UV filter overview: Characteristics and applications

Fig. 3 compares the characteristics of two filters with different blocking width and depths in linear and logarithmic scale. For filter 2 the blocking is for a broader wavelength range and about 2 orders of magnitude deeper. For a wider and deeper blocking a filter design with more coating layers is needed. Accordingly, absorption as well as scattering increases at lower wavelength and the transmission gradually decreases from about 300 to 250nm. Generally, a thicker coating is accompanied by increased costs due to a longer duration of the coating process.

In Fig. 4 the feasibility of a narrow band pass filter is demonstrated, here at 248 nm. 248 nm is a typical Excimer laser wavelength. For cost-efficient applications a broadband UV light source combined with a bandpass filter at 248 nm has the potential to replace the excimer laser which is very complex, has a large footprint and high costs.

The next example shows the utilization of longpass filters in UV Raman spectroscopy. The filters must have very steep transmission edges in order to separate the low Raman signal from the strong scattered laser wavelength (Fig. 5). The transmission profiles for the longpass filters at 213, 220 and 266nm are displayed in Fig. 6. The diagram on the right demonstrates how the filter edges can be finetuned by changing the angle of incidence (AOI) on the filter.

In Fig. 7 a very demanding UV bandpass filter with passband from 270-400 nm is displayed which was employed on the Mars Rover mission in 2020 (Fig. 8). Mars samples were analyzed in the UV region with strong blocking of environmental light from the sun in the VIS to NIR region up to 1200nm. Due to the wide spectral range from about 200-1200nm the filter design is very complex and consists of about 400 layers. A tight controlled, high-precision coating process is indispensable for production of such kind of filters.

The last example in Fig. 9 shows a patterned filter which consists of 4 different regions and is used as an order sorting filter in UV spectroscopy: In grating-based spectrometers higher diffraction orders lead to artefacts in the measured spectra. These artefacts can be blocked by so called order sorting filters (OSF). Typically, such order sorting filters consist of one uncoated zone and two or three coated zones. The uncoated zone transmits the light with shortest wavelength whereas the coated zones are transmitting the longer wavelengths while blocking higher diffraction orders of shorter wavelengths. The individual transmission/filter profiles of the zones are displayed in Fig. 10. It is evident that zone 1 has a transmission in the UV-C range downto 130nm. This is realized by using crystalline CaF2 as a substrate instead of standard Fused Silica. Therefore the photolithographic structuring process was especially adapted to the sensitive nature of the CaF2 substrate material.