The default value is 170.

Valid values are 0 to 884.

The radius used is non-linear. A Radius of approximately 172 corresponds to 2/3 of the spectrum and will emulate the same range of dropout that a color scanner would generate. Larger values will drop a greater range of color, while smaller values will drop a narrower range of color.

Colors in the inner half of the radius will drop fully to white while colors in the outer half radius will shade to black.

Reference