Infrared Multilayer interference filter manufacture: supposed longwave limit
J S Seeley, R Hunneman, A Whatley
Applied Optics, Vol 18, No 20, Pages 3368-3370 (1979)
The extension of multilayer technique to the longer IR wavelengths is hampered by various difficulties, the most important of which are (1) ever-increasing number of layers and manufacturing operations involved, (2) transparency in layer and substrate materials, (3) physical quality of the layers and their substrates. The filter type that most clearly illuminates these problems is the spectrophotometer lowpass filter cutting on at a particular wavelength, transmitting for one octave, but rejecting all shorter wavelengths. As the wavelength is increased, more subsidiary multilayers are necessary added to extend the rejection region since the reflective bandwidth per multilayer is less than one octave (usually). In our experience the complexity of depositing a succession of subsidiary multilayers on one surface of a substrate is unacceptable beyond about sixty layers in all.