To create a green screen, you will need either green cloth or green paper. You can even use green butcher paper from your school's workroom. The goal is to have no wrinkles and no highlights on the green--if you spend a lot of time and money setting it up, the effect will look better. The trickiest part of setting up a green screen is to light it evenly. If one part is darker, the software thinks that it is a different tone, and the green will remain in the video. I recommend either the $9 work lamp from a hardware store or the $100 soft box from Tube Tape. As long as the green is decently flat and decently lit, it will disappear.
Try to keep the actor away from the green screen. Also, make sure your actor is not wearing any green, or it will disappear, too. Once you've recorded the green screen footage, you need software that can key out (or get rid of) the green. You can remove the green on Apple computers with iMovie or on iOS devices with Green Screen Mobile Effects (free app). Record the background or find a photo, then drag the greens screen footage on top of the original clip. It's common to see teachers changing the background for tutorials or news shows. Other possibilities include changing backgrounds to a fictional place, "traveling" to a real place, or using student art. The background can be a still image, which is easier, or a video clip.
Once you show your class a few examples, they will amaze you with ways to integrate green screen clips into your classroom. I recommend showing them professional reels and student-created clips. It could also be useful to teach your students about the history of green screen technology. How else have you seen green screens used in the classroom?
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A special thank you to Bill for composing this post for my site. Congrats on getting accepted into the Google Teacher Academy - well deserved!