You might even consider modeling the wall (including the opening and the bars) and then a cell behind it (kind of like an open box on its side with the opening turned toward the window). That way you could put a backlight in the box to give you an outline of the prisoner without highlighting the prisoner's face.

Or may be you could....

Or may be you could....

Or may be you could....

"That's part of the fun isn't it!" said Bobasaur with a grin.

Before they made me they broke the mold!
http://home.roadrunner.com/~kflach/