The Science of Smoke

Smoke, haze, atmos, whatever you want to call it, anyone who knows me knows that I’m a big fan. But how does it work and what is the purpose of smoking up a set?

 

Aerial perspective

At the most basic level, smoke simulates a natural phenomenon called aerial perspective. If you look at – for example – a range of mountains receding into the distance, the further mountains will appear bluer, lighter, less contrasty and less colour-saturated than the nearer mountains.

An example of aerial perspective

This effect is due to light being scattered by particles naturally suspended in the air, and by molecules of the air itself. It is described by the scary-looking Rayleigh Equation:

We don’t need to get into what all the variables stand for, but there are a few things worth noting:

  • The symbol on the far right represents the angle between the incident light and the scattered light. In practice this means that the more you shoot into the sun – the more the air you’re photographing is backlit – the more scattering there will be. Place the sun behind your camera and scattering will be minimal.
  • is the distance from the particle that’s doing the scattering, so you can see that scattering increases with distance as per the Inverse Square Law.
  • Lamda (the sort of upside-down y next to the x) is the wavelength of the light, so the shorter the wavelength, the more scattering. This is why things look bluer with distance: blue light has a shorter wavelength and so is scattered more. It’s also why shooting through an ultraviolet filter reduces the appearance of aerial perspective/atmospheric haze.

 

How smoke works

An Artem smoke gun

Foggers, hazers and smoke machines simulate aerial perspective by adding suspended particles to the air. These particles start off as smoke fluid (a.k.a. “fog juice”) which is made of mineral oil, or of a combination of water and glycol/glycerin.

In a smoke machine or gas-powered smoke gun (like the Artem), smoke fluid is pushed into a heat exchanger which vaporises it. When the vapour makes contact with the colder air, it condenses to form fog.

A hazer uses compression rather than heat to vaporise the fluid, meaning you don’t have to wait for the machine to heat up. The particles are smaller, making for a more subtle and longer-lasting effect.

As a general rule, you should use only hazers for interior cinematography, unless there is a story reason for smoke to be present in the scene. Outdoors, however, hazers are ineffective. An Artem or two will work well for smaller exterior scenes; for larger ones, a Tube of Death is the best solution. This is a long, plastic inflatable tube with regularly-spaced holes, with a fan and a smoke machine (usually electric) at the end. It ensures that smoke is distributed fairly evenly over a large area.

A Tube of Death in action on the set of “The Little Mermaid”

 

The effects of smoke

Just like aerial perspective, smoke/haze separates the background from the foreground, as the background has more smoke between it and the camera. The background becomes brighter, less contrasty, less saturated and (depending on the type of smoke) bluer, making the foreground stand out against it.

Since smoke also obeys the Rayleigh Equation, it shows up best when it’s backlit, a bit when it’s side-lit and barely at all when front-lit.

Here are some of the other things that smoke achieves:

  • It diffuses the image, particularly things further away from camera.
  • It lowers contrast.
  • It brightens the image.
  • It lifts the shadows by scattering light into them.
  • If it’s sufficiently thick, and particularly if it’s smoke rather than haze, it adds movement and texture to the image, which helps to make sets look less fake.
  • It volumises the light, showing up clear shafts of hard light and diffuse pools of soft light. (For more on this, read 5 Tips for Perfect Shafts of Light.)
  • Backlit smoke in front of a person or an object will obscure them, concealing identity.
Heavy smoke (from an Artem) pops Lyanna (Dita Tantang) out of the background in “Ren: The Girl with the Mark” (dir. Kate Madison).
Backlit smoke through a roof of branches creates magical shafts of light in “Ren: The Girl with the Mark”.
The final day/sunset look. From each side an orange-gelled and a pink-gelled par can light the backdrop. A 2K tungsten fresnel provides backlight, while a 650W fresnel with a cucoloris provides dappled light on the tree and tarsier. An LED panel off right supplies fill, and a second panel is inside the cave with a turquoise gel.
The colour-washed infinity cove in the background of this music promo for Lewis Watson’s “Droplets” (dir. Tom Walsh) is softened and disguised by smoke.
Haze gives the LED panels their glowing appearance in this video for “X, Y & Z Rays” (dir. Tom Walsh) by Revenge of Calculon.
This torch beam in “Above the Clouds” (dir. Leon Chambers) shows up so well because the set is heavily fogged.
Smoke backlit by an HMI creates the blue background glow against which the heroes of “The First Musketeer” (dir. Harriet Sams) stand out.
Haze creates the shafts of light from HMIs outside the windows, and adds to the gothic feel of “Heretiks” (dir. Paul Hyett).
The Science of Smoke