Chroma Key Lighting Overview

General Concerns
(1) Use soft lighting and lighting instruments that produce or have a wide spectral range; the instruments should produce soft light evenly, over a large area.

(2) Any light striking the chroma key should be even and less bright than the light striking your foreground objects or actors.

(3) High light as needed, using key, side, back, or kicker lights.

(4) The Inverse Square Law of Light is very important when lighting any setup, but especially with chroma keys.

We’ll first use the foot-candle as a unit of measurement. (One foot-candle is the amount of light given off by one candle from a foot away.) One hundred foot-candles of light at 20 feet from the light source will be four hundred at 10 feet from the light source, or 25 foot-candles of light from 40 feet away from the light source. (See the Chroma Key Link section for the formula.)

What this law does is help the lighting person to control lighting ratios. Using the foot-candle example above, a front light source (key light(s) that produces 100 foot-candles of light on the talent because it’s 10 feet in front of the talent, will cast 25 foot-candles of light on the key if there is another 10 feet of distance between the key and the backside of the talent. The ratio is a 4:1 because 100 (foot-candles) divided by 25 (foot-candles) is 4. (See diagram # ___________). In other words, there is four times as much light on the talent as on the key, and that's good. The talent should be brighter than the key. The usual ratio is either 4:1 or 3:1, depending upon the talent’s skin tone and clothing brightness.

Many directors of photography and lighting directors measure light in f-stops. (For a detailed explanation of the f-stop, see the link entitled, “A Tedious Explanation of the f/stop,” and pay special attention to the first and last columns in the table.) The point is this. If the director asks for an f 5.6 on the talent because it gives the depth-of-field that’s needed, and you as the lighting director put the amount of light on the talent that produces the best look for that f-stop requirement, the chroma key should meter at f-2.8, which is a 3:1 lighting ratio. (The camera would need to open up to f-2.8 to get a good image if the camera’s director, theoretically, wanted to shoot the green key by itself, but the main object is the talent at f-5.6 with more light. The camera is mainly set at 5.6 for the talent. Setting the camera at 2.8 would cause the talent to be overly bright by that 3:1 ratio.

(5) Depending upon the flesh tone (or reflective ness) of your talent or object in the key, the more or less light you may need on them.
Remember that there is give and take with most things, especially chroma key lighting. If your director requests low key lighting with no backlight, additional software might be needed in post-production editing sessions.