Tone Correction - Epson PERFECTION 1640 SU Reference Manual

Click the eyedropper button beneath the slider, and the mouse
pointer changes to an eyedropper. Move the eyedropper over the
preview image, and click a dark area. The eyedropper "picks up"
the color from that area, and uses it as the shadow for the entire
image. This effect shows up immediately in the preview image.
Click Cancel to restore the original image.
Threshold:
Available only when Pixel Depth is set to Black & White and
Halftone is set to None in the Image Type dialog box. The scanner
scans gray shades as either black or white using this setting.
You can select from 0 (lightest) to 255 (darkest) by moving the
slider.

Tone Correction

Understanding EPSON TWAIN Features
3-23