Thorium fluoride ThF4 has been used over the last several decades as a low index coating material for many optical applications such as in anti-reflection (AR) coatings, high reflective coating, beam splitters, coating dichroics and optical filters. ThF4 has excellent optical and mechanical properties but unfortunately is slightly radioactive. New guidelines from many government agencies require that ThF4 shall not be used as coating material for safety reasons. Also new stringent requirements on the disposal of radioactive material in many states within the USA have made it economically difficult to use. This paper therefore deals with AR coatings which do not have radioactive materials in them. Three types of coatings based on applications have been discussed in this paper. These are namely (1) AR coatings for long wavelength infrared region (LWIR), (2) AR coatings for 1.06 micrometer and long wavelength infrared regions (LWIR), and (3) AR coatings for multiple wavelength regions namely VIS/NIR/MWIR (medium wavelength infrared region) and VIS/NIR/MWIR and LWIR. The spectral performance, mechanical and environmental durability, and rain-erosion test data is presented in this paper.
The new military requirements that call for the use of eyesafe lasers for range finding capabilities on aircraft, ships, tanks, and other vehicles has put an extra burden on the transmission of conductive coatings. Originally, the window coatings have low resistivity, less than 7.0 ohm/square, for EMI control were required to transmit up to 1.06 micron. With the new eyesafe lasers for range finding, the requirements of near infrared transmission have been pushed toward longer wavelengths. The windows are now required to have good transmission at 1.543 microns (eyesafe laser wavelength). The traditional indium-tin oxide coating has poor transmission varying from 20% to 50% due to free carrier absorption. A new conductive coating developed at HDOS has good transmission in photopic and xenon tracker regions and it has good transmission at 1.543 micron. This coating also has good environmental durability. The spectral performance, environmental durability, rain erosion resistance, acid resistance, and sand abrasion resistance of this coating are also significant.
A high performance, electrically conductive indium-based film for application to visible/near IR windows has been developed. It provides a low sheet resistance of 7 ohms/square along with high optical transmission (88% average from 400 to 1200 nm with the back surface AR coating). These films have been shown to be highly durable. Their combined performance is superior to the best conductive films reported in the literature. This new film is production ready and can be applied to substrates up to 20 inches in diameter.
Conventional ZnS, clear ZnS, ZnSe, and ZnS/ZnSe sandwich materials along with 8 to 12 micrometers anti-reflection (AR) coatings have been used as windows for forward looking infrared (FLIR) thermal imaging electro-optical sensors (such as those incorporated on PAVE TACK, F-18, and LANTIRN pods). Conventional ZnS also has been used as dome material for IR Maverick missiles and other missile applications. All of these systems have separate windows/systems for target designation, rangefinding, and low light level television (LLLTV) applications. New generation system require that a single window provide multispectral capabilities to perform various functions. A graded index AR coating developed at Hughes Danbury Optical Systems (HDOS) provides the multispectral capabilities and is highly durable for subsonic aircraft and missile applications. The spectral performance, durability, rain- erosion, and some sand and dust data of such a coating are presented in this paper. The data is also presented for this coating in conjunction with grids for EMI attenuation. The transmission of the coating as a function angle of incidence is also presented.
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