Metallic mesh coatings are used on visible and infrared windows and domes to provide shielding from electromagnetic
interference (EMI) and as heaters to de-fog or de-ice windows or domes. The periodic metallic mesh structures that
provide the EMI shielding and/or resistive electrical paths for the heating elements create a diffraction pattern when
optical or infrared beams are incident on the coated windows. Over the years several different mesh geometries have
been used to try to reduce the effects of diffraction. We have fabricated several different mesh patterns on small
coupons of BK-7 and measured the transmitted power and the diffraction patterns of each one using a CW 1064 nm
laser. In this paper we will present some predictions and measurements of the diffraction patterns of several different
mesh patterns.
Metallic mesh thin film coatings have been used for many years to provide electromagnetic interference (EMI) shielding
on infrared windows and domes. The level of EMI shielding effectiveness (SE) of metallic mesh coatings when used in a
high frequency application is understood and characterized. Conversely, the level of SE of these metallic mesh coatings
when used in a low frequency application has been called into question. In a recent study, we applied an appropriately
designed metallic mesh coating to a sapphire window, mounted that window in a fixture, and tested the SE of the
window assembly over a frequency range that envelopes the various military platforms covered in MIL-STD-461 (10
kHz to 18 GHz) for a radiated emissions test. The test plan was devised in such a way as to independently assess the
individual contributions of the aperture, the mounting, and the metallic mesh coating to the total shielding. The results of
our testing will be described in this paper. Additionally, the test results will be compared to the predicted SE for both the
aperture and the metallic mesh coated window in order to validate the predictive model. Finally, an assessment of the
appropriateness of the use of metallic mesh coatings for EMI shielding in a low and/or broad range frequency
application will be made.
Metallic mesh thin-film coatings have been used for many years to provide electromagnetic interference (EMI) shielding
on infrared windows and domes. During the fabrication of these conductive, micron-sized mesh patterns, mesh voids or
holes in the mesh pattern occasionally occur. Voids in the mesh degrade the EMI shielding or insertion loss of the mesh
coating. In the past, we have shown that a small number of 1-mm voids do not degrade the insertion loss significantly
for 20-dB insertion-loss mesh coatings. In this paper, we present a theory that provides an approximation for the number
and size of mesh voids that can be tolerated without degrading the EMI shielding properties of a mesh coating. We also
measured the insertion loss of several typical metallic-mesh coatings with and without voids and compared the results
with our simple insertion loss model. Our analysis shows that tens of very small voids may have only minimal impact
on the EMI shielding properties of a metallic mesh coating. Even a single 3-mm diameter void may not degrade the
shielding properties significantly.
The use of wire grid structures to block EMI radiation is well known. For the past several years Battelle
has been developing photolithographic techniques and computer models to predict the performance of wire
grid structures on IR transmitting windows. The amount of IR radiation transmitted through non-resonant
grid structures is limited to the percent open area of the window. As the wires of the grid structure are
placed closer together the area of the window not obscured by the grid decreases. Resonant grid (mesh)
structures have the potential for superior JR transmission at specific wavelengths, while providing adequate
EMI attenuation. The polarization of the IR radiation becomes important when non-normal angles of
incidence are considered. A computer model has been developed to predict the performance of wire grid
structures on windows for various angles of incidence and polarization. This model has been experimentally
verified for a number of cases and has been shown to be useful in designing windows incorporating EMI
shielding. The results of the modeling for several cases, along with the experimental verification, are
presented. The limitations of such techniques are also discussed.
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