Kind of like a fill light but much more subtle, a bias light illuminates the subject just shy of perception, but when combined with a transient source such as a practical fire light, the image appears to be entirely illuminated by the practical fire light, with all its gradations and dynamics.
If you think in terms of characteristic curves of a film or sensor, a bias light brings the subject level up to the bottom of the linear portion of the curve, or just to the top of the toe. Any additional light, such as from a practical fire source, catapults the exposure into the linear portion of the curve with its resultant steeper rise of response in relation to exposure.
I learned this trick from the late Frank Stanley, ASC. He taught a workshop at the International Film Workshops in Maine (now the Maine Media Workshops+College). Frank was demonstrating the ubiquitous night exterior cigarette lighting, lighting setup. He demonstrated the bias light by positioning a small soft light just out of frame and in-line with the pending match light. He adjusted the light level so that it’s effect was barely visible as it played upon the actor on his final mark. However, when our actor emerged from a dark and foggy alley, strode to his mark and lit up a cigarette, his face was illuminated in a vibrant flare of match fire.
I had the opportunity to resurrect this technique on a recent shoot. I needed to capture an actor performing with fire poi. In order to get the fire balls to pop on camera and also illuminate her face, I parked my truck about fifty feet away and hit her with the high beams. While barely noticeable in the image, the headlights augmented the existing fire light just enough to achieve full exposure and create the illusion that her face is lit solely by the poi balls.
Take a look at this snippet.