LIGHTING A CHROMA KEY (BLUE OR GREEN SCREEN)
Green / Blue screen is a very fascinating element for all kind of media productions because it opens the way to a wide imagination and the creation of impossible shots. The typical misconception is that greenscreen shots must be flat lit. This misunderstanding arises from the need to light the background evenly. However, the foreground should be lit very carefully in a manner that will match the composited background. In fact, inconsistency between foreground and background lighting is a major visual cue that will give away a composited greescreen shot.
There are two lighting setups for a chroma: one for the background and one for the subject in the foreground. Both setups are critical to the effectiveness of the final product – the background lighting for a successful composite and the foreground lighting to sell the integration of the subject with the composited background. Both composited background and foreground need to use the same lighting angles and have the same perspective. If either one is off, no matter how clean the key is, the viewer will feel that something doesn’t belong to the scene.
Many chroma-key composites are intended to look fake and not simulate reality. Weatherman-style shots, in which informational graphics appear behind the host, really can be lit any way you want. However, for realistic shots, you need to know what the composited background will be and how the lights fall in it. Attention to detail is what makes the shot believable.
Is it green or blue better? The real issue is what color you need to have in the foreground. If the subject needs to wear blue shirt, use green. If green needs to appear in the foreground, use blue. When all else fails, use any other color that isn’t needed in the foreground. Blue and green are the favorites because they do not appear in human skin tones, a color you can’t change very easily. However, virtually any color can be used, including red and yellow.
Lighting the background
By far, the first step in successfully lighting a Chroma Key project is the most important one. Lighting the background properly can save you hours of needless frustration and problems.
You can take one or two distinct courses when lighting the background. The one you pick will depend on the keying hardware/software you use. The difference is spill-reflected green or blue light from the background that appears on the foreground subject. This is the great problem of a simple chromakey, and it must be reduced on the set through lighting techniques.
When an actor stands in front of a large, intensely lit bluescreen or greenscreen, two factors come into play to create spill. One is radiosity, the green or blue light that radiates off the screen onto nearby objects or people. The second is reflection on the skin or other surfaces from the screen because it acts as a large reflecting card.
Both of these problems are minimized by the application of the inverse square law – create come distance between the subject and the screen! This is a good idea generally because it is easier to light the foreground subject when there is some distance between subject and background. Radiosity drops off rapidly and is usually completely insignificant on a subject 8 or 10 feet away from the background. Reflections aren’t as affected by distance, however, so other methods must be used. A very simple technique is to light the background at a lower level than the foreground. Remember that the more lumens you put on the background, the more the color will reflect on the subject’s face, hair, and clothing. Many people resist this suggestion because they’ve always heard that greenscreens must be lit brightly. This is a simple not so. The hardware (or software) can typically key out any color and range you specify and can remove dark green as easily as bright green. For classic chromakey usage, if you’re using a light meter, the light on the background should be about half that you use as key on the foreground.
Although intense bright lighting is not necessary, even lighting is. The more even the lighting on the background, the less latitude is needed in the software while pulling the matte. The less latitude needed, the less likely you are to encounter problems with blue halos or vanishing ears. The theoretical ideal key would have a perfectly even color in the background, with none of that color appearing in the foreground. The closer you can get to this ideal through careful, even lighting, the better the end result will be. This is where fluorescent instruments really excel and beat incandescents hands down every time.
Typically, you’ll still have some spill to deal with, usually on shoulders and hair. The classic approach, and one that works when set up right, is to use a backlight gelled to a color opposite that used for the background. For a blue background, use straw; for green, use magenta. Doing this just right is a bit of an art because, to the eye, the colored light is quite noticeable. However, on-camera, the right setup of intensity and color virtually eliminates the spill without seeming magenta or straw. It’s best to watch the effect of the colored backlight on-camera rather than with the eye.
Lighting the Foreground
A number of problems crop up when lighting a green- or blue –screen shot. Set objects can cause particular problems, especially if they have shiny surfaces. A table with a varnished top can reflect the green background so that the top literally vanishes in the composite, leaving teacups and saucers floating in air. Even many tablecloths have a surface that is shiny enough to cause a problem, although usually to a lesser degree. Shiny objects of any sort can cause this problem. When possible, use very flat finishes for bluescreen shots.
Often, green invisible set pieces are included, especially when using the newer virtual sets. A plywood cube of the proper height painted green lets the actor set objects down on a virtual surface. However, these set pieces present peculiar problems because they are difficult to light evenly, and they are in the foreground zone, where you might be using a contrasty lighting scheme. The top will catch more light, and the object will cast shadows that must be eliminated either live or on post. One solution is to use a darker shade of the key color on the top of the set piece than on the sides. Another is to use traditional lighting controls – flags or scrims – to control the amount of light on the top of the set piece.
When the blue floor must be visible (or rather invisible in the composite), it must be lit carefully to preserve the even color, while interfering as little as possible with the lighting of the subject. It is especially important that lights not be pointed straight down or angled toward the camera from rear position because it will create specular highlights on the floor that will appear in the composite as veiling – blue or green-tinted areas. Lights for the floor should be nearer the camera so that light is bounced back to the camera, without the hot specular highlights.
In some cases, backlighting the subject is essential and will create those annoying hot spots on the floor. When this situation is inescapable, a polarizing filter on the camera can eliminate the glare from the floor. A similar trick that is used sometimes in film but works just fine in video as well is the use of a beamsplitter and a glass-beaded reflective material. A half-silvered mirror is mounted at 45º angle in front of the camera lens, and a green or blue light is reflected from it toward the reflective material. The end result is quite good.
A large studio with a hard cyc light is not necessary for basic green screen work. Ordinary fluorescent worklights (with electronic ballast) illuminate the background, and the subject is illuminated with a softbox and a Fresnel as backlight.
These are just a few of the most common specialty lighting situations you’ll run into. In point of fact, every setup is a specialty situation. You should regard each setup as a new and unique challenge. Try different looks and new solutions all the time. Although you’ll find basic solutions that work well, you need to avoid falling into the rut of lighting every interview exactly the same way and every product just like the one before.