"Hard" IBS (Ion-Beam Sputter) Coating Technology

Conventional narrowband interference filters are fabricated in two steps - one to determine the center wavelength and bandwidth of the filter, and a second step to determine its blocking capabilities. The first step is created by depositing up to 50 layers of quarter-wave optical thickness (QWOT) and halfwave optical thickness (HWOT) dielectric materials of alternating high and low refractive indicies. The bandwidth of the filter is determined by the ratio of high and low index materials, the number of layers, and the number of "cavities" created by HWOT layers and adjacent stacks. On an alternate substrate, metallic film layers are deposited for broad-spectrum blocking. Finally, these two sections are scribed, laminated, cut, and mounted. Additional colored glass materials may be inserted between the two layers for further blocking.

This conventional method leads to three fundamental problems. First, the colored glass and metallic film blocking layers reduce transmission at all wavelengths, including the design passband wavelengths, limiting the overall signal through the filter. Typical transmissions range from 25 to 50%, depending on center wavelength. Second, the construction of the filters (several layers of glass, several layers of epoxy, and dielectric materials) is quite complicated, making it difficult to use these filters in imaging applications. With so many layers of different materials, accurate ray-tracing through the filter is impossible, and repeatable construction of the filters is difficult and costly. Loose tolerancing on the overall thickness and on the center wavelength is commonplace. Finally, because the epoxy and often the coating materials themselves have poor mechanical and optical durability and are prone to humidity-induced swelling and degradation, the overall reliability of the filters is limited.