High-Dynamic-Range Absorption Spectroscopy of Polymer Sheets and Slabs by Integrating Sphere Illumination and Detection

Abstract

The optical absorption coefficient α of a homogeneous medium is typically determined by measuring the regular transmittance of a plane-parallel optical structure (e.g., a solid sheet or slab). Especially for low absorption, this technique is unsatisfactory as it depends on precise knowledge of the reflectance of the slab/sheet's faces and/or it needs a very large thickness. Using a specially designed integrating sphere for full-field illumination and detection, we eliminate the dependency on the knowledge of reflectance and increase the sensitivity orders of magnitude at very low absorption. We describe this method analytically and validate it experimentally by comparing the proposed method to the regular ones for different polymeric sheets and slabs. We measure the absorption coefficient of polystyrene (PS) and polycarbonate (PC) sheets and polymethylmethacrylate (PMMA) slabs in the spectral range of 400-2500 nm. With the presented integrating sphere procedure, the absorption coefficient α can be reliably obtained, covering four orders of magnitude in a single sample measurement.