Epson PERFECTION 1640 SU Reference Manual page 86

Save Preview Image and Settings:
Select this check box to automatically save the preview image and
all the image settings when you exit EPSON TWAIN (quit
scanning). The next time you restart EPSON TWAIN, the image
and settings will reappear.
The Image Type, Destination, Tone curve settings you made are
automatically saved whether this check box is on or off.
Fast Preview:
Enables high-speed loading of the preview image. If this is
selected, the preview image is scanned in the Draft mode. If this
is not selected, the preview image is scanned in the Best mode,
and you can get accurate values for the Highlight, Shadow, and
Gray Balance Intensity by sampling with the eyedropper in the
preview image.
Do not use this feature (do not select the check box) when quality
is more important than speed.
Automatic Preview:
Preview automatically starts when EPSON TWAIN is launched.
Auto Exposure on Preview:
Displays the preview image with automatically optimized
exposure settings. The result is the same as if you clicked the Auto
Exposure button.
Note:
This check box appears dimmed and is not available when the
ICM/sRGB check box is selected.
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Understanding EPSON TWAIN Features