According to onstagelighting.com, a gobo is a piece of metal designed to produce patterns of light by having different shapes cut into it. Design patterns can be anything from abstract shapes to specific images. Once the gobo is chosen, it is inserted into the focal center of the spotlight, known as the gate. The gate is located between the bulb of the lamp and the lenses. The pattern of the gobo is then reflected onto the stage to create different lighting effects.
Onstagelighting.com explains the three different types of gobos. The most traditional gobo typically used for stage lighting is the metal gobo. The pattern of the gobo is cut out of the metal sheet to provide the lighting effect. A gobo holder is used to hold the metal gobo in place inside the spotlight. The metal gobo is one of the least expensive ways to create designs and patterns of stage lighting.
The glass gobo uses a pattern that is etched into the glass using aluminum or another type of coating that helps to block out the light. Glass gobos can create more intricate images than metal gobos, and must be inserted into a fragile gobo holder. Glass gobos must be inserted properly in order for the image to be projected correctly, since their design dictates they can only be used when facing one way. They must be heated slowly and gently within the spotlight, or they may crack.
The most expensive gobos are full-color and dichroic gobos. They are made from expensive and fragile glass and are diverse in the patterns and images they are able to project.
According to onstagelighting.com, there are a wide variety of gobo patterns and designs to choose from. These include abstract patterns and shapes, specific logos and text or other designs such as clouds, stars or the moon. Depending on the lighting needs of the production, gobos can produce mood by providing textures of light such as blotchy patterns or simulating leafy patterns shining through the trees. Other gobo designs include light streaming in from a window or street signs to create a city scene.