Epson Perfection 640U Reference Manual page 57

Gamma:
The Gamma control allows you to adjust the mid-tones in an
image. You can select from 50 (darkest) to 500 (brightest) by
moving the slider.
Highlight:
The Highlight control allows you to adjust the brightness level of
the brightest parts of an image. You can select from 61 to 490 by
moving the slider.
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 bright area. The eyedropper "picks
up" the color from that area, and uses it as the highlight for the
entire image. This effect appears immediately in the preview
image. Click Cancel to restore the original image.
Shadow:
The Shadow control allows you to adjust the darkness level of the
darkest parts of an image. You can select from 0 to 60 by moving
the slider.
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 appears immediately in the preview image.
Click Cancel to restore the original image.
Threshold:
Available only when Image Type is set to Text/Line Art. 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.
Understanding EPSON TWAIN Features
3-11