Theatrical smoke and fog
Theatrical smoke and fog, also known as special effect smoke, fog or haze, is a category of atmospheric effects used in the entertainment industry. The use of fogs can be found throughout motion picture and television productions, live theatre, concerts, at nightclubs and raves, amusement and theme parks and even in video arcades and similar venues. These atmospheric effects are used for creating special effects, to make lighting and lighting effects visible, and to create a specific sense of mood or atmosphere. Recently smaller, cheaper fog machines have become available to the general public, and fog effects are becoming more common in residential applications, from small house parties to Halloween and Christmas.
Image:2007 Texas Longhorns football team entry.jpg|thumb|right|Fog is used for dramatic effect as the 2007 Texas Longhorns football team enters the field of play.
Theatrical fog and theatrical fog machines are also becoming more prevalent in industrial applications outside of the entertainment industry, due to their ease of use, inherent portability and ruggedness. Common popular applications for theatrical fog include environmental testing as well as emergency personnel and disaster response training exercises.
Militaries have historically used smoke and fog to mask troop movements in training and combat, the techniques of which are technologically similar to those used in theatre and film.
Health harms can be caused by short- and long-term exposure to artificial fogs. Some types of fog are less healthy than others. Handling the generating equipment also has health risks.
Types of effects
There are generally 4 types of fog effects used in entertainment applications: smoke, fog, haze, and "low-lying" effects.Smoke
Smoke effects refers to theatrical atmospheric effects produced either by pyrotechnic materials, such as Smoke Cookies, and pre-fabricated smoke cartridges; or other, flammable substances such as incense or HVAC smoke pencils or pens.Smoke is differentiated from other atmospheric effects in that it is composed of solid particles released during combustion, rather than the liquid droplets that fog or haze are composed of.
Fog
Fog is created by pumping one of a variety of different glycol or glycol/water mixtures into a heat exchanger and heating until the fluid vapourises, creating a thick translucent or opaque cloud. Devices specifically manufactured for this purpose are referred to as fog machines.An obsolete method for creating theatrical fog on-stage is to use a device known as a thermal fogger, initially designed for distributing pesticide, which aspirates a petroleum product, ignites the fuel to create a flame, and then heats a mixture of air and pesticide to create a dense fog. This technique is similar to the smoke generators used by military forces to create smoke screens, and is generally only used outdoors due to the volume of fog produced and the petroleum fuel required. For theatrical purposes the pesticide is typically replaced with glycol, glycol/water mixtures, or water.
Image:Smoke Machine.jpg|thumb|right|A thermal fogger and fan on a movie set
"Low-lying" fog effects can be created by combining a fog machine with another device designed specifically for this purpose. As the fog exits the fog machine it is chilled, either by passing through a device containing a fan and ice, or by passing through a device containing a fan and compressor similar to an air conditioner. The result is a relatively thick fog that stays within a few feet of the ground. As the fog warms, or is agitated, it rises and dissipates. Several manufacturers of theatrical fog fluid have developed specially formulated mixtures specifically designed to be used with, intended to provide thicker, more consistent fog effects. Although these chilling devices do not use carbon dioxide, the specially formulated fog fluid does create denser fog than regular fog fluid.
Haze
Haze effects refer to creating an unobtrusive, homogeneous cloud intended primarily to reveal lighting beams, such as "light fingers" in a rock concert. This effect is produced using a haze machine, typically done in one of two ways. One technique uses mineral oil, atomized via a spray pump powered either by electricity or compressed CO2, breaking the mineral oil into a fine mist. Another technique for creating haze uses a glycol/water mixture to create haze in a process nearly identical to that for creating fog effects. In either case the fluid used is referred to as haze fluid, but the different formulations are not compatible or interchangeable. Glycol/water haze fluid is sometimes referred to as "water based haze" to avoid ambiguity.Smaller volumes of haze can also be generated from aerosol canisters containing mineral oil under pressure. Although the density of haze generated and the volume of space that can be filled is significantly smaller than that of a haze machine, aerosol canisters have the advantages of portability, no requirements for electricity and finer control over the volume of haze generated.
Carbon dioxide and dry ice
, stored in compressed cylinders, is used in conjunction with theatrical fog machines to produce "low-lying" fog effects. When liquid CO2 is used to chill theatrical fog, the result is a thick fog that stays within a few feet of the ground. As the fog warms, or is agitated, it rises and dissipates. Several manufacturers of theatrical fog fluid have developed specially formulated mixtures specifically designed to be used with CO2, intended to provide thicker, more consistent fog effects. Effect duration is determined by the heating cycle of the theatrical fog machine and consumption rate of liquid CO2.A large billowing fog plumes are created from the condensation of liquid that dry ice is submerged into. As dry ice is submerged into a bulk of liquid, the pure CO2 gas bubbles are formed, then the bulk liquid molecules start to evaporate at the surface of the bubbles into the gas bubbles. The evaporated liquid molecules are later condensed within the bubbles creating a fog which lead to more evaporation of liquid molecules into gas bubbles based on LeChatelier ’s principle. The fog is released through an electric solenoid valve to control timing and duration. When the solenoid valve is closed, the fog rapidly disperses in the air, ending the effect nearly instantaneously. This effect can be used for a variety of applications, including simulating geysers of steam, in place of pyrotechnics, or to create an instant opaque wall for a reveal or disappearance during magic acts.
Image:DryIceMachine.JPG|thumb|right|Generic dry ice machine made from a 45gal. drum
Dry ice effects are produced by heating water to or near boiling in a suitable container, and then dropping in one or more pieces of dry ice. Because carbon dioxide cannot exist as a liquid at atmospheric pressure, the dry ice sublimates and instantly produces a gas, condensing water vapour and creating a thick white fog. A fan placed at the top of the container directs the fog where it is needed, creating a rolling fog that lies low to the ground. As the submerged dry ice cools the water, the amount and duration of fog produced will be reduced, requiring "rest" periods to reheat the water.
Dry ice can also be used in conjunction with a fog machine to create a low-lying fog effect. Dry ice is placed inside an insulated container with an orifice at each end. Fog from a fog machine is pumped in one side of the container, and allowed to flow out the other end. Although this technique does allow an individual to create low-lying fog "on the cheap", the volume of low-lying fog produced is typically less, and is more susceptible to atmospheric disturbances.
Nitrogen
is used to create low-lying fog effects in a manner similar to dry ice. A machine heats water to at or near the boiling point, creating steam and increasing the humidity in a closed container. When liquid nitrogen is pumped into the container, the moisture rapidly condenses, creating a thick white fog. A fan placed at the output of the container directs the fog where it is needed, creating a rolling fog that lies low to the ground. These types of machines are commonly referred to as "dry foggers" because the fog created by this method consists solely of water droplets, and as it dissipates there is little to no residue left on any surfaces. Dry Fogger is also a trademarked name for a particular brand of this style of fog machine. Liquid air can be used instead of nitrogen.Historical usage
The Globe Theatre reportedly used smoke effects during performances for atmosphere and special effects.On 23 March 1934, Adelaide Hall opened at Harlem's Cotton Club in The Cotton Club Parade 24th Edition. In the show Hall introduced the song "Ill Wind", which Harold Arlen and Ted Koehler wrote especially for her. It was during Hall's rendition of "Ill Wind" that nitrogen smoke was used to cover the floor of the stage. It was the first time such an effect had ever been used on a stage and caused a sensation.
Smoke testing
When using smoke machines, a common test is to fill the venue to the full capacity with smoke to see if there are any smoke detectors still live, or if there are any leaks of smoke from the venue sufficient to set off detectors in other parts of the venue being tested. This practice is known as a smoke test.Smoke machines are commonly used in the testing of larger HVAC systems to identify leaks in ducting, as well as to visualize air flow.