Very longwavelength lowpass interference filters

The extension of metallic mesh interference filter techniques to wavelengths longer than 2000 m is discussed. The problems associated with design considerations and removal of leaks is considered in some detail. Successful filters were produced with edges at 6.3 cm^–1 and 4.3 cm^–1. When used in series and with a specific optical system, good rejection of unwanted radiation was achieved.