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.