Multi-spectral (optical/IR and RF) characterization of omnidirectional reflective dielectric thin film for sensor applications

Material characterization is important for the design and development of multi-spectral sensors. In this paper, we present the measurement and analysis of the optical properties of commercial multilayered dielectric films with high reflectivity in the visible (VIS) and near infrared (NIR) regimes. Using a UV/VIS/NIR spectrometer, we measured the reflectance and transmittance spectra of single, two-, and three-sheets of films at different incidence angles. Our results show that both reflectance and transmittance vary with incidence angles as well as the number of combined sheets of the films. For the NIR film, reflectance decreases from 90% to 70% as the angle changes from 70° to 50°, while for VIS film, the decrease is only from 90% to 85%. A low transmittance with less than 1% is found for VIS film, while that of NIR film is below 20%. We also find that the NIR film shows more spectrum variation with wavelength and combining more sheets can result in a 10% higher reflectance. For both NIR and VIS films, the measured permittivity is about 2.5 ∼ 2.7 between 2 GHz and 12 GHz.