Epson PERFECTION 1640 SU Reference Manual page 79

Tone curve editor:
The editor allows you to customize tone correction. Drag points
on the tone curve to adjust the tone value. You can edit up to five
tonal elements.
Output options:
If you select the check box for a specific point, such as 1/4 Tone
or Midtone, you can move that point and it remains where you
leave it; it is not affected by the movement of other points on the
curve. If the check box is cleared, the point becomes affected by
the movement of other points.
Channels options:
You can select the combined red, green, and blue (RGB) channel
to modify all colors, or select these colors separately.
Note:
If Pixel Depth is set to Black & White, 14-bit Gray, or 8-bit Gray
in the Image Type dialog box, you can only select the combined RGB
channel option.
Save/Delete/OK/Cancel/Help buttons:
After you correct tone, type a name for your custom settings in
the tone correction list and click Save.
To delete a setting, select the name of the setting you want to
delete in the tone correction list and click Delete.
To accept the defining Tone Correction or close the dialog box,
click OK.
To cancel the operation, click Cancel.
To access the online help, click Help.
Understanding EPSON TWAIN Features
3-25