These are similar to the opaque projectors, except that you can place your material (textbook or diagram) directly on top of the screen for projection to a screen.  They are fixed to the desktop. 

The Elmo presenters are connected to a Kramer switch and provide three different ways of presenting: through the glass, on to the glass and from the glass.  You can also use transparencies and 3D objects  much like the Avery portable document cameras.  The difference is that Elmo Video Presenters are fixed to our computer system.