I use the LevelsLinear filter and base it on image stats:
ImageStatistics stats = new ImageStatistics(sourceImage);
LevelsLinear levelsLinear = new LevelsLinear {
InRed = stats.Red.GetRange( 0.90 ),
InGreen = stats.Green.GetRange( 0.90 ),
InBlue = stats.Blue.GetRange( 0.90 )
};
levelsLinear.ApplyInPlace(sourceImage);
You can play with the range to tweak the result.