light helps brighten the dark shadows made by the key light, and makes the subject more visible and clear. Positioned slightly near the camera and on the opposite side of it as the key light, the fill light points as directly to the front of the subject as possible. The fill light is rarely brighter, or as bright, as the key light, so it does not create its own conflicting shadows on the subject.
Backlights point at the back of the subject from behind it and show the outline of the subject. In order for the light to have the best effect, the light is usually of hard quality, so enough of it can hit the edges of the subject, or the shoulders and hair of a human subject. Outlining the subject with a backlight also creates distance between it and the background, creating three-dimensional space on film.