To be more accurate, it's called a chromakey, though green screen is used so often it's used rather synonymously. Red and blue work just as well under certain circumstances, but are usually avoided for reasons we'll get into in a moment. You can also use luminance to create this key effect, where anything over (or under) a specific brightness will be taken out. Technically this works slightly differently though, so the powers that be have given it a different name, luminance key. Nevertheless the premise is the same, and that's really what we're concerned with.

Green for the Green

Let's give a brief overview of how to use a green screen. A solid color is placed as a background and lit as evenly as possible. Your foreground subject is then placed in front of this backdrop. The rules are simple: the subject must not contain any of the background color (green), must not be hit by any green light, and must never cross the edges of the green screen backdrop. Anything that is to be rendered "invisible" must be colored and lit as close to the background as possible.

Once shooting is done, the image is taken into the edit system or green screen software, where the background color is targeted and removed, leaving a transparent area in its place. This transparency can then be substituted with whatever is desired, real or imaginary. That's the basics. It sounds easy but there's actually a good deal of both art and science behind it. For the rest of our time together we're going to concentrate specifically on green screen materials, but extensive details on all things green screen can be found at www.videomaker.com/learn

Too Green or not too Green?

Only primary colors are used for keying, because they do not contain elements of any other color. If you tried to key on purple for example, anything blue and green would be subject to removal. Green is the color most often used when dealing with people because it is not very prominent in anyone's skin. Blue can be substituted on those occasions when there is green in the subject that cannot be replaced, (like a plant). Likewise red can be used when green and blue cannot. Red is not a very popular key color though, predominantly because it is a prominent skin pigment.

There are actually specific shades of green, blue and red that have been determined to be optimal for the process. Manufacturers of chromakey products have zeroed in on these colors and created a variety of products to help you produce the best effects possible (though again, red is all but nonexistent these days). Green screen materials can consist of paint, cloth, paper or cardboard, and more, each with its own benefits and quirks. It is also common for people on a limited budget to make their own green screen backgrounds out of consumer-ready material. Take a look at what chromakey materials are out there and contrast them with some popular homemade counterparts.

Fabricating your Green Screen

Probably the go-to system when one thinks of a chromakey effect, fabric backdrops are a great balance of durability, quality and versatility. Green screen cloth is easily stretched across an adjustable frame and can be positioned or carried as needed. The right fabric offers the perfect balance of reflection without glare, which will minimize lighting headaches big time. Fabric is more durable than other options, and ages well. It can be sized and shaped simply by folding, and can be draped over objects for concealment. It is, however, prone to wrinkles, whose dark lines can be a big headache. Professional green screen fabric is often comprised of a thick cotton or cotton/muslin blend on a foam backing (think neoprene or felt). It is more resistant to creases than a generic fabric, and great care is taken to dye the fabric evenly. Most fabrics are washable, but permanent staining is always a risk.

In general use, it can be hard to prevent shifting and stretching, so it's not the best choice if you're going to be skateboarding through your key. Large fabric setups can be heavy and difficult to transport. It's also one of the most expensive selections, especially when you factor in the rigging equipment you'll require to hang it.

The obvious improvised solution is to nip out to your local fabric store. You can use anything from a green sheet or blanket to a nice custom cut swath of cotton muslin. The more you pay for your fabric, the happier you'll be with the results, and you can attach some stitched loops or grommets for hanging your green screen fabric. There will come a point however, when your time and effort will equal the cost of the professional system. It will also be harder to create larger home-brewed screens without seams.