The testing reported in this paper operationalized the material requirement: An infrared transparent dome material
must be at least as good as magnesium fluoride in rain tests and substantially better than magnesium fluoride in sand
tests. Sand erosion test conclusions, based on changes in midwave infrared transmission, are that Cleartran<sup>TM</sup> with the
protective coating system tested is not substantially more resistant to large grain sand erosion damage than magnesium
fluoride. ALON<sup>TM</sup> and spinel are substantially more resistant to large grain sand erosion damage than magnesium
fluoride. There is no significant transmission difference due to small grain sand erosion observed between any of the
tested coupons. Qualitative analysis of coupon damage after exposure to an artificial rain field on a whirling arm
showed that ALON<sup>TM</sup> and spinel are at least as rain erosion resistant as magnesium fluoride, but the coated Cleartran<sup>TM</sup>
coupons delaminated rapidly under the same rain test conditions. Testing coupons exposed sequentially to the milder
sand condition followed by the whirling arm rain erosion test demonstrated that magnesium fluoride rain resistance is
diminished in the combined test, but that ALON<sup>TM</sup> and spinel retain their robust resistance. Coated Cleartran<sup>TM</sup>
delaminated under the combined conditions as well. It is noteworthy that the results reported for the midwave infrared
range also apply to the near infrared region above 1 micron.
The dome or window on a sensor suite seems, at first glance, to be a relatively low tech item. In reality, it can be one of the most costly items in the system. The choice of materials is highly dependent on the sensor, the anticipated operating conditions, and other requirements such as electromagnetic interference or radar cross section issues. The situation is further complicated when multiple sensor bands are used. Some dome materials are suitable for visible or near infrared applications, some for midwave infrared applications, and others for long wave infrared applications. Materials are also available which can be used for dual band sensors such as visible and midwave sensors.
The Army is currently developing a tri-mode seeker containing semi-active laser, midwave infrared, and millimeter sensors all using a common aperture. This added complexity is nowhere more apparent than in the missile dome. Optically transparent infrared optical materials tend to have relatively high dielectric constants. Millimeter wave radomes typically have low dielectric constants. Electromagnetic shielding for an optical seeker frequently consists of some type of grid that serves as a wideband filter. Electromagnetic shielding for millimeter wave missiles may use complicated frequency selective surfaces that block all but the frequency of interest. Unfortunately, those frequency selective surfaces tend to be predominantly metal and are opaque in the optical regime.
This paper will discuss the unique requirements that are placed on a tri-mode seeker as well as efforts to meet those requirements.