A NOVEL SEMIEMPIRICAL METHOD TO DETERMINE THE INTERNAL REFLECTANCE OFTHE POLYMERIC COATING LAYER IN THE MID-IR REGION

This study presents a semi-empirical method to determine the internal reflectances of the polymeric coating layer in the IR waveband, An opt ical system including IR spectroradiometer, integrating sphere and bla ckbody source was also designed as the measurement system. The interna l reflectance can be determined by solving the simultaneous equations of transmittance and reflectance derived by the multi-reflection model for a free standing coating Nm. The transmittance and reflectance of the free standing coating layer can be obtained by measuring the trans mittance and reflectance of the coating/substrate (composite layers) s ystem and then resolving it via the plane parallel plate model. Experi mental results indicate that the internal reflectance of the free stan ding polymer film is negligibly small. However, the internal reflectan ce of the polymeric coating backed with an aluminum substrate is highe r than that of the standing polymer film and corresponds to the intern al transmittance. Observations involving polymer binders with differen t IR transparencies confirm the above events. The correlation between internal reflectances and internal transmittances can be used to predi ct the reflectances of polymeric coating systems accurately. (C) 1997 Elsevier Science B.V.