Atmospheric effects are often bundled together under the terms "fog" and "smoke," although they fall into four basic categories--smoke, haze, fog and heavy fog, each with different applications and peculiarities.
Smoke can be produced by burning anything from sandalwood to auto tires. Given enough time or assistance to diffuse, smoke hangs in the air for prolonged periods and provides an excellent medium for defining beams of light. It works well for revealing laser beams, shafts of sunlight or a followspot in a smoky nightclub.
Despite its continued use in the worship practices of many faiths, the indoor use of smoke in public places is not intrinsically safe. While not all smokes are toxic, most contain carcinogenic agents and all smokes can produce respiratory irritation. Smoke can be produced in almost any color by burning the appropriate mix of materials. However, most colored smokes are toxic. Breathing equipment and high-volume air-exchange systems are essential components in any smoke-effect system.
Smoke effects are generally produced by the controlled partial combustion of a fuel material. Total combustion of carbon-based fuels such as gasoline, charcoal or natural gas produces little more than a nearly invisible blue flame, some carbon dioxide and some water vapor.
Smoke production systems generally use an overload of fuel in the combustible mixture to leave unburned material, which appears as smoke. Smoke systems are usually made from a heat source such as an electric element or a charcoal burner and some sort of smoke-producing fuel mix in the quantity required for the effect.
Smoke candles, smoke flares and similar devices incorporate the entire combustion process into a single mix which, once ignited, cannot be controlled in either output or duration.
Haze, fog and low fog are all vapor effects, formed by suspending microscopic droplets of water or an edible fluid in the air. The vapor is produced either by heat or mechanical processes that involve no combustion, and thus no carcinogenic products. Unlike smoke, vapors only come in plain white. If you need colored haze or fog, the effect must be achieved with colored light.
Haze is a very fine effect that hangs in the atmosphere like smoke, although it tends to dissipate more rapidly. It can be substituted for smoke in many applications. Fog is a more dense version of haze, with larger droplets that diffuse more light, but being heavier, they settle out of the atmosphere more rapidly.
Fog is ideally suited for giving shape and form to the beams of light from your rig. It's almost inconceivable that you would use moving lights in a game show or music segment without some fog in the atmosphere to help sell the effect. Fog is also the effect that adds to the mystery of night scenes, especially on location.
Low fog is the stuff of dream sequences, mysterious lands in children's shows and ballroom dancing. It is produced either by pumping fog through a chiller unit or by the use of dry ice or liquid nitrogen to generate a chilled water vapor fog. Low fog dissipates very quickly and requires a lot of effort to maintain.
The fluid used in fog systems can be any material that is capable of being easily vaporized and condensed. Water ought to be an ideal material, being both cheap and safe. However, it has a low refractive index, producing little light scattering and so making it a less visible fog than other materials. It's also very volatile at normal temperatures, and disappears far too quickly for most applications.
The fluids most commonly used are members of a family of low-volatility alcohols known as glycols. In addition to their low volatility, glycols are very water soluble and can therefore be mixed with water to produce fluids with a wide range of diffusion and volatility characteristics. Glycol fog fluids are frequently referred to as being "water-based," despite the fact that some varieties contain no water at all.
Glycols have been a benign part of our chemical environment for more than a century, being regularly used in cosmetics, inks, antifreeze and as food additives. Several glycol-based effects systems have been approved as safe by Equity and some even have endorsements from opera singers or entire opera companies whose members are quite prepared to sing while engulfed in fog effects.
Members of the glycol family used in atmospheric effects include: propylene glycol, triethylene glycol, diethylene glycol, dipropylene glycol, 1,3-butylene glycol, polyethylene glycol, triethyline glycol, monopropylene glycol, and glycerol, which is also known as glycerin.Most of them are quite oily in texture and can pose a dangerous slip hazard if spilled or allowed to condense on floors and set decks.
Some haze machines produce tiny vapor droplets by forcing an aerosol of droplets of a low-volatility fluid, such as glycerin or mineral oil, through a mechanical system that smashes them into smaller pieces. These much smaller droplets are separated off and expelled from the generator. These are the "cracker" systems produced by several manufacturers.
Most haze and fog systems work by pumping an aerosol of fog fluid through a heating chamber. The large aerosol droplets are completely vaporized by the heating, only to re-condense into microscopic droplets when hitting the cooler air as they exit the chamber.
Different mixes of glycols and water can produce slow or fast dispersing fogs from the same machine. The temperature of the heating chamber is matched to the vaporization temperature of the fog fluid. Using the wrong fog fluid mix can lead to either burning the fluid and thereby producing smoke, or not completely vaporizing it so that fluid is sprayed out of the machine.
Many low fog systems use a chiller device to make the fog more dense than air and thus make it low-lying. Cryogenically based low fog systems use a mix of hot water vapor and either carbon dioxide from dry ice or liquid nitrogen to produce clouds of chilled water vapor.
The fogs produced by liquid nitrogen and dry ice systems have a substantially reduced oxygen content and require careful attention to the ventilation of low-lying areas.
There are a vast number of smoke, haze, fog and heavy fog generation systems available--from fog generators that can literally fit in your pocket, to haze makers that can fill a concert stage for a whole performance on less than a teaspoonful of fluid, to low fog machines based on refrigeration plants substantial enough to keep an open field looking foggy throughout an entire night of continuous shooting.
Most systems have remote controls that enable the setting of both the volume and the duty cycle of effect production. Many of these are driven by DMX512 signals and can be controlled as part of the general lighting cueing system via a console operator or show control program.
There is a whole generation of concert LDs out there who won't even get the crew to strike up the moving lights until the atmospherics are pumping out their magic.