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The importance of optical applications in the middle-ultraviolet spectral region — between about 200 and 320 nm — continues to increase with the advent of new excitation light sources in this range, also identified as the UVC (200 to 280 nm) and UVB (280 to 320 nm) bands. Examples of new sources include more efficient mid-UV lasers, AlGaN LEDs and more conventional broadband sources such as high-pressure mercury and xenon arc lamps with improved efficiency and longer lifetimes.
A major impediment that continues to make the mid-UV an optically challenging spectral region is the lack of durable optical filters with adequate performance. However, optical filters constructed from thin-film coating materials that have relatively low absorption and spectral dispersion yet high reliability at these short wavelengths would have significant advantages over components such as the diffraction gratings used in monochromators and spectrophotometers.
In general, grating-based systems are not as selective and do not achieve as much throughput as filter-based systems, and only filters allow direct imaging. Until recently, optical filters for the mid-UV have exhibited poor performance in terms of transmission, spectral selectivity and durability. Optical filters made with soft-coated thin-film materials and/or multiple laminated absorbing and transparent glass substrates cannot withstand the intense energies of illumination at mid-UV wavelengths. Hybrid metal-dielectric filters also have poor optical damage thresholds, and inadequately low transmission and edge steepness.
Optical filters with good performance in the mid-UV are crucial for enabling important new instruments based on applications including biochemical absorption, fluorescence and spectroscopy, as well as for realizing the full potential of industrial applications in areas including UV sterilization, and semiconductor and electronics manufacturing.
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