Colm Boran , Dec 21, 1999; 09:59 p.m.
I don't own an HP scanner, but in all likelihood, the $99 transparency adapter is not passive. It probably has it's own light source and shines light down through the slide onto the CCD below. It should be able to throw more light through the slide than the passive one which is only reflecting light from below. This, in turn, should reduce the digital noise in the shadow areas of the image.