It is clear that overfilters and underfilters are dual in the sense of the duality principle.1999, Edward R. Dougherty, Jaakko Astola, Nonlinear Filters for Image Processing, page 180:
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A filter that is positioned beneath something.
The paving downstream consisted of 18x18x8-inch blocks laid on edge over a brick base which in turn was underlain with a gravel or brick chip filter. Since there was some controversy as to whether an underfilter was necessary, the first test was made with the filter omited.^([sic])1967, Journal of the Institution of Engineers Pakistan:
Since there is no underfilter, the top flow line emerges on the downstream slope.1988, Árpád Kézdi, László Rétháti, Handbook of Soil Mechanics, page 61:
2 more examples
A single sheet of filter paper is used as an underfilter.1992, Margaret M. Manson, Immunochemical Protocols, page 445:
In addition, the flow from the underfilter, which had previously been clear, became cloudy, indicating that the filter had failed to block particles from the core.2004, R.F. Craig, Craig's Soil Mechanics, →ISBN, page 430:
It is not possible to overfilter your tank but marine aquaria are frequently underfiltered.1966, William P. Braker, Know how to Keep Saltwater Fishes, page 19:
You don't want to underfilter. Nor do you want to filter beyond what is economically justifiable.1978, Hydraulics and Pneumatics:
1 more example
Underfiltering by using too high a cut-off frequency leaves too much noise in the reconstructed images (bottom row).1994, Douglas Van Nostrand, Selected Atlases of Cardiovascular Nuclear Medicine, →ISBN, page 35: