Epson PERFECTION 1640 SU Reference Manual page 76

Exposure:
The Exposure setting lightens or darkens an image. You can select
from -10 (darkest) to 20 (brightest) by moving (clicking and
dragging) the slider.
The Exposure and Highlight controls are related. When the
Exposure setting is changed, the Highlight value is automatically
set.
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 shows up 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.
3-22
Understanding EPSON TWAIN Features