Abstract

There is a need in science and technology for short- and long-wavelength cut-off filters. For the visible range of wavelengths there is a very complete range of short-wavelength cut-off filters based on absorption in coloured glasses and organic dyes. Many companies provide such filters and the properties of the filters supplied by them have been summarized and compared in the past.¹ Unfortunately, a corresponding set of long-wavelength cut-off filters does not exist for the visible and adjacent spectral regions. In the past, filters based on the interference of light in thin films have been used for this purpose. Such filters can be constructed with a cut-off at any desired wavelength, and almost any desired rejection can be achieved provided that a sufficiently large number of layers is used.² The disadvantage of these devices is that, in order to achieve a rejection over a wide range of wavelengths, a number of contiguous stacks have to be used and consequently the number of layers in the resulting system can be quite large. Another disadvantage is that the range of wavelengths over which the transmittance is high can be relatively narrow, unless special designs are used, in which case the number of layers required is even larger.³

© 1998 Optical Society of America