In ScanGear (scanner driver)'s Advanced Mode tab, click 
(Tone Curve Settings).
NoteClick Defaults to reset all adjustments in the current window.
Each dot of an image is a mixture of Red, Green and Blue in various proportions (gradation). These colors can be adjusted individually as a "channel."
Master
Adjust the Red, Green and Blue combined.
Red
Adjust the Red channel.
Green
Adjust the Green channel.
Blue
Adjust the Blue channel.
NoteOnly Grayscale will be displayed in Channel when Color Mode is Grayscale.
With ScanGear, scanning images via a scanner is the input, and displaying to a monitor is the output. "Tone Curve" shows the balance of tone input and output for each Channel.
In Select Tone Curve, select a tone curve from No correction, Overexposure, Underexposure, High contrast, Reverse the negative/positive image and Edit custom curve.
The midtone data of the input side is stretched toward the highlight of the output side, resulting in a bright-toned image when viewed on a monitor.
The midtone data of the input side is stretched toward the shadow of the output side, resulting in a dark-toned image when viewed on a monitor.
The highlight and shadow of the input side are enhanced, resulting in a high-contrast image.
The input and output sides are reversed, resulting in a negative-positive inverted image.
You can drag specific points on the Tone Curve to freely adjust the brightness of the corresponding areas.
