Interference filters for the far infrared.

Perforated metal plates (grids) of various kinds are used for the construction of transmission filters for the far ir. Examples are given of filters with low pass, high pass, bandpass, and bandstop characteristics with steep slopes. These filters are the optical equivalents of microwave waveguide filters. They can be designed by the same theoretical procedures as those to meet a wide variety of different specifications, at least in principle. Actually, losses and constructional tolerances limit the performance. Measurements at oblique incidence indicate that the filters will find useful application in light pipes too. Furthermore, they may prove advantageous in short millimeter wave systems that employ optical techniques.