I would recommend getting them scanned from a scannner/printer with a black peice of paper behind it [or just a dark colour]. Letting you get as much of the pencil as you can without turing up the exposer afterward, if need be.
1. A higher exposer shows the pencil better.
2. Something that I have done before with some success.
3. An old example of mine. - https://www.newgrounds.com/art/view/soup101/ng-city