Epson Expression 1680 - Manual (Page 12)

1-4 Optimizing Scanning
2. Click the eyedropper button under Shadow.
The pointer changes to an eyedropper, with movement
restricted to within the Preview window.
3. Move the eyedropper to the location you want to select as the
shadow point, and then click.
The brightness of the pixel at the point you click is set as the
shadow level, and the other parts of the image are adjusted
accordingly.
4. To change the shadow level of the point you selected, move
the Shadow slider left or right, or enter a value between 0 and
60 in the text box.