Coating Rules of Thumb
With thin-film design software becoming cheaper and more widespread, some system designers also take on the coating design task. In general, a coating obtained by using the software “optimize” feature will fall short of one produced by a skilled designer in the areas of tolerance and manufacturability. In either case, there are a few important rules of thumb to consider regarding coating performance.
Coating “features” (the wavelength at which peaks and valleys occur, band location, etc.) will shift toward lower wavelengths as the angle of incidence (AOI) is increased. This means that bandwidth and tolerance values need to be adjusted to take into account the wavelength shift created by the AOI range. The amount of wavelength shift is a function of the coating design and the materials used. A design with a larger proportion of high-index materials will show less angle shift than one dominated by low-index materials.
To a first approximation, coatings behave identically with both s- and p-polarized light at zero AOI. Polarization-specific effects occur at non-zero AOI with diverging values for the reflectance of the s- and p-polarized components. In the case of UAS imaging systems, designers try to minimize polarization effects since the imagers are not polarizationsensitive. In other optical coating applications, such as polarizing beamsplitters, the polarization effects can be exploited to attain a system goal.
Antireflection coatings (AR, ARC) show the greatest reduction in reflectance at a single wavelength or over a very narrow wavelength range. In general it is difficult to attain very low reflectance over more than a 2X range in wavelength. In order to get very broadband performance, the coating designer must strike a compromise between reflectance level and bandwidth. This is the approach used by Deposition Sciences, Inc. (DSI) in designing broadband antireflection (BBAR) coatings for UAS application. The graph in Figure 2 shows BBAR coating performance with a cone-angle average of fifteen degrees. Figure 3 indicates the angle performance of new UAS BBAR coatings from DSI.
Environmental Performance and Specifications
Optics on UAS can be subject to demanding environmental requirements, not only in the service environment but also in field deployment. Airborne service requirements include extremes of temperature, exposure to fuels, chemicals, solvents, foreign object debris (FOD) during ground operations, in-flight hazards such as dust and grit impact, and meteorological (rain, hail) impact. Field deployment risks include all the above plus conditions specific to sea or land exposure such as salt fog, humidity, mold and mildew, and handling during maintenance. A detailed discussion of environmental requirements is given in MIL-C-675C, MIL-F-48497, MIL-PRF-13830B, and MIL-F-48616. For up-to-date information, users should consult the DOD ASSIST service located at https://assist.daps.dla.mil/.
DSI’s new line of broadband anti-reflection coatings are intended for use on sapphire substrates for multispectral applications. Designed for the harsh operating environments of unmanned aircraft system applications (e.g. gimbal windows and detector assemblies), the wide-angle BBAR coatings are environmentally stable and perform to MILC48497 and MIL-F-48816 standards.
EMI/RFI and Conductive Coatings
Conductive coated optics are often part of the EMI/RFI design of a sensing system. The requirements could be to limit emissions, to prevent onboard systems from interfering with one another, or to address an electronic countermeasures (ECM) threat from an adversary. The transparent conductive oxide (TCO) thin-film materials are often used to provide electrical conductivity on an optical surface. The most well-known of these is indium-tin oxide or ITO, although various other materials are available. ITO has DC resistivity as low as 2 to 4 times 10-6 Ohm-m, but shows increasing free-carrier absorptance as the wavelength increases. This effect starts to appear as soon as 1 to 1.2 microns in the NIR; in the MWIR, ITO behaves as a “free electron” metal. For MWIR and LWIR use, conductive grids are still the main choice for RF rejection if system considerations permit. At present, there is no known material with adequate infrared (MWIR, LWIR) transparency and adequate electrical conductivity for electromagnetic shielding.