Semi-Active Laser (SAL) guidance systems were developed starting in the mid-1960's and today form an important class of precision guided weapons. The laser wavelengths generally fall in the short wave infrared region of the spectrum. Relative to passive, image based, infrared seekers the optical demands placed on the domes or windows of SAL seekers is very modest, allowing the use of low cost, easily manufactured materials, such as polycarbonate. This paper will examine the transition of SAL window and dome science and technology from the laboratory to battlefield, with special emphasis on the story of polycarbonate domes.
The story of germanium's development as an infrared dome and window material exemplifies the manner
in which the complementary efforts of industry, academia and government can work together to solve
scientific and technical challenges. Germanium is one of a few materials which have been used for infrared
transparent windows in both the long-wave 8-12 micron region, as well as the mid-wave 3-5 micron region.
Due to its legacy as one of the first commercial semiconductors, germanium became available in high
purity as a bulk commodity. This paper will examine the history of how germanium came to be used as an
infrared window and what enabling technologies have supported its use.
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 CleartranTM with the
protective coating system tested is not substantially more resistant to large grain sand erosion damage than magnesium
fluoride. ALONTM 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 ALONTM and spinel are at least as rain erosion resistant as magnesium fluoride, but the coated CleartranTM
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 ALONTM and spinel retain their robust resistance. Coated CleartranTM
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 aluminum oxynitride phase known as ALON is prepared as a ceramic with promising physical and
optical properties for infrared transparent domes and windows. It was first reported in the late nineteen fifties. James
McCauley of the Army Materials and Mechanics Research Center (AMMRC) began work in the nineteen seventies that
led to a firm foundation for understanding the phase diagram of AlN-Al2O3, which in turn led to the first transparent
ceramic ALON samples. Technology transfer from the AMMRC to Raytheon Corporation resulted in further
development of the material, which was qualified for missile dome applications. In the nineteen nineties a renaissance in
interest in ALON occurred when ALON composites were shown to have superior qualities as transparent armor.
The first decade of the twenty first century has seen Surmet Corporation acquire ALON optical ceramic technology
from Raytheon Company and transition the research and development technology to production.