Everyone is familiar with optical filters that select a single color from a visible light beam. These can simply be colored glass pieces that transmit the desired hue and absorb all others. But light can also be filtered with a coating composed of several layers of different materials, typically certain oxides. Such multilayer coatings, which can be transmissive or reflective, are called interference filters. To select a narrow portion of the visible spectrum—to identify a material through its emitted or absorbed light band, for instance—requires narrowband filters.

Interference filters for visible light can be purchased off-the-shelf. But for applications ranging from defense to biology, scientists and technologists need to select virtually any part of the electromagnetic spectrum. One can find high-performance narrowband filters when moving from the visible to the near-IR or the near-UV. The only requirement is materials that are transparent to a wavelength of interest, with appropriately tailored layer thicknesses. Radiation then reflects at each interface of the coating, and the reflected components interfere constructively to produce the desired color.

Unfortunately, materials in nature do not behave the same way over the whole spectrum. Most materials that are transparent in the near-UV are not so in the far-UV. In fact, no material is transparent at wavelengths just below 105nm, where electrons absorb energy and transition from the valence to the conduction band.