Hello.
Maybe I am on the wrong place, I am not a photographer, but I am searching for really large photo transparencies (square meters) which can be used with light behind them, appearing a real vision, like looking through a window.
How are they made and what materials and techniques are used?
Thanks
Noel

Go to a sign maker ask them for materials suitable to back lighting, if you don't need absolute transparency. ("...which can be used with light behind them..." makes me think you don't need absolute transparency...) You'll find plenty products/solutions and services suitable to your needs.

Sounds like duratran material. http://www.duratran.com/