Microtek ScanMaker 6400XL Reference Manual page 75

You can also use the Eyedropper tools to define the two end points of the
Highlight (White) and Shadow (Black). Simply move the cursor toward the
prescan image while the AIC White/Black Points dialog box is on display. Notice
that the cursor automatically transform itself into Black Eyedropper tool
(default).
The Black Eyedropper tool appears when
you move cursor toward the prescan
while AIC White/Black Points dialog box is
on display.
Use this tool to manually define your
Black (Shadow) Point setting.
Press Alt key to switch tool into White
Eyedropper and use it to assign the White
(Highlight) Point setting.
Release Alt key to revert back to Black
Eyedropper tool.
Enable this check box to
show simulated effect of
Reference: The Settings Window
the adjustment on the
prescan image
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