Light Source: Using Diffusion

Light-diffusing material (just plain diffusion in professional jargon) softens directional light by scattering it. In the short version, that's all there is to tell, but selecting, mounting and fine-tuning diffusion is one of the gaffer's subtler arts. The rest of this column is the long version. We'll see how to determine the lighting look you want, match diffusion media to the lights you're using and deploy all this stuff to good effect.

Placed in front of a light source, diffusion discombobulates the directional light rays that pass through it, scattering them widely. The resulting beam appears softer and more general, with shadows lessened or even completely absent. This softer light has several advantages. It looks more natural and more like daylight or uniform overhead office lighting. It flatters subjects by diplomatically suppressing pits, crags, wrinkles and other facial blemishes. It reduces uncomfortable glare in a subject's eyes. By minimizing shadows, it can conceal the fact that a subject is lit movie-style, by two or more instruments. (Nothing gives the game away like a matched pair of key and fill light shadows behind a subject.)

In addition to softening, diffusion widens the pattern of the light beam and reduces light intensity. The broader beam lets you light a larger area per instrument, but the lower light level means you may need more units to achieve a given light level, so that's often a wash. The key point here is that diffusion will always reduce the light intensity on a subject.

Finally, diffusion reduces undesirable patterns, such as lamp filaments, the lamps themselves and the lenses in front of them. These often place noticeable shadows right in the middle of the light beam, where it's most visible to the camera. Diffusion usually solves this problem.

Why not use self-diffusing soft boxes instead and dispense with diffusion entirely? The answer is control. You can never make a soft light any harder, but you can soften a spotlight progressively until you get exactly the diffusion you want. And when you're traveling light (or cheap) a unit that works both hard and soft can do the jobs of two instruments.

As noted, soft boxes are already completely diffused and umbrellas can't usually be softened, but every other common type of lighting instrument can benefit from the soothing influence of diffusion material.

Fluorescent banks are already quite soft and they can be ultra-creamy when covered with spun glass. One of the best materials is the milk plastic panels sold for fluorescent ceiling lights. These can add significant weight, however.

Scoops are large, open, circular floods often used in studios. Because they throw soft but distinct shadows, they're usually fitted with big rings holding diffusion material.

Broads are floodlights, just big enough to hold a long halogen lamp, a half-cylinder reflector behind it and a barn door on each side. Because of their small size, broads throw obvious shadows and their beams can be unpleasantly mottled. The workhorses of location lighting, broads can be much improved by judicious diffusion.

Spotlights are small light sources throwing hard-edged, directional beams. Although you can often move the lamp back in the instrument (closer to the reflector) to soften the light a bit, this approach makes the barn doors less effective. You will probably spend much of your light-diffusing efforts on spotlights.

What will you diffuse these instruments with? As always: it depends. Milky plastic filter material creates a very soft effect, but make absolutely sure you get the strongly fire-resistant plastic specifically intended for movie and theatrical lighting. If you'll be supporting the diffusion on separate C-stands you can (carefully) use standard milk plastic 2x4 fluorescent light panels.

Spun glass is a pure-white cousin of pink insulation batting (with the same nasty habit of leaving fiberglass stickers in your hands). Supplied in large, thin, flexible sheets, it is perhaps the most versatile diffusion because you can cut it to any size, double or triple it in thickness and place it wherever you want. Like plastic filters, you can buy spun glass from theatrical supply houses. Or you can get furnace air filters at your local hardware store. One caution: the glass material will not easily burn, but it will melt.

White sheeting is useful for tenting, a special form of ultra-diffusion. Once, I even covered a picture window with a pale yellow sheet, softening the sunshine streaming in and changing the color temperature from daylight to (roughly) tungsten.

Finally, translucent white umbrellas can turn a soft umbrella setup into an ultra-soft box. Simply install the white brolly in place of the silver or gold one, reverse the unit and aim the umbrella at the subject so that the light shoots through it.