Low-key lighting is a style of lighting for photography, film or television. It attempts to create a chiaroscuro effect. In traditional photographic lighting, three-point lighting uses a key light, a fill light, and a back light for even illumination. Low-key lighting requires only one key light, optionally controlled with a fill light or a simple reflector.
Low key light accentuates the contours of an object by throwing areas into shade while a fill light or reflector may illuminate the shadow areas to control contrast. The relative strength of key-to-fill, known as the lighting ratio, can be measured using a light meter. Low key lighting has a higher lighting ratio, e.g. 8:1, than high key lighting, which can approach 1:1.
The term "low key" is used in cinematography to refer to any scene with a high lighting ratio, especially if there is a predominance of shadowy areas. It tends to heighten the sense of alienation felt by the viewer, hence is commonly used in film noir and horror genres.
HIGH KEY LIGHTING
High-key lighting is a style of lighting for film, television, or photography that aims to reduce the lighting ratio present in the scene. This was originally done partly for technological reasons, since early film and television did not deal well with high contrast ratios, but now is used to suggest an upbeat mood. It is often used in sitcoms and comedies. High-key lighting is usually quite homogeneous and free from dark shadows.
In the 1950s and 1960s, high-key lighting was achieved through multiple light sources lighting a scene—usually using three fixtures per person (left, right, and central) —which resulted in a uniform lighting pattern with very little modeling. Nowadays, multiple hot light sources are substituted with much more efficient fluorescent soft lights which provide a similar effect.
The advantage to high-key lighting is that it doesn't require adjustment for each scene which allows the production to complete the shooting in hours instead of days. The primary drawback is that high-key lighting fails to add meaning or drama by lighting certain parts more prominently than others.
Most recently, shows with bigger budgets moved away from high-key lighting by using lighting set-ups different from the standard three-point lighting. Part of the reason for this is the advent of new lighting fixtures which are easier to use and quicker to set up. Another reason is the growing sophistication of the audience for TV programs and the need to differentiate.
The term "high-key" has found its way from cinema into more widespread usage, for example referring to an event that requires much organization or is subject to a great deal of publicity.