Selecting suitable optical material is important when designing aerospace optical systems.
Schott and Ohara offer a variety of i-line glasses with enhanced blue transmittance which are successfully utilized in space applications. Advanced melting techniques and purer raw materials are used in developing these i-line glasses, improving the internal transmittance and homogeneity values. Proper radiation filtering is necessary for the protection of the optics. These glasses are commonly used in semiconductor applications; therefore, they are melted frequently and readily available.
Another consideration when choosing optics for aerospace is radiation resistance. Exposure to high-energy radiation changes the transmittance of optical glass. Adding cerium to the composition improves the glasses’ resistance to radiation. By doing this, the transmittance edge is shifted to longer wavelengths but stabilized against radiation to a high degree. The higher amount of cerium in the composition, the greater the stabilizing effect and the more resistant the glass is too high radiation. In general, Schott radiation-resistant glasses are inquiry glasses and may not be in stock.
These radiation-hardened glasses tend to have a longer life cycle in space, but exhibit lower transmission in the blue compared to their undoped counterparts (i.e. N-BK-7 vs. BK7G18 – SCHOTT Technical Information #42). For optical systems requiring high blue transmittance, i-line glasses and Fluoride Crystals (CaF2, BaF2, MgF2) can be used with proper radiation filtering that protects the optical elements. However, for optical systems that are going to be exposed to high radiation over long time periods, cerium stabilized glasses are the preferred solution. Popular materials are listed in the chart below.
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- Fused Silica
- Barium Fluoride
- Calcium Fluoride
- Magnesium Fluoride
- Germanium **
- ZnS Multispectral / Cleartran **
- *HT: High Transmittance
**With proper filtering to block UV radiation, IR crystals can be used