The Dark Knight Rises Real Bat-Signal?

You wouldn't. You'd get a spotlight with maybe a dark blur in the middle. A projector is more like what the signal would need to be.

 

Yeah, they show in The Dark Knight what it would look like, a mess, which is why Gordon was able to pass it off as a "defective piece of equipment".

 

They attempted to use one for real if I remember Nolan saying somewhere, and it just didnt work.

 

It actually looks much better in the movie than it would in real life even though it's messy.

 

It would want to for a movie. Realism or not.

 

You could probably get a sharper bat symbol in the sky if you used lots of laserspointers instead of one large light though, the reason it's so fuzzy is because the light spreads, with a bunch of lasers you'd get a lot more focus I think. Then again unless you have something to project it on it'll probably be fuzzy anyway so...

Still, when Batman sees the big spotlight on the police HQ's roof shining I doubt he cares wether it's projecting a bat, cat or a mermaid, he'll get the jist of it...