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Light Source: The Illusion of Diffusion

Without diffusion, our movie talent would look old and rough, our shiny cars would reflect intense lights and our sets would fill up with hard splotches of light, with little blending. Without diffusion, our world would light up like a room with a bare light bulb hanging from the ceiling not a pretty sight. So how do you create the world you see on your television and in movie theaters, in which every actor has skin as smooth as silk and the cars all look like they came off the showroom floor? It is all part of the illusion of diffusion, the lighting designer's greatest weapon against old age, rough skin and shiny objects. In this column we will look at the various types of diffusion available to videographers and describe many different setups for using diffusion.

An advantage that you have when you are working with light is its tendency to travel in a straight line. Shine it on a reflector at an angle and it will bounce off at an exact complementary angle. While this characteristic of light is usually good, it does have a tendency to make light coming from an extremely bright bulb very hard. By hard, I mean that the shadow it casts has a very defined edge between the shadow and the light. This sharp edge is called the shadow edge transfer. This area of transition between the lit area and the shadow area defines whether a light is hard or soft. The sharper the edge between the two areas, the harder the light.

Keep in mind, shadows are not a bad thing. They give your on-screen images a three-dimensional look. However, shadows also show the texture of a surface. If your talent is aging and has lines on his face, the shadows cast by the texture of the wrinkles will make his skin appear very rough and old. Any imperfections will be highlighted. While this is great if you are doing an interview with an old cowboy with lots of character in his face, your leading lady will not be too thrilled when the new wrinkle she found that morning is seen in all its glory due to your hard light setup.

Diffusion material takes a beam of light and spreads it out by bending the light beam through the diffusion material. The more dense the diffusion material, the wider the spread of light. The actress's face would look years younger under diffused light because the light would spread more evenly over her face and not cast the shadow of her newly found wrinkle.

Let's look at an example that you can see without setting up lights. If you walk outside on an overcast day, do you see your shadow? Probably not. The clouds act as a huge diffuser of the sun. As the clouds thin, you begin seeing soft shadows and as the clouds disappear, you will see well-defined shadows. You can easily duplicate this effect in the studio with a variety of diffusion materials and a little bit of practice.

There are many tools you can use to diffuse light. The easiest and least time-consuming are scrims, gels and frosts.

A scrim is a wire mesh screening that is placed over the front of a light to reduce the intensity of the light and slightly diffuse it without changing its color temperature. These mesh screens can cover the entire light or, as half-scrims, cover only half of the light source. Scrims are very handy when you are trying to get two light intensities from one source. By placing a half-scrim on a light and rotating the scrim to the top, you can light your subject with the bright light from the bottom of the instrument and reduce the light on the background from the top half of the light. One word of caution: metal scrims get very hot, handle them with care.