http://tips.newtek.com/using-your-smartphone-to-measure-green-screen-lighting/
Regardless what kind of green screen setup you’re using or how you’re lighting it, you need to ensure that your screen is evenly-lit from the camera’s perspective. That means the screen behind your subject is all the same color and brightness. This can be harder than you may think, especially using a pop-up green screen and portable lights. But thankfully, with the advent of portable devices like smartphones and tablets, developers have been busy making useful tools to help you get a fantastic green screen lit, even before you set up your camera!
