Fadeout on the whole frame

I try to get a fade on the whole scene, in order to get an effect who let you see the old images behind but… I can’t find how to do that.

Any help ? Should I skip to beginners ?

Blend a big quad over the whole screen. Gradually increase its alpha

many thanks !
I had already thought about that way… But I thought that there is something faster…

But I will try this!