What is a gobo? It's anything that controls or changes the beam of light as it is on the way out of your light source. The word comes from "goes between" or "goes before optics". Gobos are typically used in theater or stage lighting to create an effect. They control light by blocking it in some areas, by allowing the addition of color, or by diffusing it. They are used often with ellipsoidal spotlights that have a moveable lens for sharp or soft focusing.
Gobos are made from either thin metal or glass, commercially. Glass gobos can be colored or textured to create effects. Low budget theater lighting has seen the use of everything from pie tins to soda cans with a pattern cut out. Essentially anything that will not burn or melt in close proximity to a very hot light source has been used at one time or another to create a gobo.
For the architectural or interior photographer, as well as some studio portrait photographers, these creative lighting tools can come in very handy. Metal gobos come in a huge variety of patterns, including windows, blinds, doors, trees and leaves, and abstract shapes. Imagine the possibilities for interior photography! You can throw a light pattern across a room, giving the visual impression that there is a window somewhere just out of view in the image, when in fact there was none. Or you can use the technique with an abstract pattern to break up a blank and monotonous area (a stairwell, occasional) and add interest to an otherwise awkward area.
It is also useful for adding light without having it look like a spotlight was placed on an object. When used with an ellipsoidal light, a creative photographer can not only use the internal barn doors to flag the light beam, but also use the gobo to spread out a broken pattern with either a very soft edge or by adjusting the focus of the beam, Create a very hard edge. This gives the option of simulating either soft, early morning light or mimicking the feeling of bright sunlight at noon. Add a dimmer switch to the equation and you can easily expand your possibilities.
Given the complexity and variety available from gobo manufacturers, you are limited only by your imagination … and budget. While the metal gobos themselves are reliably inexpensive, the lights and accessories are not. But having these tools at your disposal on a location shoot gives you options not possible any other way. Catalog product photography [http://www.accent-photography.net/catalog_photography.html] goes from "same old thing" to something with depth and dimension, and for the advertising photographer it opens up a brochure range of possibilities for photographing large Products as well as adding what you need on a cloudy day for an outdoor location shoot. Your local theater lighting supply company or Barbizon Lighting are good places to start. Ask for a catalog of gobos, sit back and let your imagination run wild. Just make sure you have sturdy stands and sandbags. Even the smaller ellipsoidal lights are heavy and need good support.