The background is now ready set, your cast is in color appropriate wardrobe but it’s not quite time to roll camera. Like any set your ‘green’ screen needs lit, though unlike a normal set, you can’t get away with much less than ideal lighting.
The light on the background, in this case the green screen, needs to be as even as possible. When setting your lights, the aim is to reduce shadows and remove hot spots on the screen. Hot spots occur when an area of the screen is receiving a greater concentration of light than another area. Shadows most often occur when the actor or subject throws a shadow on the background due to one or more of the lights. Will there be much movement by the actors? If so, do a walk through before hand to look for potential issues of strong shadows.
Being able to adjust the intensity of individual lights helps to reduce hot spots and shadows, as does diffusing the light with frosted gels, softbox lights, white cloth or even layers of white tissue paper. Of course, the biggest contributor to reducing hot spots and shadows is light placement.
I typically use a 4-5 light set up. When using 4 lights, I have 2 to either side of the screen near to the screen itself and heavily diffused. These back lights are angled so that they not only evenly light the screen, but also provide a back/rim light to help separate the subject from the background. The idea is that outside of a few inches at the green screen edge that are hot, the remainder of the screen is evenly lit. The other 2 lights are placed much farther back from the screen, with the primary/Key running the hottest and the secondary/Fill as a fill for the subjects shadowed side. These lights need to be angled so that they do not throw shadows directly behind the subject. The shadows that are cast should fall to the left or right of the subject whenever possible and should not be too harsh. The fifth light, when used, is a top down light from above and slightly behind the subject works as a hair light, though I typically only use this if the scene the characters are going into would have light falling on them from above.
Just as important as attempting to match the lighting of the scene the subject will be placed into, is keeping the lighting even. However, this is not always possible. When keying, I have typically found in post that it is easier to key out slight to moderate shadows than it is to remove highlights (hot spots). Also, when in post, opacity masks or garbage mattes can help to remove trouble areas from your key (more on this in future posts).
And lastly, on the subject of lighting, have a shot list and preferably a storyboard as well. To keep consistent light from scene to scene, it can be very difficult to maintain the direction, for example, the sun is coming from if you do not have the storyboard to help you keep that consistent. The shot list means you don’t have to necessarily make more than a few changes to your lighting, by keeping similarly lite shots grouped on the shot list.
Remember, you don’t have to have the most expensive lights, particularly when it comes to your screen back lights. The main thing where they are concerned is giving an even light. Color temperature is not paramount with the back/screen lights. I use a pair of $20 work lights with layers of white tissue paper for my screen lights, and reserve my LED’s for lighting the subject.