The ELMO document camera has become an indispensible teaching tool in my room. Image quality is spectacular, even when showing student algebraic solutions lightly written in pencil.
I also use a pen tablet whenever I'm using the document camera. I'll often have students hand me their classwork then go up to the board and talk through their solutions to the rest of the class. I then add supportive colored coded commentary and illustrations to an electronic overlay of that students classwork during their presentation.
Combining the document camera with the pen tablet facilitates rapid turnaround of sample student work illustrating both correct solutions, and the often more helpful, incorrect solutions that the class then identifies and indicates the necessary revisions.
