The Scan to Gray function is only intended for black and white scanning. Bitonal scans viewed at extents without this feature are mostly invisible as lines made from just a couple of pixels will not display properly on a normal monitor. Scale to Gray uses grayscale to display the image at low zooms gradually changing to bitonal as the image nears 100% zoom. This allows the operator to confidently decide that the scan is complete i.e. the title block has been captured in its entirety, without necessarily zooming right into the drawing. The default state is ON for this reason.