On the evolution of NIR-reflecting materials

 
During both world wars near infrared sensitive film was used for obtaining sharp aerial photographs of objects for military reconnaissance since short wavelength atmospheric haze is reduced. It was also used to discriminate between foliage and camouflaged objects. Camouflage textiles and coatings were continuously improved to get a better match to the surrounding background not only in the visible but also in the near infrared range (NIR). Around 2000, the importance of the near infrared range of the solar spectrum to mitigate urban island heating effects was recognised and soon exploited by the paint industry. Since then advertisements on "cool coatings" appeared. These coatings are supposed to reflect a good portion of the NIR-radiation that is provided by the sun and translates directly into heat. In this presentation, examples of camouflage materials and NIR-reflecting pigments will be discussed. Reflectance spectra, heat-up curves and some surprising photographic effects along with the physics behind will be explained to provide a better understanding of this interesting matter.