ABSTRACT
Filters in which the primary characteristic is an abrupt change between a region of rejection and a region of transmission are known as edge filters. Edge filters are divided into two principal groups, longwave pass and shortwave pass, the designations being descriptive. Note that sometimes the terms high pass and low pass may be employed. We advise against these alternative designations because it is not always clear whether they refer to wavelength or frequency and so can be ambiguous. To the edge filters we add the notch filter that consists of a rejection region surrounded by regions of high transmission on both sides. This is sometimes termed a minus filter, and sometimes a band-stop filter, but most frequently a notch filter.
