Create a "diffusion" material, like cinema/stage filters

Hey ! Thank you very much for your answer !

You’re right, I finally succeeded to create something not that bad, thanks to Subsurface material (that’s always not really it) :

http://image.noelshack.com/fichiers/2015/46/1447256305-blingbling2.jpg

But I always have a problem with the lights. If I’m moving, the lights don’t look the same at all :

http://i.imgur.com/Wmixqdm.gif

You can also see that the exterior faces of the walls don’t have this light problem. Strange.
Here is a light plan :

http://image.noelshack.com/fichiers/2015/46/1447258601-blingblingplanlumiere.jpg

Thanks for the help !