Atmospheric Effects

Definition

A beam of light becomes visible only when particles in the air scatter part of it toward the viewer, and atmospheric effects supply those particles. A hazer produces a fine, even suspension that can stay in the air for an hour or more and reveals beams uniformly; it is the usual support for moving-light and laser effects. A fog machine produces denser, shorter-lived clouds for reveals and dramatic moments. Low fog and cryogenic effects use carbon dioxide or liquid nitrogen to cool the fog so that it stays close to the floor, and it dissipates as it warms and rises.

The guide

Density is managed during the show. With too little, the beams disappear; with too much, sightlines are lost and the haze reduces the contrast it was meant to reveal. Heavy output can set off smoke detectors, so its use is coordinated with the venue and its fire safety system. Performers and crew breathe the atmosphere throughout the show, so the lowest density that achieves the effect is used. In North America, ANSI E1.5 covers fog made from aqueous glycol and glycerine solutions, and ANSI E1.23 covers the design, execution and maintenance of atmospheric effects. Carbon dioxide and nitrogen displace oxygen and collect in low, enclosed spaces such as orchestra pits, which creates a risk of asphyxiation.

The purpose of atmosphere is to make the beams visible, and density is reduced as soon as it starts to hide performers or scenery.

The rule

Atmosphere is used at the lowest density that makes the beams visible.

Sources

ESTA Technical Standards Program : liste des normes publiées, dont ANSI E1.5 et ANSI E1.23 · tsp.esta.org
ANSI E1.5, ESTA : fumées théâtrales produites à partir de solutions aqueuses d'alcools di- et trihydriques (glycols, glycérine) · tsp.esta.org
ANSI E1.23-2020, ESTA : conception, mise en œuvre et maintenance des effets atmosphériques · webstore.ansi.org
Fog machine (Wikipédia, pointeur secondaire) : machines à fumée, fumée lourde au CO2 et à l'azote liquide · en.wikipedia.org
Haze machine (Wikipédia, pointeur secondaire) : brume fine qui peut tenir une heure ou plus, contre quelques minutes pour la fumée · en.wikipedia.org
Gouvernement de l'Ontario, Safety guidelines for the live performance industry, fumées et brumes : le CO2 et l'azote déplacent l'oxygène, ventilation des zones basses et confinées comme les fosses d'orchestre · ontario.ca
HSE, ETIS3, Smoke and vapour effects used in entertainment : carboglace et azote liquide, risque d'anoxie · hse.gov.uk