Most scanners of this nature work on a reflective-light scheme. The scanner bounces light off the media and an image is generated from the "camera"
A transparency adapter passes light "through" the media. This light is what is required since determining the color and intensity of the media depends on the amount of light that is transmited and not reflected.
Think of the transparencey adpater as the cold light behind a flat panel, the media as the crystals in the matrix, and your eyes as the camera. If you tried to veiw the LCD without the cold light, you wouldnt see much ...even if you had a ton of light reflecting off the LCD screen.
BTW, thats a pretty nice scanner to use if you do end up getting the transparency adapter.
I hope thats what you were wanting to know.