Lighting
The target lighting level for a blue or green screen varies slightly with respect to the background plate that needs to be matched, but in general should be around .70 of the full range of the green channel. The red and blue channels, of course, should be as low as possible.
The above image was taken of a digital green background with standard light on a 4:2:2 camera. As can be seen, the RGB values at the sampling point are:
- R: 0.46
- G: 0.70
- B: 0.25
These are fairly typical values that would be seen. Obviously, the separation between the red and green channels is less than desired, but the above lighting can still generate a functional key.
The above image was taken with the same 4:2:2 camera and green background, but was lit by gelled lights (gel: Lee 738 JAS Green) to restrict the spectrum of light falling on the background. As can be seen in the image, the separation between green, red, and blue channels is much, much better:
- R: 0.28
- G: 0.70
- B: 0.20
This would give a fantastic key, with excellent separation between foreground and background, and very little matte cleanup needed.
Fixtures
Gelling lights works, but reduces the light output significantly. A better way to get colored light in larger spaces is to use the special Kino-Flo bulbs.
The bulbs themselves are quite inexpensive, about $20 for a 4’ safety coated green lamp. To avoid flicker, fluorescent bulbs must be used with electronic ballasts, which pulse the lighting at a frequency of several kilohertz, far above the speed of camera. Older magnetically ballasted fixtures operate at the same rate as the power in the wall (60Hz) and will cause the image to flicker.
The matching Kino-Flo fluorescent fixtures have electronic ballasting, and are rugged for heavy stage use. However, if the fixtures are going to see relatively gentle use, standard hardware store fluorescent T12 fixtures with electronic ballasts can be used, like the $30 Lithonia 1241DP.
Blue Screen Lighting
Green screen lighting is much more forgiving, and this is one of the reasons why green screens are so much more common than blue screens at present. The despill operation on a green subject resolves to a grayish fringe, which can be easily retinted to blend into a given background.
The despill operation on a blue screen subject resolves to a harsh magenta edge, and this is extremely difficult to remove, and explains a lot of the ugly dark lines around older composite images. Motion blur is also a problem with blue screens with inadequately separated light; this will show up as magenta streaks whenever someone moves quickly in the scene.
The solution is generally to light blue screens with gelled lights (Lee 079 Just Blue), or color matched Kino-Flo fluorescent bulbs. When lit with this type of lighting, Digital Blue screens will provide color channel differences similar to that of the lit green screen to the left, and will not have the magenta artifacts mentioned above.