From AMS Glossary
- Any process by which the Fourier components of a temporally or spatially varying output are different in amplitude or phase from the corresponding components of an input.
Cellophane is a (temporal) filter: The amplitudes of the spectral components of white light (input) incident on the cellophane are different from those of the light it transmits (output). A thermometer is a low-pass filter; all components of air temperature are fed into the thermometer but it passes only the low-frequency components and suppresses the high-frequency components. Optical devices of all kinds are also spatial filters: They may transmit each spatial component of a pattern differently.