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Building a Home Video Studio

A 12 part series of articles on the making of aviaTV.com

by Darren Smith, CFII/MEI

Green Screen (aka Backdrops, Part II)

Chroma key compositing (aka chroma keying aka green screen) is a technique for compositing two images or frames together in which a color (in this case green) is removed, revealing another image behind it. This technique is also referred to as color keying and often uses a particular shade of green or blue. These colors are the furthest away from skin tone which regardless of race, are different contrasts of pink. According to Wikipedia, green is currently used as a backdrop more than any other color because image sensors in digital video cameras are most sensitive to green. The magic happens in the video editing software as it searches for the particular shade of green and replaces it with the desired background.

It is commonly used for weather forecast broadcasts on your local news (picture right). The weather man looks like he is standing in front of a large map in the studio. Actually its a big wall painted blue or green.

The biggest challenge when setting up a green screen is even lighting. You have to avoid any chance of a shadow because its a darker color to the camera and might not register. You must have as narrow a color range as possible in chroma keying. To the right, you can see those shadows and how they appear to be darker shades of green. It's something that must be avoided. Professional green screens have special lights called kino flo lights which give the green color a bump so as to remove any of the other parts of the visual spectrum.

I decided to make my own "kinoflow" lights using workshop lights from home depot and using a very high color temperature light bulb. The higher the color temperature, the less red spectrum and these babies (pictured right) are 6500 kelvin - the whitest (almost blue) bulbs commercially available. I purchased some bulb protectors from B&H Photo (NYC) in Pea Green and installed both into the shop light. These are mounted vertically on a standard studio light stand. To see more about this, take a look at the page on Lighting to see how I made these home made studio lights. Cost to make the green screen backdrop lights was $32 each plus $20 (set of 2) for the green tinted bulb protectors from B&H Photo. Total on lights $104

Then it was on to creating the backdrop. I had to create a green wall also but not do any semi-permanent damage that a painted wall might offer. So I repeated the same trick from the Backdrops page and purchased a 6 foot by 9 foot canvas drop cloth from home depot. I bought the heaviest fabric weight they had available. Cost of the canvas drop cloth, $10.

When you unpackage these drop clothes the first thing you notice is how wrinkled they are. So just like in the white one I made about a month ago, I took the clothes iron outside and ironed the whole thing on a table covered with a blanket. After ironing, I spread it out on a huge tarp. I put two coats of white primer (1 gallon $15) to give my green paint the best chance of adhering to the cloth. I'm starting to go through quite a few of the roller brushes, so add $8 for a set of 3 roller brushes to put the primer and paint on the canvas backdrop.

The whole reason I've been using canvas for the backdrops is because it's a relatively smooth surface which can be manipulated to produce the results I'm looking for. I couldn't use the textured wall as a green screen surface because it wasn't perfectly smooth. Next, it's time to paint the ready canvas it's final color.

This paragraph contains the secret of low cost, high quality green screen. Here it is... this is the big secret. I used Behr interior flat latex paint. The custom tinted paint is a stock color at your home depot called Sparkling Apple. There was nothing more special to it than this. This particular shade of green seems to be the most green you can get. And that's what it takes for editing computer to do its magic. It goes after that "super green" and replaces it with other video.

	Total cost for the magical "super green" paint was $20. Sure you can go online and find a more expensive brew, but I'm on a budget. As you can see from the label on the left, its not returnable, so be sure you really wanna do this. I had some grommets left over from the sand bag project, so I installed one in each corner. As you can see from the final product (below), I have a green screen for a fraction of the cost the professionals pay.
Before		After

Wall anchors & screws to mount the canvas drop cloth to the wall cost me about $2

Total cost for this phase of the project: $159 lights & green backdrop

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