Optical Studies of Bio-inspired Materials for Camouflage
Sammanfattning: In the ongoing sensor-camouflage duel, new functionalities and sensing abilities are continuously incorporated in detector devices, requiring new capabilities on the camouflage side. The aim of this work is to contribute to improved camouflage including low polarization detectability in wavelength regions with both visible and invisible light. To find new surfaces that can be used as camouflage, we seek for materials that in a spectral design perspective meet the requests of military utility. We have represented this with a 'ladder model' which step by step connects the system and requirements aspects from a basic material level to a useful application.The focus in this study is mainly on the first rungs of the 'ladder' which are addressing aspects of spectral design and systems engineering as well as requirements of camouflage materials. We have stated six criteria for evaluating camouflage materials and their interactions with light. The criteria are related to reflection, gloss, degree of polarization, emissivity, broadband and dynamic properties.The included papers can be divided into two parts. The first part is related to aspects of the evaluation criteria for camouflage and the second part presents studies of bio-inspired materials and their performance related to the evaluation criteria.In connection with the presentation of the 'ladder model' for military utility and the six criteria for camouflage, several materials divided into the four categories: pigments, thin film coatings, multidimensional structures and metamaterials are surveyed. We also studied higher system design levels evaluating the visual and thermal contrast by inspecting images of an object's surface in a background. Important terms and parameters at this level were detectability and the lightness of a color. Furthermore a literature review of polarimetric environmental background properties was performed together with a reference materials study.Camouflage properties of several organic material surfaces have been studied. The biopolymeric materials range from beetle cuticle to cellulose based foams and protein fibres. In addition to white structures, we also investigated dynamic properties and initiated work using organic dyes to expand the use of the studied materials to camouflage applications. The main characterization techniques were reflection spectrometry, scatterometry and Mueller-matrix ellipsometry. These methods were used with an aim to mainly investigate the first three camouflage criteria (reflection, gloss, degree of polarization) but also touching on emissivity and dynamic coloring. Overall, the academic approach have been balanced with the requests and limits given by the military utility. The results will contribute to better camouflage by using advanced bio-inspired materials.
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