A gobo is a stencil or template placed inside or in front of a light source to control the shape of the emitted light. Lighting designers typically use them with stage lighting instruments to manipulate the shape of the light cast over a space or object—for example to produce a pattern of leaves on a stage floor. The term "gobo" has come to sometimes refer to any device that produces patterns of light and shadow, and various pieces of equipment that go before a light. In theatrical lighting, however, the term more specifically refers to a device placed in 'the gate' or at the 'point of focus' between the light source, called a lamp and the lenses. This placement is important because it produces a crisp, sharp edged pattern or design. Gobos placed after the optics do not produce a finely focused image, and are more precisely called "flags" or "cucoloris".
Derivation
The exact derivation of gobo is unclear. It is cited by some lighting professionals as "goes before optics" or, less often, "goes between optics". An alternative explanation is "graphical optical black out." The term is traced back to the 1930s, and originated in reference to a screen or sheet of sound-absorbent material for shielding a microphone from sounds coming from a particular direction, with no application to optics. The treatment of the word as an acronym is recent and ignores the original definition in favor of popular invention. There are many examples of acoustic gobos. The word most likely is a derivative of "goes between."
Use
Gobos are used with projectors and simpler light sources to create lighting scenes in theatrical applications. Simple gobos, incorporated into automated lighting systems, are popular at nightclubs and other musical venues to create moving shapes. Gobos may also be used for architectural lighting, as well as in interior design, as in projecting a company logo on a wall.
Materials
Gobos are made of various materials. Common types include steel, glass, and plastic. Steel gobos or metal gobos use a metal template from which the image is cut out. These are the most sturdy, but often require modifications to the original design—called bridging—to display correctly. To correctly represent the letter "O" for example, requires small tabs or bridges to support the opaque center of the letter. These can be visible in the projected image, which might be undesirable in some applications. Glass gobos are made from clear glass with a partial mirror coating to block the light and produce "black" areas in the projected image. This eliminates any need for bridging and accommodates more intricate images. Glass gobos can also include colored areas, whether by multiple layers of dichroic glass glued on an aluminium or chrome coated black and white gobo, or by newer technologies that vary the thickness of the dichroic coating in a controlled way on a single piece of glass—which makes it possible to turn a color photo into a glass gobo. Glass gobos generally offer the highest image fidelity, but are the most fragile. Glass gobos are typically created with laser ablation or photo etching. Plastic gobos or Transparency gobos can be used in LEDellipsoidal spotlights. These "LED Only" plastic gobos can be full color, but are far less delicate. They are new to the market, as are LED lights, and their durability and effectiveness vary between brands. In the past, plastic gobos were generally custom made for when a pattern requires color and glass does not suffice. However, in a "traditional" light fixture, the focus point position of a gobo is extremely hot, so these thin plastic films require special cooling elements to prevent melting. A lapse in the cooling apparatus, even for seconds, can ruin a plastic gobo in a tungsten-halogen lighting instrument.
Patterns
Theatrical and photographic supply companies manufacture many simple and complex stock patterns. They also can produce custom gobos from customer artwork. Generally, a lighting designer chooses a pattern from a manufacturer's catalog. Because of the large number of gobos available, they are generally referred to by number, not name. Lighting technicians can also hand cut custom gobos out of sheet metal stock, or even aluminum pie tins. Gobos are often used in weddings and corporate events. They can project company logos, the couple's names, or just about any artwork. Some companies can turn a custom gobo out in as little as a week. Designers also use "stock" gobo patterns for these events—for example for projecting stars or leaves onto the ceiling.
Mechanics/optics
The gobo is placed in the focal plane of the lantern. The gobo is inserted upside-down and back-to-front. The lighting instrument inverts the projected image.
Common sizes
Other usages
The term "gobo" also is used to describe black panels of different sizes or shapes placed between a light source and photographic subject to control the modeling effect of the existing light. It is the opposite of a photographer using a "reflector" to redirect light into a shadow, which is "additive" lighting and most commonly used. Use of a gobo subtracts light from a portion of an overall shaded subject and creates a contrast between one side of the face and another. It allows the photographer to expose with wider open apertures giving soft natural transitions between the sharp subject and unsharp background, called bokeh.
