To create a special lighting effect, you may need to place a lighting instrument or some other piece of technical gear on the stage. You may also have cables that power the device running across the stage within the audience's view. Masking techniques hide these technical elements from the audience so they do not become distracted by the technology and can instead enjoy its effects and immerse themselves in the performance.
A broad variety of materials can be used to hide lighting instruments and effects machines, but the three basic types of masking are hard, soft and inset masking. Hard masking refers to masking made of any hard material such as wood, metal or plastic. Soft masking generally refers to fabric, which may be placed on some sort of wood or metal frame but is not solid. Inset masking is the practice of creating lowered spaces within the stage floor. Lights are placed in these sunken spaces and aim upward at the performers or scenery.
Soft masking is the most common type of masking in theaters. All of the curtains you see hanging in a standard proscenium theater (the type where the stage is framed by a wall, rather than surrounded by seating on three or more sides) are masking. The curtains hanging down the sides, known as "legs" or "tormentors," hide lighting effects that are placed on poles or other devices known as "trees" or "ladders." The curtains that run across the top of the stage, known as "borders" or "teasers," hide lighting effects that are hung above the stage.
Hard masking is generally used for specific onstage effects. A piece of scenery is designed to fit around the light, so all the audience sees is a set piece such as a rock or bush, but the side of the set piece facing the stage is open, allowing lighting effects to shine through. This is often used to simulate the effects of fire or flickering television light.
Inset masking requires a stage with removable segments. Lighting effects placed in the depression created by inset masking may be left uncovered or the opening may be covered by clear or translucent heat-resistant plastic so actors can walk over them freely.
Fire safety is always a concern when dealing with theatrical lighting effects because most theatrical lights range between 575 and 2,000 watts, which creates a significant amount of heat. All soft masking must be made of inherently flame-retardant fabric or treated with National Fire Protection Agency-approved flame retardant solutions. Wood and metal masking, whether above stage or inset, must allow adequate heat ventilation and be kept a safe distance from the light, with distances varying according to the lamp's wattage and the exact material. Woods may not be treated with oil-based paints or stains. When in doubt, ask your local fire marshal to inspect the stage and offer recommendations; if you do this, be prepared to receive a list of things you must fix before opening night or risk having the performance shut down.
When a lighting effect is placed onstage, one of the prime considerations after hiding the effect from the audience is whether or not the masking itself block's the audience's view of the performance. The view from a particular seat is known as a "sightline." Once the masking is in place, have someone check the sightline from a variety of seats in the audience, paying particular attention to seats in the first few rows, which are most likely to have their views obscured with a raised proscenium stage. If blocking the view for a few seats is unavoidable, the show's producer may elect to sell those seats at a discounted price as "obstructed view seating."