Image Processing Reference
The NL Filter
Another filter that helps reduce noise contamination is the NL filter (NL =
nonlinear)( menu Filters > Enhance > NL Filter ).
Specifically, this filter was conceived to reduce the noise in pictures.
You have a choice of three operating modes: Alpha trimmed mean, Optimal
estimation, and Edge enhancement .
The NL filter with the result in the preview window
Alpha trimmed mean: Use this setting to reduce noise. The filter works
as a blur tool that smudges the pixels. You can get good results if you keep the
Alpha values low and set Radius larger than 0.5.
Optimal estimation: This setting applies a soft-focus effect that blurs
the pixel noise. However, its algorithms work so that clear edges don't get
smoothened. The idea behind this function is that strong contrasts and edges
are intended and probably belong in the image. If you choose the mode
Optimal estimation, the NL filter will work similar to the Selective Gaussian
Blur filter. The starting values for Alpha as well as the Radius parameter should
be set at 1.0 for our example picture. Based on this value, you can continue
adjusting the settings until you get an optimal result.