In addition to the familiar optical philtres for modifying spectral energy distribution, and thereby colour rendition, optical philtres are also produced as low-pass philtres for spatial detail in an optical image, eliminating high-frequency information that would exceed the Nyquist limit of the system and produce excessive aliasing. Many of these philtres are cut from optically birefringent crystals and function by providing multiple images slightly displaced one form another so that fine detail is blurred (i.e., low-pass filtered).