Multilayer thin film -based hyperspectral camouflage coatings
Tinus, Tuomas (2022)
2022
Master's Programme in Science and Engineering
Tekniikan ja luonnontieteiden tiedekunta - Faculty of Engineering and Natural Sciences
This publication is copyrighted. You may download, display and print it for Your own personal use. Commercial use is prohibited.
Hyväksymispäivämäärä
2022-06-27
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tuni-202206095585
https://urn.fi/URN:NBN:fi:tuni-202206095585
Tiivistelmä
Camouflage has a very important role on a modern battlefield, and developments in camouflage have always contested with developments in imaging technology. Hyperspectral imaging solutions in use today have the capability to distinguish between natural and camouflage materials based on the difference in manner in which they reflect light - the spectral signature. As a result, there is a pressing need for developing new camouflage materials which can compete with hyperspectral imaging in a wide spectral range.
Dielectric multilayer thin films are a well-developed optical coating technology that allows for creation of efficient reflective and antireflective coatings, gratings, filters, etc. Optical properties of such multilayer thin films can be adjusted with very high precision using specialised analysis and optimization tools to vary thickness and material of the layers.
In this thesis, hyperspectral camouflage materials are developed on the basis of dielectric multilayer thin films. While similar materials have been proposed previously for two-dimensional substrates, this work offers a drastic improvement in optical performance via a solution applicable to three-dimensional objects, namely coating with atomic layer deposition (ALD) method. ALD and more conventional ion beam sputtering techniques are used to deposit TiO2 and Al2O3 thin films with distinct properties, and ellipsometric data is employed to numerically optimise and analyse multilayer structures. Simulation results have produced designs with close adherence to the target material spectra in the 400–750 nm, 750–1800 nm wavelength ranges and their combination, on quartz glass and Si substrates. On metal substrates, this work shows the need for intermediate layers or system-scale solutions for acceptable performance.
Multilayer thin films deposited on quartz glass have shown close adherence to the numerical results, giving good reproduction of spectral features of natural materials in all wavelength ranges. The narrower the range - the lower the required complexity of the multilayer stack, and experimentally better spectral response across the range. The biggest disadvantage of the thin film multilayer approach is high gloss, which is undesirable in a camouflage material, so methods of gloss reduction were investigated. When deposited on sanded glass substrates gloss was reduced proportionally to increased surface roughness, however the quality of spectral response was reduced. Chemical etching of substrates with HF after sanding has resulted in change of surface morphology, decreased gloss and further decline in quality of reproduction of natural material spectra. Hyperspectral imaging has shown that the reduction of gloss and consequental increase in diffuse reflection has resulted in a camouflage material that, in certain conditions, improved on the capabilities of current camouflage technology.
Dielectric multilayer thin films are a well-developed optical coating technology that allows for creation of efficient reflective and antireflective coatings, gratings, filters, etc. Optical properties of such multilayer thin films can be adjusted with very high precision using specialised analysis and optimization tools to vary thickness and material of the layers.
In this thesis, hyperspectral camouflage materials are developed on the basis of dielectric multilayer thin films. While similar materials have been proposed previously for two-dimensional substrates, this work offers a drastic improvement in optical performance via a solution applicable to three-dimensional objects, namely coating with atomic layer deposition (ALD) method. ALD and more conventional ion beam sputtering techniques are used to deposit TiO2 and Al2O3 thin films with distinct properties, and ellipsometric data is employed to numerically optimise and analyse multilayer structures. Simulation results have produced designs with close adherence to the target material spectra in the 400–750 nm, 750–1800 nm wavelength ranges and their combination, on quartz glass and Si substrates. On metal substrates, this work shows the need for intermediate layers or system-scale solutions for acceptable performance.
Multilayer thin films deposited on quartz glass have shown close adherence to the numerical results, giving good reproduction of spectral features of natural materials in all wavelength ranges. The narrower the range - the lower the required complexity of the multilayer stack, and experimentally better spectral response across the range. The biggest disadvantage of the thin film multilayer approach is high gloss, which is undesirable in a camouflage material, so methods of gloss reduction were investigated. When deposited on sanded glass substrates gloss was reduced proportionally to increased surface roughness, however the quality of spectral response was reduced. Chemical etching of substrates with HF after sanding has resulted in change of surface morphology, decreased gloss and further decline in quality of reproduction of natural material spectra. Hyperspectral imaging has shown that the reduction of gloss and consequental increase in diffuse reflection has resulted in a camouflage material that, in certain conditions, improved on the capabilities of current camouflage technology.