What is chromakey? We've all seen it performed a million
times over the years when the TV meteorologist stands in
front of the weather map on the nightly news.

This is done by electronically selecting a color (or
chrominance value) in the scene and then making it
transparent (or "keying" it out) to allow a background to
show through. Keying is not limited to chromakeying
(based on a color value), but can also relate to brightness
(luminance).
Chromakey Defined
Here are the 2 most common problems when using a Green or Blue
Screen backdrop:

1. Uneven backdrop lighting
2. Reflected spill onto the foreground subject.

Here is how we recommend you set up your lighting.

Step 1 Light The Chromakey Backdrop:

When lighting a chromakey backdrop, you want the lighting on the
backdrop to be as evenly as possible, with no shadows.Your
backdrop lights should be behind the foreground subject.

Step 2 Light The Foreground Subject:

You will want to light the subject with its own set of lights. You want
as much separation between the foreground subject and the
chromakey backdrop as possible.The more separation you have
between the foreground subject and the chromakey backdrop, the
better.This reduces the chance of the foreground shadows hitting
the chromakey backdrop. It will help minimizes the "blue or green
fringe" from light that is reflected off the chromakey backdrop.
(called reflective spill)

Step 3 Neutralize Any Color-Cast

To neutralize any color-cast from light reflected off the backing. I
use a soft back-light from each side to light up each side of the
foreground subject, helping to neutralizing the reflected spill from
the chromakey backdrop.Note If you take the time to light your
foreground and chromakey backdrop correctly. No "blue or green
fringe" on the object or the persons clothing, hair or skin, then even
the most basic chromakey hardware or software will give you good
results.

Rules For A Good Key

Make sure your lighting is even and well balanced on the
chromakey backdropShoot with the iris of your camera at full open
Keep the subject as far forward of the chromakey backdrop as
possibleLight the subject separatelyPrevent shadows from hitting
the chromakey backdropMake sure the subject does not contain
the same color as your chromakey backdrop!
Green Screen Equipment & Lighting Tips
GreenScreen Techniques
Chroma Key / GreenScreen Software
Using PhotoKey - Photography / Magazines